The Law of Conservation of Mass:
The total mass of the substances does not change during a chemical reaction. The
number of substances might change, their properties might change, but the total amount
of matter must remain constant. The moral is: matter can be neither created nor
destroyed. The law of conservation of mass can be explained as : atoms are neither
created nor destroyed during a chemical reaction, they just rearrange to form a new
substance.
C5H12
(l) +
8O2 (g)
5 CO2 (g) +
6 H2O (l)
We took a hydrocarbon (C5H12) known as pentane, added some oxygen to it and with an
input of energy we generate CO2 gas and water.
The Law of Definite Proportion/Constant Composition:
Constant composition refers to the elemental composition by mass of a given compound
being the same for all samples of that compound. This means that every, every, every
time water is decomposed, there are 88.8 grams of oxygen present for every 11.2 g of
hydrogen. And a 100-gram sample of NaCl always contains 39.3 grams of Na and 60.7 g
of Cl.
How does this work?
NaCl is made up of 1 Na and 1 Cl.
The molar mass of Na = 22.9
The molar mass of Cl = 35.4
Thus Na =
22.9
22.9  35.4
*
100 = 39.3 grams
Constant composition/definite proportion is explained by assuming that atoms combine
in fixed ratios when they form a compound. Thus, if one oxygen atom combines with 2
hydrogen atoms to form water, then all samples of water must have the same
composition
The Law of Multiple Proportions:
In different compounds containing the same elements, the masses of one element
combined with a fixed mass of the other element are in the ratio of the smallest whole
numbers.
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Consider SO2 and SO3. Both compounds contains only sulfur and oxygen atoms. Sulfur
oxygen for every atom of sulfur. The formulas suggest that for a given mass of sulfur,
the masses of oxygen in SO3 compared to SO2 will be in a 3/2 ratio. Other examples
include CO and CO2, FeCl2 and FeCl3, UF3 UF4 and UF6.
How does this work?
SO2
molar mass of S = 32.06
molar mass of O = 15.99
grams of S in a 100 g sample = 50.1
grams of O in a 100 g sample = 49.9
gramsO
=1
gramsS
SO3
molar mass of S = 32.06
molar mass of O = 15.99
grams of S in a 100 g sample = 40.1
grams of O in a 100 gram sample = 59.9
gramsO
= 1.5
gramsS
Since we do not want decimals (remember we are looking for the ratio of the smallest
whole numbers!!) for ratios we must get rid of the 0.5 and turn it into a whole number. If
we multiply a 0.5 by 2 we get 1 (for 0.33 multiply by 3 to get the new whole number, for
0.25 multiply by 4, for 0.75 multiply by 4 etc . . .)
So – comparing the O in SO3 to the O in SO2 for a given 1 atom of sulfur results in 3 O
atoms (1.5*2) to 2 O atoms (1*2). REMEMBER!! Whatever we do to one ratio number we
must mathematically do to the other. Therefore a 1.5:1 ratio is equal to 3:2 ratio if we
want the answer in only whole numbers (which we DO!!)
It is very important to begin thinking in terms of moles when examining chemical reactions as it
clarifies the amount of each substance present. Comparing molar masses does not clarify the
ratio of each substance present. Thinking in terms of masses does nothing but show us that the
mass during the chemical reaction will be conserved (the grams at the beginning will be the
same number of grams at the end of the chemical reaction). For example, consider the following
relationships:
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2.016 g of H2 + 38.00 g F2  40.02 g HF
1 mole of H2 + 1 mole F2  2 mole HF
This information shows that equal amounts of H2 and F2 molecules combine to form twice as a
large a population of HF molecules.
This chemical equation is made up of chemical formulas that express the identity and quantity
of substance that undergo a chemical or physical change. These chemical equations are like
sentences, which means you need to KNOW the names of the chemical species that are coming
together and undergoing a reaction. The left side of the equation shows the chemical species
that are coming together and doing the reacting, thus they are called the reactants. The
chemical species that we end up with at the end are called the products. The reactant side
shows the amount of substance present before the change and the right side shows the amount
of material produced. For the equation to accurately depict the amounts, it must be balanced.
That means the same atoms must appear on both sides of the equation and there must be the
same number of moles of each atom on both sides of the equation.
Rules for Balancing Chemical Reactions
1.) Write a skeletal equation: this will include the reactants and the products
2.) Balance the number of atoms: the number of atoms on the left side of the
arrow should be the same as the number of atoms on the right side of the arrow.
3.) Make the coefficients whole numbers:
even though we can accurately
balance a chemical equation using fractions, we will want all the numbers as
whole numbers. This means multiplying through the entire equation.
4.) Indicate the state of matter: are the reactants solids, liquids or gases? Wh
What about the products? Identify the state of matter!
Concept Test
Balance the following equations:
_________H3PO4 + ________NaCN  ________HCN + ________Na3PO4
__________Na + __________Cl2  __________NaCl
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Keep in mind that when you are balancing the equations that the coefficient operates on all the
atoms in the formula. Just like the parentheses when we write a chemical formula applies to all
atoms inside the parentheses, the coefficient operates on all atoms in the formula. You cannot
change the chemical formulas to balance the number of atoms. The formulas are set by the type
of compound it is. For example, in the reactions above, you cannot change Na 3PO4 to NaPO4 in
order to “balance” the number of sodium ions! You also cannot add new elements to chemical
reactions in order to balance the reaction. You do not balance the reaction by adding 2 Na+1
ions to the reactant side so that you have 3 sodium ions on each side. Use the reactants and
products given and balance by using coefficients. A balanced chemical equation will stay
balanced no matter if you multiply or divide each coefficient by the same value. You can
multiply the system by 2 or 10 or 1000, but the atoms/ions will still be balanced. However,
generally speaking, a balanced equation should contain the lowest whole number ratio of
coefficients!
We have spent the past few weeks (!!) learning how to write name chemical species, draw
chemical species, write and balance chemical equations, balancing chemical equations, and
using the stoichiometry to relate quantities of reactants and products in the chemical reaction.
We will spend time studying the three main types of chemical reactions, acid-base,
precipitation, and redox. And we have studied the heat, and entropy associated with chemical
reactions. Another thing of importance to study is how fast does a reaction go, how much
reactant is consumed or how much product is formed in a given amount of time, are there ways
that we can speed up or slow down a chemical reaction, how do the atoms come together
during the chemical reaction so that we can determine the path in a step-by-step manner known
as the mechanism.
The study of motion is called kinetics, from the Greek work kinesis, meaning “movement.”
Kinetics deals with the speed of a reaction and its mechanism, or pathway that the molecules
take to get from reactant to product.
Chemical kinetics is the study of reaction rates, the changes in concentration per unit time.
Every chemical reaction proceeds at a different rate. Some require a very long time to consume
the reactants and are described as slow. The disintegration of an aluminum can by atmospheric
oxidation or of a plastic bottle by the actions of sunlight can take years, decades, or even
centuries. While other reactions occur in the blink of an eye. Acid base neutralization reactions
occur as fast as we can mix the materials together. Hydrogen gas and fluorine gas at room
temperature form hydrogen fluoride gas in an extremely rapid reaction. Hydrogen gas and
oxygen gas forming water at room temperature happens to be extremely slow. Under
conditions of high temperature however, the reaction occurs with much speed.
Carbon
monoxide and nitrogen monoxide, two harmful pollutants are produced in large quantities in
automobile engines. These gases can react with one another and produce CO2 and N2 gas
which are considerably more environmentally friendly than CO and NO. Unfortunately, this
reaction is very slow, regardless of the temperature, but by using a catalyst (a species that helps
the reaction along but itself is not consumed in the chemical reaction) in the form of the catalytic
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converter in your car, this reaction occurs at a much faster rate. Catalysts are invaluable
components when studying chemical kinetics or when performing chemical reactions.
Knowing how fast a reaction occurs, or being able to control the speed of the reaction is very
important in the real world. How fast can medicine act in the body? How long after you take
that aspirin does your headache go away? How long after you take your allergy medicine will
you stop sneezing? How long will the medicine stay in your system – how long does it take for
the medicine to be metabolized? How long does it take for cement to harden? Nowadays that
can be very important. Down in Corvallis, OR they actual pay people to sit and watch the
cement dry to prevent delinquent youths from writing in the wet cement. The longer it takes
the cement to dry, the more they have to pay people to literally sit there. In general, chemists
have the ability to manipulate reaction speeds to some extent. Even a little manipulation of a
chemical reaction can increase product yield, increase or decrease the speed of the reaction as
needed for a particular manufacturing process. The ability to manipulate the reaction can save
the company millions of dollars thus increasing profit – a very important thing in society today.
It was through kinetics and the study of mechanisms that the pathway leading to ozone
depletion was discovered.
The mechanism indicates that chlorine atoms from
chlorofluorocarbons (CFCs) act as the catalysts in the depletion of the ozone layer. Confidence
in the accuracy of the mechanism has led the nations of the world to take measures to reduce
and hopefully eventually eliminate the use of CFCs. Without kinetics and reaction mechanism,
the ozone layer may be even worse off than it is today.
Rate is defined as a change per some unit time. The rate of motion of a moving car is defined in
some number of miles per hour. A chemical reaction rate is expressed as a change in amount or
concentration of some reactant or product per unit time. Time can be measured in seconds,
minutes, hours, days, years, etc . . . depending on how long the reaction takes or even how long
the scientist chooses to observe the reaction.
If one can predict reaction rates, chances are one can do things to control the rate. Certain
variables are very important in controlling the rate of the reaction, such as the nature of the
reactants, concentration of reactants, temperature, surface area and the presence of a catalyst.
Under a given set of conditions (e.g. temperature and pressure) each reactant has its own
characteristic rate. This is determined by the chemical nature of the reactants. At room
temperature, hydrogen, the same reactant gas, behaves very differently chemically in the
following chemical reactions:
H2 (g) + F2 (g) → 2HF (g) [very fast]
3H2 (g) + N2 (g) → 2NH3 (g) [very slow]
There is a common reactant in each chemical equation, H2, yet the reaction rates for the two
reactions are nothing alike. This means that each reaction rate is unique. Generalizations
should not be made (meaning that every time you see H2 gas does not mean that the reaction is
always fast, or even that it is always slow). Limited predictions can be made however, they
should be done so with caution:
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Ions in solution tend to react quickly: most ionic precipitates tend to form quickly.
Think back to the chemical reactions demo when a tiny Pb(NO 3)2 crystal and some KI
crystals were placed on either side of a drop of water. When you moved the crystals into
the drop of water, almost immediately you saw the formations of the yellow PbI 2 solid.
The same reaction occurred when pre-made solutions of Pb(NO3)2 and KI were mixed
together. The bright yellow precipitate appeared almost instantaneously. Acid-base
neutralization reactions performed in lab occurred almost instantaneously. You all
monitored the changes by using phenolphthalein indicator and the color change was also
instantaneous. You did not have to wait 20 minutes or even 2 minutes to see if the
reaction was complete. Ionic reactions are fast due to the high mobility of the dissolved
ions (Brownian motion!!) and the electrostatic attraction that occurs between positively
and negatively charged ions.
Covalent molecules, especially large ones, tend to react slowly. Even in solution form,
organic reactants or other covalent reactants can take hours or days to yield an
appreciable amount of product. In organic next year, many of you will experience the
joys of organic lab, as you wait hours upon hours for your product to be produced.
Molecules with strong covalent bonds tend to react more slowly than those with weak
bonds. O2 and N2 persist in the atmosphere and do not undergo chemical reactions
because each has high bond dissociation energies. (think of their Lewis structures!)
Other molecules, such as Cl2 have weak bond dissociation energies and tend to react
much more quickly.
Higher concentrations tend to react more quickly than species at low concentrations.
Reactions can only occur when chemical species get near one another. At lower
concentrations molecules have a hard time finding one another. If they cannot find one
another, they cannot possibly be close enough to react. Imagine you want to find a
person, ANY person on this campus and there are only 10 people total here one day.
And they can be anywhere one campus. Good luck finding them! However, on a normal
school day, how easy is it to find a person – once again ANY person with whom you can
interact – pretty easy! Same with molecules that will be involved in a chemical reaction.
reaction rate  collisional frequency  concentration
The ability to “mix” affects reaction rate. The frequency of collision also depends on the
physical state of the reactants. When the reactants are in the same phase (homogeneous
reaction), such as the solid or liquid phase, the molecules are free to move around and to
collide with one another. When the reactants are in different phases (heterogeneous
reaction), then the reaction will occur at the interface between the two phases. The
greater the surface area available between the reactants, the more contact there is and the
faster the reaction can proceed. The more finely divided a solid or liquid reactant, the
more surface area is available and the faster the reaction.
Molecules must collide with enough energy in order to react. Remember that molecules
in the gas phase move faster and with more average kinetic energy as the temperature is
increased. More motion means more collisions. Many collisions result in a billiard ball
like effect. The molecules collide and actually bounce off one another. No reaction takes
place. Once enough energy is put into the system (usually in the form of increasing the
6
temperature) the molecules have sufficient energy upon reaction colliding with one
another to react. Raising the temperature increases the reaction rate by increasing the
rate of collisions and more importantly, imparting enough energy to the molecules
allowing them to react.
reaction rate  collisional energy  temperature
The effects of temperature on reaction rate are intuitively known – even now – by you!
What happens to milk if you leave it out? It spoils – probably pretty quickly as in a
matter of days if you leave it on the counter. Why do we put it in the refrigerator? To SL-O-O-W down the rate of the chemical reaction known as spoiling. What if we want a
good steak. How long would we have to wait if we took the meat out of the refrigerator
for it to cook itself on the countertop? Right, it would spoil before it cooked itself!! So in
order to cook the meat (also a chemical reaction) we put the meat on the grill – at a high
temperature in order to brown the meat!
A catalyst is a substance that increases the rate of the reaction without itself being
consumed in the chemical reaction. Catalysts are widely used in industry to speed up
reactions that would otherwise be too slow to be practical. They are also used to carry
out processes at lower and more economical temperatures. A catalyst that works for one
reaction may not work at all for another. Enzymes are catalysts for biochemical
reactions, but each enzyme is used for a specific reaction in the biochemical pathway.
There are literally thousands of enzymes present in living cells.
Increasing the temperature of a reaction increases the average kinetic energy of the molecules,
making them move faster and thus, increases their collisional frequency. We have previously
talked about how temperature affects molecules speeds. In class, each of you is taking notes,
some of you are shifting in your seats, some of you get up to sharpen your pencils, and some of
you are sleeping. There is movement. However, you are not really colliding with one another;
sometimes maybe you accidentally kick your neighbor, or reach over and tap them to ask a
question. However, if I lit a fire in the room and locked you all inside, chances are, you will
definitely be 1.) moving faster and with more energy, and 2.) colliding and running into one
another more often!
But how often molecules collide is not the only factor to consider. Because it is known that at
20oC and 1 atm the molecules in 1 mL of gas experience about 1027 collisions per second. And at
that rate, all reactions would be over instantaneously if all that mattered was that they run into
one another. In fact, the vast majority of collisions result in the molecules bouncing off one
another, much like billiard balls in a game of pool or like bumper cars at the local fair. Every
reaction that occurs has an energy threshold that must be met when molecules collide in order
for them to react. A great analogy to this is an athlete who must jump over a hurdle or a high
bar in order to complete the “race”. The minimum energy needed for the collisions to mean that
the molecules react instead of bouncing off one another is called the activation energy (E a). This
is the energy required to activate the state from which reactant bonds can be broken and
product bonds formed. At any given temperature, there are molecules with many different
7
kinetic energies, thus when they collide they collide with a wide range of collisional energies as
well. According to the collision theory, ONLY those molecules that collide with enough energy
to exceed or meet the activation energy will react to form product.
Increasing the temperature makes the molecules move faster, but more importantly, increasing
the temperature increases the number of collisions that have enough energy to exceed the
activation energy which increases the rate at which product is formed.
The activation energy can be seen on an energy diagram which shows the conversion between
reactants and products. We previously examined energy diagrams when we looked at exo and
endothermic reactions.
The “hump” between the reactants starting energy and the products that are formed is the
amount of energy needed for the reaction to occur. All reactions need a certain amount of
energy in order to overcome the hump. Even spontaneous reactions have this activation
energy. There are two activation energies present on any diagram. As chemical reactions are
reversible, there is the activation energy required for the forward direction (as written, from
reactants to products) and there is an activation energy required to convert the products back to
the reactants. Notice that one of the activation energies is larger than the other – and it depends
on whether the final energy state is lower or higher than the initial energy state. You should be
able to label an energy diagram with all the components pictured below and define/explain
what is occurring during the reaction.
energy
Endothermic Reaction
products
Ea reverse
Efinal > Einitial
reactants
Ea forward
time
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Exothermic Reaction
reactants
Ea forward
Efinal < Einitial
Ea reverse
products
time
The activation energy value will change from reaction to reaction. Thus, the value of E a and the
temperature will affect exactly how many molecules have enough energy to be converted from
reactants to products. The rate of the reaction will depend on how long it takes for the
molecules to have sufficient energy to make it over the hill to become products. If not many
molecules make it, then the amount of product produced will be small. If many molecules
make it into the product form, then the amount of product produced can be substantial.
Notice that the height of Ea forward and Ea reverse are different. The smaller the Ea the faster
that reaction will proceed comparatively speaking. That means that for an endothermic
reaction, the reverse reaction will occur faster than the forward reaction. And for an exothermic
reaction, the forward reaction will occur faster than the reverse.
Larger Ea => smaller k values => slower reaction rates
Smaller Ea => larger k values => faster reaction rates
There is a final component for the collision theory, and that is molecular structure. Even though
there are millions upon millions of collisions occurring per second, not very many occur with
substantial energy in order to exceed the activation energy to become products. In addition to
having the correct amount of energy, the molecules MUST collide in such a way that is
favorable for the reaction to occur – meaning they must come together like pieces in a puzzle.
This is often termed correct orientation. The molecules must be orientated in such a manner
that the region of the molecule that is undergoing the reaction meets with the correct region of
the other molecule so that the reaction can actually take place. Much like putting a puzzle
together. You may have the LAST two pieces of the puzzle in your hands, but they do not fit
9
together in some haphazard way do they? No, the little indentations and outcroppings only fit
together and lock into place in one way. The same is true with molecules. They fit together in a
specific way in order for the reaction to take place.
The puzzle piece can only fit ONE way in order to complete the picture
The same is true of molecules, they must meet in a certain way in order to
react and create the final picture (the products)
The probability of the molecules having the correct energy AND the correct orientation makes
investigating reaction rates a pretty complicated thing. In fact, for one chemical reaction [ NO (g)
+ NO3 (g) → 2NO2 (g)] , only 1 out of every 167 collisions results in the correct orientation and
energy that will lead to the creation of product. For some chemical reaction, orientation is not
the all important factor in determining if the reaction will take place, the frequency and
activation energy are more important. For other chemical reactions, especially biochemical
reactions, orientation is the major bottleneck in determining if the chemical reaction will occur.
In fact, for some biochemical reactions, orientation is so important that only 1 in a million
molecules are correctly orientated and have sufficient energy to react. Thus, it is a GOOD thing
that there are 1027 collisions per second! And it makes our functioning bodies quite amazing!
Imagine a jar of water open to the atmosphere. We know, that after some time has passes, that
the water will all evaporate and disappear. But what if we put a lid on the water container?
The water does not disappear, in fact, it could stay in the jar for years. We do, however, see
little beads of water form along the upper walls and on the top of the jar as the water
evaporates, and then condenses inside the container. The gas molecules are not free to “fly
away” and thus the volume change of water is minimal (depends on how tight we have that
lid!!), or perhaps even constant!
On the molecular level (which is where we should be thinking now) chemical and physical
changes invariably are composed of two processes, with each process undoing the work or the
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progress of the other. When one process occurs more rapidly than the other, we see something
happen - a gas is evolved, a precipitate forms etc. . . When both processes occur at the same rate
we think that nothing is happening – and we oftentimes make the mistake of saying that
“nothing is happening”. In actuality, the system is at equilibrium such that the forward and the
reverse reactions are occurring at the same rate. We will now investigate this state when
equilibrium exists and we will see that equilibrium systems are actually very active and very
much “alive”.
A lot of reactions happen and then they stop – this is examining reactions that “go to
completion”.
However, not all reactions go to completion. Reversible reactions reach the
equilibrium state when the forward and reverse reactions proceed at the same rate and to the
same extent. Rate is a tricky concept. Chemical equilibrium is actually unconcerned with the
amount of time that it takes for a reaction to reach the equilibrium state. We will be examining
the concentrations of components once they reach the equilibrium state. Later on, we will be
examining how long it takes for reactions to proceed, but now, we are unconcerned with how
long. At equilibrium, we are studying the EXTENT that the reaction has proceeded – how much
of each species is present. If it took 10 years to get to equilibrium or 10 seconds, that does not
matter!
Dynamic equilibrium is a state in which no net change takes place between two opposing
processes occurring at the same rate. We have already examined equilibrium in terms of vapor
pressures (liquid gas equilibrium). There are two requirements for equilibrium to take place:
the process must be reversible (one that goes backwards as well as forwards)
the system must be closed (a system from which substances cannot escape)
True equilibria are rarely found in nature because few real systems are entirely closed. In an
open container, the products tend to disperse and the reverse reaction cannot take place.
In the state of equilibrium, the concentration of the reactants and the concentration of the
products no longer change with time. It appears to us – as the observer on the macroscopic
scale – that there is nothing going on.
Since no changes occur in the concentrations of reactants or products in a reaction system at
equilibrium, it may appear that everything has stopped. However, this is not the case. On the
molecular level, there is frantic activity. Equilibrium is not static but is a highly dynamic
situation. The concept of chemical equilibrium is analogous to the flow of cars across a bridge
connecting two island cities. Suppose the traffic flow on the bridge is the same in both
directions. It is obvious that there is motion, since one can see the cars traveling back and forth
across the bridge, but the number of cars in each city is not changing because equal numbers of
cars are entering and leaving. The result is no net change in the car population.
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Changes in matter are accompanied by a change in energy. Remember, nature is lazy!! Physical
and chemical changes occur because that is the path of least resistance. The production and
usage of energy that occurs in reactions have enormous impacts on society. People are making
big bucks as they try to find new ways to harness energy (think of nuclear energy, solar energy,
and fossil fuels).
Thermodynamics refers to the study of heat and its transformations while thermochemistry is
the branch of thermodynamics that deals with the heat involved in chemical reactions. Where
does the heat change come from??
Chemical reactions are accompanied by energy changes. Some reactions release energy to the
surrounding, some reactions absorb energy. A reaction that releases or gives off heat energy is
termed an exothermic reaction. A reaction that absorbs or needs heat energy in order to occur
is termed an endothermic reaction.
Energy can be converted from one form into another. Sunlight falling on a surface is absorbed
and converted into heat energy. The kinetic energy of running water and the thermal energy of
steam are converted into electrical energy by turbines. The stored potential energy of fuel is
converted into mechanical energy by automobile engines. Conservation of Energy tells us that
energy is neither lost nor gained, it just changes from one form to another. In fact, we know that
systems cycle from potential to kinetic and back again. What was once kinetic, then becomes
potential again. Thus is the cycle of energy.
First Law of Thermodynamics: energy can be neither created nor destroyed, it can only be
converted from one form to another.
To make a meaningful observation about this conversion of energy, we must first make
assignments. What substance/species/situation are we studying? This becomes the system.
Everything else, are the surroundings. These are “arbitrary” assignments. It all comes back to
point-of-view. In order to explain our observations, we must choose a point of view. Therefore,
we make an assignment that this is the system. Everything else will be the surrounding. The
system could be the plant growing in your yard, and its surroundings would be the ground, the
air, and everything else around it. The system is usually the object or reaction that we are
studying. The surroundings would include the container, the atmosphere, and the observer.
The first law of thermodynamics tells us that the system and surrounding will interact such that
the energy lost by the system will be absorbed by the surroundings. Or the energy absorbed by
the system will be absorbed from the surroundings.
Many chemical events occur spontaneously, meaning on their own, without the input of
additional energy. Other events only occur when there is some sort of intervening action. We
can oftentimes anticipate spontaneous occurrences, for example, ice melting outside of the
freezer is no surprise, living things grow older and die. We know that tomorrow is not
yesterday, thus we already know the natural direction of many events. The first law of
thermodynamics tells us that tomorrow’s energy will be the same as today’s which is the same
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as yesterday’s. From an energy standpoint, all days are equal, so events might as well runs
backwards or forwards, but they do not. Milk does not unspoil, meat does not cook itself.
Time is a one-way trip.
Chemical equilibrium is defined as meaning that the forward chemical reaction or event occurs
at the same rate as the reverse chemical reaction or event. The rate of evaporation (a phase
change from a liquid into a gas) equaled the rate of condensation (a phase change from the gas
into the liquid) for our water in the closed jar example.
A reversible chemical reaction is somewhat analogous to a tug of war between two children. If
one child is stronger than the other, then a net driving force exists and the rope (reaction) will
move in the direction of the stronger child. If the two children are evenly matched against one
another, there may be momentary shifts in one direction or the other but the rope will maintain
its equilibrium position.
A spontaneous change in a system, whether a chemical, physical, or a change in location, is one
that occurs by itself under some set of conditions (e.g. temperature, pressure, etc) without the
ongoing input of energy. Water spontaneously freezes at -5oC ad 1 atm. It spontaneously melts
at 80oC and 1 atm. Some spontaneous changes need a little input of energy initially, but once
they get rolling, it continues without the need for this extra energy. Fires are an example of this
type of spontaneous process. After you take the match away from the wood in your fireplace,
the fire will keep on blazing if there is wood present.
In contrast, a nonspontaneous change requires the constant input of some form of energy in
order to maintain the chemical, physical, or change in location that they system is undergoing.
A book falls off the table spontaneously (if left balancing precariously on the edge), but what are
the chances that the book will spontaneously put itself back on top of the table? Not so good.
It is important to know that: if a reaction or change is spontaneous in one direction, it is not
spontaneous in the reverse direction.
We have talked about the first law of thermodynamics and the energy of the reaction –
specifically we were looking at heat. But is the first law of thermodynamics able to explain
which reactions are spontaneous or not? The answer is no. It can only account for energy
transfer, it cannot predict or explain why or which direction the reaction will proceed. For
example, ice melts in your hand. It absorbs the heat energy from your hand and melts. So why
doesn’t the melted ice give off that heat is absorbed and refreeze in your hand? The first law
says that the energy of the system will be conserved if that would happen – so why doesn’t it
happen? Obviously there must be other variables to consider.
It was believed, for many years, that if a reaction was exothermic or endothermic, that one could
predict whether or not a reaction was spontaneous. For many years, it was believed that
exothermic reactions were spontaneous while endothermic reactions were nonspontaneous.
And while many exothermic reactions are spontaneous, there are several real world simple
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examples of spontaneous endothermic reactions. Ice melting for example. Ice placed in your
hand removes the heat from your hand and uses that heat energy to change from a solid to a
liquid. Water vaporizing is also an endothermic reaction. It takes energy for water to change
from the liquid to the gas phase. However, even at room temperature and normal pressures,
some water molecules are vaporizing! Salts dissolving in water (NaCl, NH 4NO3) and turning
into ions is also an endothermic reaction that is spontaneous.
Examining these examples carefully shows us some things in common:
solid  liquid  gas
ionic solid and water  ions in solution
Think about the order and disorder associated with solids, liquids, gases and ions in solution.
As we proceed from the solid state to the liquid state to the gas state disorder increases (or order
decreases). It appears that order, or lack thereof might be important for determining if a
reactions is spontaneous or not.
Therefore, there must be other factors that we must consider when determining if a reaction is
spontaneous or not.
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Which situation would you expect to happen spontaneously?
(I)
(II)
Adding food coloring to water shows the color dispersing (randomizing) its molecules into the
liquid water. The food coloring does not stay as one big blob in the water. By increasing it’s the
disorder of its molecules and the water molecules the color is able to permeate throughout the
entire water solution. Would you expect that the color food coloring molecules would ever
conglomerate back together into a blob in the water? Would this be creating order or disorder if
the molecules did just that? Which situation is favored – order or disorder?
Which situation would you expect to happen spontaneously?
(I)
(II)
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Placing ice on the countertop shows the solid, ordered water molecules melting and turning
into liquid water molecules. Is this increasing or decreasing the order of the water molecules?
Would you expect the liquid water molecules to group back together on the countertop at room
temperature and turn back into solid water? If they did, would this be creating order or
disorder out of the water molecules? Which situation is favored – order or disorder?
Entropy is a measure of disorder, which is related to the number of arrangements that are
possible for a system. A disordered system has many ways of being arranged and a high
probability of existing. An ordered system has few ways of being arranged and a low
probability of existing. For example, if you spill a glass of water it is highly improbable that the
water will fall to the floor in the shape of the glass because many more arrangements of the
water molecules are possible if the water spreads out over the floor. The water molecules are
more disordered on the floor than in the glass.
A spontaneous change has a natural tendency to occur without outside intervention. The
driving force for a spontaneous process is an increase in the entropy (and disorder) of the
universe.
There is a natural tendency for nature to become more disordered. Think about all the work
you have to do to keep your room clean, to keep the kitchen clean. It would and is so much
easier to just throw the dirty clothes on the floor, to leave the dishes in the sink. It is so hard
and takes so much work to actually put the clothes away or in the dirty clothes hamper and to
wash the dirty dishes and then put them away. Too much work  These are two classic easy
examples to illustrate that disorder – or more disorder is favored. Nature constantly moves
towards more disorder.
Second Law of Thermodynamics: all spontaneous changes are accompanied by an increase in
the entropy of the system/surroundings. This increase in entropy of the system/surroundings
means that the final entropy is greater than the initial entropy which means that the entropy of
the universe also increases!
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