NIES
Rates of Reactions
Mrs. Shymaa Taha
What is a chemical reaction?
A chemical reaction is the breaking down of bonds in reactants and the
formation of new bonds in products.
The rate of a reaction is a measure of how quickly a reactant is used up,
or a product is formed.
Collision theory
For a chemical reaction to happen:
•
reactant particles must collide with each other
•
the particles must have enough energy for them to react
A collision that produces a reaction is called a successful collision.
The activation energy is the minimum amount of energy needed for a
collision to be successful. It is different for different reactions.
The temperature, concentration, pressure of reacting gases, surface
area of reacting solids, and the use of catalysts, are all factors which
affect the rate of a reaction.
Measuring rates
Different reactions can happen at different rates. Reactions that happen
slowly have a low rate of reaction. Reactions that happen quickly have a high
rate of reaction. For example, the chemical weathering of rocks is a very slow
reaction: it has a low rate of reaction. Explosions are very fast reactions: they
have a high rate of reaction.
Reactants and products
There are two ways to measure the rate of a reaction:
1. Measure the rate at which a reactant is used up
2. Measure the rate at which a product is formed
The method chosen depends on the reaction being studied. Sometimes it is
easier to measure the change in the amount of a reactant that has been used
up; sometimes it is easier to measure the change in the amount of product
that has been produced.
1
The mean rate of reaction can be calculated using either of these two
equations:
𝑚𝑒𝑎𝑛 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 =
𝑚𝑒𝑎𝑛 𝑟𝑎𝑡𝑒𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 =
𝑞𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 𝑢𝑠𝑒𝑑
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
𝑞𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
Measuring mass
The change in mass of a reactant or product can be followed during a
reaction. This method is useful when carbon dioxide is a product which leaves
the reaction container. It is not suitable for hydrogen and other gases with a
small relative formula mass, Mr.
Bubbles formed indicate that a gas is produced so the mass of reactants will
decrease.
2
The units for rate are usually g/s or g/min
For example, the graph below could be used to calculate the average rate
over any period of time
3
Using this graph, we can calculate the average rate between 30 seconds and
120 seconds.
𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 =
𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑚𝑎𝑠𝑠
𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑡𝑖𝑚𝑒
𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 =
3.1 − 1.4
120 − 30
𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 =
1.7
90
𝑎𝑣𝑒𝑟𝑎𝑔𝑒𝑟𝑎𝑡𝑒 = 0.0189𝑔/𝑠
The unit that rate is measured in depends on the measurable quantity. Since
a change in mass is measured in grams and a change in time in seconds in
this example, the unit of rate would be grams per second (g/ s). Similarly, if a
change in concentration is measured (in mol/ l), then rate will have the unit
moles per litre per second (mol/ l s) or a change in volume measured in cubic
centimetres, centimetres cubed per second (cm3/ s).
4
Measuring volume
The change in volume of a reactant or product can be followed during a
reaction. This method is useful when a gas leaves the reaction container. The
volume of a gas is measured using a gas syringe, or an upside
down burette or measuring cylinder.
The units for rate are usually cm3 s-1 or cm3 min-1.
Two ways to measure the volume of a gas produced in a reaction
5
Graphs
The rate of reaction can be analysed by plotting a graph of mass or volume of
product formed against time. The graph shows this for two reactions.
The steeper the line, the greater the rate of reaction. Faster reactions where the line becomes horizontal - finish sooner than slower reactions
The gradient of the line is equal to the rate of reaction:
•
the steeper the line, the greater the rate of reaction
•
fast reactions - seen when the line becomes horizontal - finish sooner than
slow reactions
Units for rates of reaction
The rate of a chemical reaction can also be measured in Mol /s
6
• Factors affecting the rate of the chemical reactions:
1) Effect of concentration and pressure:
The rate of a chemical reaction can be changed by altering the concentration
of a reactant in solution, or the pressure of a gaseous reactant. If the
concentration or pressure is increased:
•
•
•
The reactant particles become more crowded
There is a greater chance of the particles colliding
The rate of reaction increases
Compared to a reaction with a reactant at a low concentration (if a solution) or
pressure (if a gas), the graph line for the same reaction but at a higher
concentration or pressure:
•
•
Has a steeper gradient at the start
Becomes horizontal sooner, showing that the reaction time is less
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Practical
Investigate the effect of changing the concentration on the
rate of a reaction by measuring the production of a gas.
Calcium carbonate reacts with dilute hydrochloric acid:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon
dioxide
CaCO3(s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g)
The volume of carbon dioxide gas produced can be measured using a gas
syringe.
Method
1. Support a gas syringe with a stand.
2. Using a measuring cylinder, add 50 cm3 of dilute hydrochloric acid to a
conical flask.
3. Add 0.4 g of calcium carbonate to the flask. Immediately connect the gas
syringe and start a stop watch.
4. Every 10 seconds, record the volume of gas produced.
5. When the reaction is complete, clean the apparatus.
6. Repeat steps 1 to 5 with different concentrations of hydrochloric acid.
8
Measuring the volume of carbon dioxide using a gas syringe.
Record the results in a table.
Volume (cm3)
Time (s)
0
0
10
…cm3
20
…cm3
Analysis
1. For each concentration of hydrochloric acid, plot a graph to show:
•
volume of gas (cm3) on the vertical axis
•
time (s) on the horizontal axis
•
draw a curve of best fit
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2. For each concentration of acid, calculate the mean rate of reaction until the
reaction stops:
𝑚𝑒𝑎𝑛 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛(𝑐𝑚3 /𝑠) =
𝑡𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑔𝑎𝑠 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑(𝑐𝑚3 )
𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒(𝑠)
3. Describe the effect of increasing the concentration of acid on the mean rate
of reaction. Use your graphs and calculations in step 2 to help you.
Question
Describe how you can tell that the reaction is complete.
Answer
No more gas is produced, and the line on the graph becomes horizontal.
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Hazards, risks and precautions
Evaluate the hazards and the precautions needed to reduce the risk of harm.
For example:
Hazard
Possible harm
Possible precaution
Hydrochloric acid
Causes skin and eye
Wear eye protection
irritation
Fizzing in the
reaction mixture
Acidic spray which
may damage skin
and eyes
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Use a large conical flask so there is plenty of
space inside; do not look over the top when
adding the calcium carbonate
2) Effect of temperature:
The rate of a chemical reaction can be changed by altering the temperature. If
the temperature is increased:
•
•
•
•
•
The reactant particles move more quickly
They have more energy
The particles collide more often, and more of the collisions are successful
The number of particles with activation energy increases.
The rate of reaction increases
Compared to a reaction at a low temperature, the graph line for the same
reaction but at a higher temperature:
•
•
Has a steeper gradient at the start
Becomes horizontal sooner, showing that the reaction time is less
12
Practical - Investigate the effect of changing the temperature
on the rate of a reaction. (Investigate the rate of reaction by
colour change)
Sodium thiosulfate solution reacts with dilute hydrochloric acid:
sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur
dioxide + sulfur
Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
The sulfur forms a cloudy yellow-white precipitate during the reaction. The
time taken for this to achieve a given cloudiness provides a way to measure
the reaction time. This type of reaction is a precipitation reaction where an
insoluble substance is formed (precipitate).
Method
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1. Using a measuring cylinder, add 50 cm3 of dilute sodium thiosulfate
solution to a conical flask.
2. Place the conical flask on a piece of paper with a black cross drawn on it.
3. Using a different measuring cylinder, add 10 cm3 of dilute hydrochloric
acid to the conical flask. Immediately swirl the flask to mix its contents,
and start a stop clock.
4. Look down through the reaction mixture. When the cross can no longer
be seen, record the time on the stop clock.
5. Measure and record the temperature of the reaction mixture, and clean
the apparatus as directed by a teacher.
6. Repeat steps 1 to 5 with different starting temperatures of sodium
thiosulfate solution.
Results
Record the results in a table. This table gives some example results.
Temperature of reaction mixture (°C)
Reaction time (s)
Reaction rate 1000/s
18
80
12.5
29
57
17.5
42
32
31.3
49
20
50.0
Analysis
1. Calculate 1000/time for each temperature. This value is proportional to the
rate of reaction.
2. Plot a graph to show:
•
reaction rate (/s) on the vertical axis
•
temperature (°C) on the horizontal axis
•
draw a curve of best fit
14
A graph showing reaction rate to temperature.
Question
Describe the effect of increasing the temperature of the reaction
mixture on the rate of reaction. Use your graph to help you.
Answer
The rate of reaction increases as the temperature increases. The rate
increases by a greater amount at higher temperatures.
Question
Suggest a reason why the same person should look at the black cross
each time.
Answer
Different people may decide that they cannot see the cross at different
amounts of cloudiness, leading to errors in deciding when to take the reaction
time.
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Hazards, risks and precautions
Evaluate the hazards and the precautions needed to reduce the risk of harm.
For example:
Hazard
Possible harm
Possible precaution
Hot sodium
thiosulfate
solution
Burns to the skin
Do not heat above 60°C
Sulfur dioxide
Can cause irritation to the eyes
and lungs, particularly to people
with asthma
Make sure the room is well
ventilated, avoid breathing directly
over the top of the flask
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3) Effect of surface area:
The rate of a chemical reaction can be raised by increasing the surface area
of a solid reactant. This is done by cutting the substance into small pieces, or
by grinding it into a powder. If the surface area of a reactant is increased:
• more particles are exposed to the other reactant
•
there are more collisions
•
the rate of reaction increases
Compared to a reaction with lumps of reactant, the graph line for the same
reaction but with powdered reactant:
• has a steeper gradient at the start
• becomes horizontal sooner
This shows that the rate of reaction is greater when the surface area is
increased.
Explosions
An explosion is a very fast reaction that releases a large volume of gaseous
products. There is a danger of explosion in factories that handle powdered,
flammable substances. These substances include custard powder, flour and
powdered sulfur.
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4) Effect of light:
Reactions that are initiated by light are called photochemical reactions.
The brighter the light, the faster the reaction.
Photograph in a tray of developing fluid
Photography
One example of a photochemical reaction is the use of silver halide salts (eg
silver chloride) in black and white photography.
Silver chloride is sensitive to light and breaks down to form metallic silver,
which appears black. This is because the silver ions, Ag+, become
silver atoms, Ag. The brighter the light falling on the photographic film or
paper, the faster the reaction - and the darker that part of the (negative) image
appears.
Photosynthesis
In photosynthesis (the chemical change that occurs in the leaves of green
plants), light energy is absorbed by the green pigment chlorophyll. This allows
the reaction between carbon dioxide and water to take place,
producing glucose and oxygen.
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5) Effect of catalysts:
The rate of a reaction can be increased by adding a suitable catalyst. A
catalyst is a substance which speeds up the reaction but is unchanged at the
end of the reaction.
Only a very small amount of catalyst is needed to increase the rate of reaction
between large amounts of reactants.
A catalyst is specific to a particular reaction:
• different catalysts catalyse different reactions
• not all reactions have suitable catalysts
This table summarises some common catalysts used in industry and the
reactions they catalyse:
Catalyst
Reaction catalyzed
Iron
Making ammonia from nitrogen and hydrogen
Platinum
Making nitric acid from ammonia
Vanadium(V) oxide
Making sulfuric acid
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The effect of catalyst on the rate of a chemical reaction
A picture of a particles of gaseous molecules or molecules/ions in
solution undergoing a chemical changes on the surface of a catalyst
•
Catalysts increase the rate of a reaction by reducing the activation
energy.
•
The activation energy is the minimum amount of energy required
to start a reaction.
•
They have the advantage of bringing about reactions at normal
temperatures and pressures which would otherwise need more
expensive and energy-demanding equipment.
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Therefore at the same temperature, using a catalyst, more reactant
molecules have enough kinetic energy to react compared to the
uncatalysed situation.
The catalyst does NOT increase the energy of the reactant molecules!
Neither does a catalyst increase the frequency of reactant particle collisions.
Many solution or gaseous catalysed reactions involve a solid catalyst.
The reactant molecules are adsorbed onto the surface, and this 'sticking on to
the surface' enables the bonds of the reactant molecules to be more easily
broken.
This is actually what 'lowering of the activation energy' means at the
molecular level.
Although a true catalyst does take part in the reaction and may change
chemically temporarily, but it does not get used up and can be
reused/regenerated with more reactants. It does not change chemically or
get used up in the end.
21
Experimental methods for investigating the effect of a catalyst on the
rate of a chemical reaction
•
The apparatus can be used to investigate how the speed of the
decomposition of hydrogen peroxide varies with different
catalysts.
o
o
o
o
o
o
o
Oxygen gas is given off which can be collected in the gas
syringe and its volume is used to measure how fast the reaction
is going.
In the diagram above, the white 'blobs' represent oxygen gas
being evolved and the grey lumps the catalyst powder.
hydrogen peroxide == catalyst ==> water + oxygen
2H2O2(aq) ====> 2H2O(l) + O2(g)
MnO2 Manganese(IV) oxide (manganese dioxide'), is a very
effective catalyst, but can also try other transition metal oxides
as catalysts like CuO copper(II) oxide.
The variables to be kept constant are - the concentration of the
hydrogen peroxide solution, the volume of the hydrogen
peroxide, the same amount of catalyst (ideally of the same
particle size) and the temperature of the reaction mixture.
You must also swirl the flask gently to ensure a good mixing as
the reaction proceeds.
22
•
GraphA (for a faster reaction) represent using a catalyst, but in GraphB
(a slower reaction) a less effective catalyst is used.
23
Activation energy, catalysts and reaction profiles
Reaction profile
diagram
Comments related to the reaction activation
energy and use of a catalyst
An exothermic reaction with a small activation
energy
The reaction may go very well without a catalyst
at a practical temperature, perhaps even at room
temperature
An exothermic reaction with a moderately high
activation energy.
This reaction might benefit from using a catalyst
if a suitable one is available
An endothermic reaction with a big activation
energy
This reaction would benefit from using a catalyst
e.g. to avoid using an excessively high
temperature, catalysts used to crack crude oil into
useful fractions
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Enzymes
Enzymes are biological catalysts – protein molecules that speed up chemical
reactions. They catalyse chemical reactions in living cells such. Examples of
these chemical reactions are:
•
•
•
Respiration
Photosynthesis
Protein synthesis
catalase is an enzyme that Catalyzes the breakdown of potentially harmful
hydrogen peroxide to water and oxygen. Important in respiration metabolism
chemistry.
2H2O2(aq) ==> 2H2O(l) + O2(g)
The optimum pH value for catalase to work effectively is approximately 7.
Zymase is an enzyme used in the production of alcohol and Amylase is an enzyme
used in the digestion of food.
Enzymes are specific. This means that they can only catalyse one reaction.
The shape of an enzyme determines how it works. Enzymes have active
sites that substrate molecules (the substances involved in the chemical
reaction) fit into when a reaction happens.
The active site has to be the right shape for the substrate molecules to fit into.
This means that enzymes have a high specificity for their substrate – a
particular type of enzyme will only work with one or a smaller number of
substrates. The animation shows how this works:
The mechanism involved is called the 'lock and key' mechanism. Just as a
lock will only accept one key, an enzyme will only accept one substrate
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Enzyme-catalysed reactions
Enzymes work best at particular temperatures and pH values.
Enzymes and temperature
At low temperatures, enzyme reactions are slow. They speed up as the
temperature rises until an optimum temperature is reached. After this point
the reaction will slow down and eventually stop. The graph shows what
happens to enzyme activity when the temperature changes.
In the example above, enzyme activity increases steadily between 0 ºC and
40 ºC. It peaks at 40 ºC (the enzyme's optimum temperature) then decreases
rapidly. This is because the enzyme gets denatured at temperatures above
the optimum.
26
Enzymes and pH
Different enzymes work best at different pH values, their optimum pH. Many
enzymes work fastest in neutral conditions. Making the solution more acidic or
alkaline will slow the reaction down. At extremes of pH the reaction will stop
altogether.
Some enzymes, such as those used in digestion, are adapted to work faster
in unusual pH conditions. For example, stomach enzymes have an optimum
pH of 2, which is very acidic.
The graph shows what happens to enzyme activity when the pH changes.
In the example above, enzyme activity increases between pH 4.5 and pH 8. It
peaks at pH 8, then decreases.
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