JOHNMAR S. DELIGERO
Chemistry/Biology 12 (Nova Scotia Curriculum)
Sino-Canadian Program
Henan Experimental High School
Zhengzhou Henan, China
http://www.bananateachersworld.wikispaces.com
What happens during a phase change?
What happens during a chemical reaction?
These changes involve changes in the potential
energy of a system, not the kinetic energy.
Enthalpy (H, also known as heat content) is the total
energy of a system, some of which is stored as chemical
potential energy in the chemical bonds.
Chemists use the term enthalpy change (ΔH) to refer to
the potential energy change of a system during a
process such as a chemical reaction or a physical change.
Enthalpy changes are measured at constant pressure.
The units of enthalpy change are kJ/mol.
a. The enthalpy change of a chemical reaction
represents the difference between the potential
energy of the products and the potential energy of
the reactants.
b. In chemical reactions, potential energy changes
result from chemical bonds being broken and
formed.
b. Chemical bonds are sources of stored energy
(potential energy). Breaking a bond is a process that
requires energy. Creating a bond is a process that
releases energy.
Chemists define the total internal energy of a substance at a
constant pressure as its enthalpy, H.
Chemists do not work with the absolute enthalpy of the reactants
and products in a physical or chemical process.
Instead, they study the enthalpy change, that accompanies a
process. That is, they study the relative enthalpy of the reactants
and products in a system.
This is like saying that the distance between your home and your
school is 2 km. You do not usually talk about the absolute position
of your home and school in terms of their latitude, longitude, and
elevation.
You talk about their relative position, in relation to each other.
- A chemical bond is caused by the attraction between the electrons and nuclei
of two atoms.
- Energy is needed to break a chemical bond, just like energy is needed to break
a link in a chain. On the other hand, making a chemical bond releases energy.
- The strength of a bond depends on how much energy is needed to break the
bond.
- When a reaction results in a net absorption of energy, it is called an
endothermic reaction.
- On the other hand, when a reaction results in a net release of energy, it is
called an exothermic reaction.
- In an exothermic reaction, more energy is released to form bonds than is used
to break bonds. Therefore, energy is released.
- The enthalpy change of a chemical reaction is known as the
enthalpy of reaction, ΔHrxn. The enthalpy of reaction is
dependent on conditions such as temperature and pressure.
- Therefore, chemists often talk about the standard enthalpy
of reaction, ΔH0rxn: the enthalpy change of a chemical
reaction that occurs at SATP (25°C and 100 kPa).
- Often, ΔH0rxn is written simply as ΔH0. The “0” symbol is
called “nought.”
- It refers to a property of a substance at a standard state or
under standard conditions.
- You may see the enthalpy of reaction referred to as the heat
of reaction in other chemistry books.
Representing Exothermic Reactions
There are three different ways to represent the enthalpy change of an exothermic
reaction.
1. The simplest way is to use a thermochemical equation: a balanced chemical equation
that indicates the amount of heat that is absorbed or released by the reaction it
represents. For example, consider the exothermic reaction of one mole of hydrogen
gas with half a mole of oxygen gas to produce liquid water. For each mole of hydrogen
gas that reacts, 285.8 kJ of heat is produced. Notice that the heat term is included
with the products because heat is produced.
H2(g) + 1/2O2(g) → H2O(l) + 285.8 kJ
2. You can also indicate the enthalpy of reaction as a separate expression beside the
chemical equation. For exothermic reactions, ΔH° is always negative.
H2(g) + 1/2O2(g) → H2O(l) ΔH0rxn = −285.8 kJ/mol
3. A third way to represent the enthalpy of reaction is to use an enthalpy diagram.
Representing Endothermic Reactions
As for an exothermic reaction, there are three different ways to represent the
enthalpy change of an endothermic reaction.
1. You can include the enthalpy of reaction as a heat term in the chemical
equation. Because heat is absorbed in an endothermic reaction, the heat
term is included on the reactant side of the equation.
117.3 kJ + MgCO3(s) → MgO(s) + CO2(g)
2. You can also indicate the enthalpy of reaction as a separate expression
beside the chemical reaction. For endothermic reactions, the enthalpy of
reaction is always positive.
MgCO3(s) → MgO(s) + CO2(g) ΔH0rxn = 117.3 kJ/mol
3. Finally, you can use a diagram to show the enthalpy of reaction.
- The enthalpy change associated with a reaction depends on the amount of
reactants involved. For example, the thermochemical equation for the
decomposition of magnesium carbonate indicates that 117.3 kJ of energy
is absorbed when one mole, or 84.32 g, of magnesium carbonate
decomposes. The decomposition of two moles of magnesium carbonate
absorbs twice as much energy, or 234.6 kJ.
117.3 kJ + MgCO3(s) → MgO(s) + CO2(g)
234.6 kJ + 2MgCO3(s) → 2MgO(s) + 2CO2(g)
- Enthalpy of reaction is linearly dependent on the amount of substances
that react.
- That is, if the amount of reactants doubles, the enthalpy change also
doubles.
- In other words, when you multiply the stoichiometric coefficients of a
thermochemical equation by any factor, you must multiply the heat term
or enthalpy expression by the same factor.
Standard Molar Enthalpy of Formation
- In a formation reaction, a substance is formed from elements in their standard
states.
- The enthalpy change of a formation reaction is called the standard molar
enthalpy of formation, ΔH0f.
- The standard molar enthalpy of formation is the quantity of energy that is
absorbed or released when one mole of a compound is formed directly from its
elements in their standard states.
- When writing a formation equation, always write the elements in their standard
states. For example, examine the equation for the formation of water directly
from its elements under standard conditions.
H2(g) + 1/2O2(g) → H2O() ΔH0f = −285.8 kJ
- A formation equation should show the formation of exactly one mole of the
compound of interest. The following equation shows the formation of benzene,
C6H6 under standard conditions.
6C(graphite) + 3H2(g) → C6H6(l) ΔH0f = 49.1 kJ
Standard Molar Enthalpy of Combustion
The standard molar enthalpy of combustion, ΔH0comb, is the enthalpy
associated with the combustion of 1 mol of a given substance. The change in
enthalpy is measured for the products and reactants in their standard states.
For example, for methane, ΔH0comb = −965.1 kJ/mol. You can represent the
standard molar enthalpy of combustion using a thermochemical equation or
using an enthalpy diagram.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 965.1 kJ
Notice that water is shown in the liquid form.
Although water is formed as vapour during a
combustion reaction, the enthalpy change for
a standard molar enthalpy of combustion is
measured with the energy change required
for products to cool to SATP taken into
consideration.
The energy released or absorbed during a chemical reaction
depends on the reactants involved.
For example, the reaction of hydrogen
and oxygen to form water releases
heat to the surroundings, while the
reaction of magnesium carbonate to
form magnesium oxide and carbon
dioxide absorbs heat from the
surroundings.
For each chemical reaction at SATP,
there is a specific enthalpy change.
What other factors affect the enthalpy change of a given chemical
reaction?
Compare the warmth you feel from a burning wooden match and
the warmth you feel from a roaring bonfire. Both involve the same
chemical reactions (the combustion of wood, primarily cellulose),
but the heat released in each case is clearly different. There is a
great deal more cellulose and oxygen involved in the reactions in
the bonfire compared to the burning matchstick. Therefore, the
amount of reactants present plays a role in the energy change.
As you know, the symbol for amount is n, and the unit is the mole
(mol). The heat released or absorbed by a system during a
chemical change can be calculated using the relationship:
- Like chemical reactions, changes of state involve changes in the potential energy
of a system only.
- The temperature of the system undergoing the state change remains constant.
Because energy is absorbed or released as heat, however, the temperature of the
surroundings often changes.
- The energy changes associated with changes of state are
important in regulating
body temperature. When you sweat, for example, the water
absorbs heat from your skin as the water vaporizes.
- Cats lick their fur, and the vaporizing liquid helps keep them
cool.
- Dogs cool off in a similar way.
The evaporating water absorbs
heat from their mouths and
helps keep them cool.
- The energy change associated with a physical change is smaller
than the energy change associated with a chemical change.
- In the case of molecular substances, for example, changes of
state involve the breaking of intermolecular forces.
- Intermolecular forces are generally much weaker than chemical
bonds. The energy released or absorbed when intermolecular
bonds form or break is much less than the energy released or
absorbed when chemical bonds form or break.
- Enthalpy changes for changes of state between liquid and gas
are greater than changes of state between liquid and solid.
For example, when a solid melts to form a liquid the
attractive forces
between particles are not completely
broken. The particles remain close together.
-
-
When a liquid changes to a gas, however, the attractive forces are
completely broken as relatively great distances separate the particles from
each other.
The difference in potential energy between a liquid and a gas is much
greater than the difference in potential energy between a solid and a liquid.
- You can represent the enthalpy change that accompanies a
change of state—from liquid to solid, for example—just like you
represented the enthalpy change of a chemical reaction.
- You can include a heat term in the equation, or you can use a
separate expression of enthalpy change.
For example, when one mole of water melts, it absorbs
6.02 kJ of energy.
H2O(s) + 6.02 kJ → H2O(l)
H2O(s) → H2O(l) ΔH = 6.02 kJ/mol
Normally, however, chemists represent enthalpy changes associated with
phase changes using modified ΔH symbols. These symbols are described
below.
• molar enthalpy of vaporization, ΔHvap : the enthalpy change for the state
change of one mole from liquid to gas.
• molar enthalpy of condensation, ΔHcond: the enthalpy change for the state
change of one mole of a substance from gas to liquid.
• molar enthalpy of melting, Δhmelt or enthalpy of fusion, ΔHfus. the enthalpy
change for the state change of one mole of a substance from solid to liquid.
• molar enthalpy of freezing, ΔHfre : the enthalpy change for the state change
of one mole of a substance from liquid to solid.
- Vaporization and condensation are opposite processes. Thus, the enthalpy changes
for these processes have the same value but opposite signs. For example, 6.02 kJ of
heat is needed to vaporize one mole of water.
Therefore, 6.02 kJ of heat is released when one mole of water freezes.
ΔHvap = −ΔHcond
Similarly, melting and freezing are opposite processes.
ΔHmelt = −ΔHfre
- Another type of physical change that involves a heat transfer is dissolution. When 1
mol of a solute dissolves in a solvent, the enthalpy change that occurs is called the
molar enthalpy of solution, ΔHsoln. Dissolution can be either endothermic or
exothermic.
- Manufacturers take advantage of endothermic dissolution to produce cold
packs that athletes can use to treat injuries. One type of cold pack contains
water and a salt, such as ammonium nitrate, in separate compartments.
When you crush the pack, the membrane that divides the compartments
breaks, and the salt dissolves. This dissolution process is endothermic. It
absorbs heat for a short time, so the cold pack feels cold.
The molar enthalpy of solution, ΔHsoln, of ammonium nitrate is 25.7 kJ/mol.
NH4NO3(s) + 25.7 kJ → NH4NO3(aq)
- Some types of hot packs are constructed in much the same way as the cold
packs described above. They have two compartments. One compartment
contains a salt, such as calcium chloride. The other compartment contains
water. In hot packs, however, the dissolution process is exothermic. The
process releases heat to the surroundings.
The molar enthalpy of solution, ΔHsoln, of calcium chloride is −82.8 kJ/mol.
CaCl2(s) → CaCl2(aq) + 82.8 kJ
You can determine the heat absorbed or released by a state change or
dissolution using the equation q = nΔH, just as you did with chemical
reactions.