1. Enthalpy Changes
Chapter 13
LEARNING OUTCOMES
Distinguish between exothermic and
endothermic reactions in terms of energy
content of products and reactants
Draw an energy profile diagram to illustrate
endothermic and exothermic change
Calculate energy changes from experiments
or experimental data
2. Exothermic reactions
When a piece of magnesium ribbon burns
in air or oxygen, heat and light energy are
produced.
This type of chemical reaction in which heat
energy is produced and given out to the
surroundings is called an exothermic change
or reaction.
An exothermic reaction is a reaction in which heat energy
is given out to the surroundings.
Enthalpy Changes
Chapter 13
3. Exothermic reactions
During an exothermic reaction,
the temperature of the
surroundings rises since heat
energy is given out by the
reaction.
Enthalpy Changes
Chapter 13
4. Other exothermic reactions
Freezing and condensation (physical changes)
Combustion of fuels
Neutralisation of acids and alkalis
Dissolving solid sodium hydroxide, anhydrous sodium
carbonate and anhydrous copper(II) sulphate in water
Adding water to concentrated sulphuric acid
Enthalpy Changes
Chapter 13
5. Endothermic reactions
An endothermic reaction is a
reaction in which heat energy is
absorbed from the surroundings.
In an endothermic reaction, the
temperature of the surroundings drops,
since heat energy is absorbed.
Enthalpy Changes
Chapter 13
When ammonium chloride crystals are dissolved in
water, there is a drop in temperature. This shows that
heat energy is absorbed from the surroundings.
6. Other Endothermic reactions
Melting and boiling (physical changes)
Thermal decomposition of compounds such as calcium
carbonate (limestone) and copper(II) carbonate
Photosynthesis
Taking a photograph with film (decomposition of
silver bromide into silver and bromine)
Dissolving ionic crystals in water, e.g.
dissolving ammonium nitrate, ammonium chloride and
sodium carbonate crystals in water
Enthalpy Changes
Chapter 13
7. Heat of Reaction
The amount of heat energy given out or taken in
during a chemical reaction is called the heat of reaction
or enthalpy change.
Enthalpy change is represented by the symbol, ∆H.
Matter contains both potential and kinetic energies. During
an exothermic reaction, heat is given out by the reactants
to the surroundings. Thus, enthalpy change is negative.
During an endothermic reaction, heat energy is absorbed
by from the surroundings. Reactants gain energy, so
enthalpy change is positive.
Enthalpy Changes
Chapter 13
8. Heat of Reaction
In an exothermic reaction, the
reactants are at a higher energy
level than that of the products.
The excess heat energy is given
out by the reactants when the
products are formed.
Since the reactants have lost
heat energy, we take ∆H as
negative.
Exothermic reaction
Enthalpy Changes
Chapter 13
9. Heat of Reaction
In an endothermic reaction,
the reactants are at a lower
energy level than that of
the products.
Heat energy is absorbed
by the reactants from the
surroundings in order to
change to the products. Endothermic reaction
Since the reactants have gained
heat energy, we take ∆H as
positive.
Enthalpy Changes
Chapter 13
10. Heat of Reaction
The negative value of ∆H means that the reaction is
exothermic.
“890 kJ” means that when 1 mole of methane burns completely
in oxygen, 890 kJ of heat energy is given out.
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) [∆H = - 890 kJ]
The heat of reaction can be included in a chemical
equation as:
Enthalpy Changes
Chapter 13
11. Heat of Reaction
The positive value of ∆H means that the reaction is
endothermic.
“222 kJ” means that when 1 mole of calcium carbonate is
decomposed, 222 kJ of heat energy is absorbed.
CaCO3 CaO + CO2 [∆ H = + 222 kJ]
Enthalpy Changes
Chapter 13
12. Bond breaking and bond making
The reason why reactions are endothermic or
exothermic is because energy is taken in or given out
when chemical bonds are broken or made.
Heat energy is absorbed to break bonds, so bond
breaking is endothermic.
Enthalpy Changes
Chapter 13
13. Bond breaking and bond making
Heat energy is given out (released) when bonds
are formed, so bond forming is exothermic.
Enthalpy Changes
Chapter 13
14. Bond breaking and bond making
So, the overall heat change in a reaction is given by:
If more energy is absorbed for breaking bonds than
released in forming bonds, the reaction will be
endothermic.
If more energy is released in forming bonds than
absorbed for breaking bonds, the reaction will be
exothermic.
Heat of reaction, ∆H = Heat absorbed Heat given out
in bond breaking in bond forming
Enthalpy Changes
Chapter 13
15. Worked example
H
+
H
O
H H O
H H
O
H H O
Since ∆H is negative, the overall reaction is exothermic.
Hydrogen burns in oxygen to form water according to the equation:
2H2(g) + O2(g) 2H2O (g)
By calculating ∆H, state whether the reaction will be endothermic or
exothermic, given the following bond energies per mole:
H-H = 436 kJ, O=O = 496 kJ, O-H = 463 kJ.
Solution:
Energy absorbed to break bonds = (2 x 436) + (1 x 496) = 1368 kJ
Energy released to form bonds = (4 x 463) = 1852 kJ
Hence, ∆ H = Energy absorbed – Energy released
= (1368 – 1852) kJ = - 484 kJ
Enthalpy Changes
Chapter 13
16. The action of a catalyst
Most catalysts increase the
rate of a reaction. A reaction
in which a catalyst is used to
increase the rate has a lower
activation energy than the
same reaction without a
catalyst
The effect of using a catalyst
can be shown on energy profile
diagrams.
Enthalpy Changes
Chapter 13
17. Calculating enthalpy changes
If the change in temperature that occurs during a reaction is
measured, the heat change, can be determined using the
formula given below
Specific heat capacity is the quantity of heat energy
required to rai se the temperature of 1 g of a substance
by 1oC.
Heat of reaction, ∆H = mass of reactant x specific heat x temperature
(g) capacity change
(J g-1 oC-1) (oC)
Enthalpy Changes
Chapter 13
18. Calculating enthalpy changes
To determine the heat of reaction,
the reaction is carried out in an
insulated container called a
calorimeter. The temperatures of
the reactants are measured before
mixing. The maximum or minimum
temperature reached when the
reactants are mixed is then
measured and used to determine
the temperature change.
Enthalpy Changes
Chapter 13
19. Three assumptions are made in calculating the heat of reaction:
The density of a dilute aqueous solution is the same as pure water, 1
g cm-3. This means that 1 cm3 of solution has a mass of 1 g.
The specific heat capacity of a dilute aqueous solution is th e same
as pure water, 4.2 J g-1 C-1. This means that it requires 4.2 J to
increase the temperature of 1 g of water by 1oC.
A negligible amount of heat is lost to, or absorbed from, the
surroundings during the reaction.
Heat of reaction, ∆H = mass of reactant x specific heat x temperature
(g) capacity change
(J g-1 oC-1) (oC)
Enthalpy Changes
Chapter 13
20. Determining the heat of solution
The heat of solution is the heat change when 1 mol of solute
dissolves in such a volume of solvent further dilution by the
solvent produces no further heat change.
When a solute dissolves in a solvent:
Bonds break between the solute particles; ionic bonds between
ions break in ionic compounds and intermolecular forces between
the molecules break in covalent substances. This absorbs energy
from the surroundings.
Intermolecular forces between the solvent molecules also break.
This absorbs energy from the surroundings.
Enthalpy Changes
Chapter 13
21. Determining the heat of solution
Attractions form between the ions or molecules of the solute and
the molecules o f the solvent, a process is called solvation. This
releases energy to the surroundings.
The reaction is exothermic if the energy absorbed to break bonds
in the solute and solvent is less than the energy released during
solvation.
The reaction is endothermic if the energy absorbed to break
bonds in the solute and solvent is greater than the energy
released during solvation
Enthalpy Changes
Chapter 13
22. Determining the heat of solution
When determining the heat o f solution, the initial temperature of
the water and the maximum or minimum temperature of the
solution must be measured. The temperature increase or
decrease and the number of moles of solute that dissolved must
then be calculated.
Chapter 13
23. Determining the heat of neutralization
Th heat of neutralization is the heat change when 1 mol of wafer
is produced in a neutralization reaction between an alkali and an
acid.
When determining the heat of neutralization, the temperature of
both solutions must be measured and used to determine the
average initial temperature. The maximum temperature of the
solution after mixing must then be measured and used to
calculate the temperature increase. Finally, the number of moles
of water made in the reaction must be determined.
Enthalpy Changes
Chapter 13
24. To determine the number of moles of water made in the reaction:
1000 cm3 NaOH(aq) contains 2.0 mol NaOH
:. 50 cm3 NaOH(aq) contains =
2.0
1000
x 50 mol NaOH
= 0.1 mol NaOH
1000 cm' H2SO,(aq) contains 1.0 mol H2SO4
:.50 cm3 H2SO4 (aq) contains =
1.0
1000
x 50 mol H2SO4
Equation: 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l)
i.e. 2 mol NaOH reacts with 1 mol H2SO4 forming 2 mol H2O(aq)
: . 0.1 mol NaOH reacts with 0.05 mol H2SO4 to form 0.1 mol H2O(aq)
0.1 mol H2O is made in the reaction.
25. To determine the heat of neutralization:
Total volume of solution = 50 + 50 = 100 cm3
:. mass of solution = 100 g
Average initial temperature =
29.4+30.0
2
oC = 29.7 oC
Final temperature = 43.2 oC
:. temperature increase = 43.2 - 29.7 oC = 13.5 oC
Specific heat capacity of the solution = 4.2 J g-1 C-1
Heat change = mass of solution x specific heat capacity x temperature change
heat evolved in forming 0.1 mol H20 = (100 x 4.2 x 13.5) J
= (5670) J
heat evolved in forming 1 mol H20 =
5670
0.1
J
=56700 1
26. Determining the heat of neutralization
Heat of neutralization, ∆H = -56.7 kJ mol-1
The heat of neutralization, ∆H, is negative because the
temperature of the reaction increased, indicating that it lost energy
to the surroundings. The reaction was exothermic.
Enthalpy Changes
Chapter 13
27. Summary
Exothermic reaction Endothermic reaction
Heat energy is given out to the
surroundings.
Heat energy is absorbed from the
surroundings.
∆H is negative. ∆H is positive.
More heat is given out in making
bonds than absorbed in breaking
bonds.
More heat is absorbed in breaking bonds
than given out in making bonds.
Enthalpy Changes
Chapter 13
28. Quick check
1. What is meant by (a) an exothermic reaction, (b) endothermic reaction?
2. What is meant by enthalpy change? What symbol is used to represent enthalpy
change?
3. What type of reaction is it when the enthalpy change ∆H is
(a) positive, (b) negative ?
4. Sodium hydroxide reacts with nitric acid according to the equation:
NaOH(aq) + HNO3(aq) NaNO3(aq) + H2O(l) ∆H = - 57.3kJ
(a) Is this reaction endothermic or exothermic? Explain your
answer.
(b) What would be the heat evolved if 2 moles of sodium
hydroxide are completely neutralised ?
5. State whether the following are exothermic or endothermic reactions:
(a) decomposing zinc carbonate,
(b) reacting potassium with water,
(c) forming hydrogen atoms from hydrogen molecules,
(d) forming chlorine molecules from chlorine atoms,
(e) respiration,
(f) photosynthesis.
Solution
Enthalpy Changes
Chapter 13
29. Solution to Quick check
1. (a) An exothermic reaction is a reaction which gives out heat energy to the
surroundings.
(b) An endothermic reaction is a reaction which absorbs heat energy from the
surroundings.
2. Enthalpy change is the amount of heat given out or taken in during a reaction.
The symbol used is ∆H.
3. (a) Endothermic (b) Exothermic
4. (a) The reaction is exothermic because ∆H is negative which means that heat
energy is lost from the reactants.
(b) Amount of heat evolved = 2 x 57.3 = 114.6 kJ
5. (a) decomposing zinc carbonate: endothermic
(b) reacting potassium with water: exothermic
(c) forming hydrogen atoms: endothermic
(d) forming chlorine molecules: exothermic
(e) respiration: exothermic
(f) photosynthesis: endothermic
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Enthalpy Changes
Chapter 13
