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
OH 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. 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
17. 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
18. 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
