Exothermic and Endothermic Reactions Notes

Q: What is enthalpy change?

\Delta H is the energy transferred in a reaction: negative if released, positive if absorbed. Enthalpy change (\Delta H) measures the overall energy change, in kJ/mol. A negative \Delta H means exothermic; a positive \Delta H means endothermic.

Q: What is activation energy and how is it shown?

Ea is the minimum energy particles need to react; on a diagram it is the height from reactants to the peak. Activation energy (Ea) is the minimum energy colliding particles must have to react. Particles with less energy will not react however often they collide.

Q: How do you calculate \Delta H from bond energies?

\Delta H is the energy of bonds broken minus the energy of bonds made. Count each bond in the balanced equation, use the bond energy table, add up bonds broken (reactants) and bonds made (products), then subtract. A negative answer means exothermic.

Cambridge IGCSE Chemistry: Revision notes

Key facts

Exothermic reactions transfer energy to the surroundings (temperature rises); endothermic reactions take it in (temperature falls).
$\Delta H$ is negative for exothermic and positive for endothermic reactions, in kJ/mol.
Activation energy $E_a$ is the minimum energy colliding particles need to react.
Breaking bonds takes in energy; making bonds releases it.
$\Delta H$ = energy of bonds broken − energy of bonds made.

Exothermic or endothermic

Exothermic reactions release energy to the surroundings; endothermic reactions absorb it.

The key idea is the direction of energy flow. Exothermic reactions release energy, felt as heat. Endothermic reactions absorb energy, causing cooling.

Illustrative energy levels. Exothermic: products end lower (energy released). Endothermic: products end higher (energy absorbed).

Exothermic Endothermic Energy transfer Transfers energy to the surroundings Takes in energy from the surroundings Temperature of surroundings Rises Falls Examples Combustion, neutralisation, respiration, condensation Melting ice, evaporation, dissolving some salts, photosynthesis

	Exothermic	Endothermic
Energy transfer	Transfers energy to the surroundings	Takes in energy from the surroundings
Temperature of surroundings	Rises	Falls
Examples	Combustion, neutralisation, respiration, condensation	Melting ice, evaporation, dissolving some salts, photosynthesis

Exothermic

Energy transfer:: Transfers energy to the surroundings
Temperature of surroundings:: Rises
Examples:: Combustion, neutralisation, respiration, condensation

Endothermic

Energy transfer:: Takes in energy from the surroundings
Temperature of surroundings:: Falls
Examples:: Melting ice, evaporation, dissolving some salts, photosynthesis

A reaction makes the surroundings cooler. It is:

Enthalpy change

$\Delta H$ is the energy transferred in a reaction: negative if released, positive if absorbed.

Enthalpy change ( $\Delta H$ ) measures the overall energy change, in kJ/mol. A negative $\Delta H$ means exothermic; a positive $\Delta H$ means endothermic. The combustion of methane has $\Delta H = -890$ kJ/mol; the thermal decomposition of calcium carbonate has $\Delta H = +178$ kJ/mol. A larger value means more energy transferred.

Energy profiles for exothermic and endothermic reactions

Thermal decomposition of calcium carbonate has $\Delta H = +178$ kJ/mol. What does this tell you?

Activation energy

$E_a$ is the minimum energy particles need to react; on a diagram it is the height from reactants to the peak.

Activation energy ( $E_a$ ) is the minimum energy colliding particles must have to react. Particles with less energy will not react however often they collide. Catalysts lower $E_a$ but do not change $Delta H$ .

On a pathway diagram, $E_a$ is the distance from reactants up to the peak; $\Delta H$ is the distance from reactants to products.

Exothermic reaction pathway diagram

Worked example

On a pathway diagram, the reactants are at 400 kJ/mol, the peak is at 550 kJ/mol and the products are at 350 kJ/mol. Find $E_a$ and $\Delta H$ .

1
$E_a$ = peak − reactants = 550 − 400 = 150 kJ/mol.
2
$\Delta H$ = products − reactants = 350 − 400 = −50 kJ/mol.
3
The sign is negative, so the reaction is exothermic.

On a reaction pathway diagram, $\Delta H$ is measured from:

Bond breaking and making

Breaking bonds takes in energy; making bonds releases it; the balance gives $\Delta H$ .

Bond breaking needs energy (endothermic) and bond making releases energy (exothermic). If more energy is released making bonds than is needed to break them, $\Delta H$ is negative (exothermic). If more is needed to break bonds than is released, $\Delta H$ is positive (endothermic).

1
Bonds in reactants break
energy is taken in
2
Atoms rearrange
new bonds are ready to form
3
Bonds in products form
energy is released
4
Compare the two
more released = exothermic

Energy changes in a reaction

Bond making is:

Bond energy calculations

$\Delta H$ is the energy of bonds broken minus the energy of bonds made.

Count each bond in the balanced equation, use the bond energy table, add up bonds broken (reactants) and bonds made (products), then subtract. A negative answer means exothermic. Bond energies are averages, so answers can differ slightly from measured values.

1
Count the bonds
in the balanced equation, reactants and products
2
Add bonds broken
energy taken in (reactants)
3
Add bonds made
energy released (products)
4
Subtract
ΔH = broken − made; negative means exothermic

Working out ΔH from bond energies.

$\Delta H$ energy of bonds broken $-$ energy of bonds made

Worked example

Calculate $\Delta H$ for $\ce{H2 + Cl2 -> 2HCl}$ . Bond energies: H–H = 436, Cl–Cl = 242, H–Cl = 431 kJ/mol.

1
Bonds broken: 1 × H–H + 1 × Cl–Cl = 436 + 242 = 678 kJ/mol.
2
Bonds made: 2 × H–Cl = 2 × 431 = 862 kJ/mol.
3
$\Delta H$ = 678 − 862 = −184 kJ/mol.

Worked example

Calculate $\Delta H$ for $\ce{CH4 + 2O2 -> CO2 + 2H2O}$ . Bond energies: C–H = 413, O=O = 498, C=O = 805, O–H = 464 kJ/mol.

1
Bonds broken: 4 × C–H + 2 × O=O = 1652 + 996 = 2648 kJ/mol.
2
Bonds made: 2 × C=O + 4 × O–H = 1610 + 1856 = 3466 kJ/mol.
3
$\Delta H$ = 2648 − 3466 = −818 kJ/mol.

Bonds broken total 678 kJ/mol and bonds made total 862 kJ/mol. What is $\Delta H$ ?

Try an exam question

Calculate $\Delta H$ for $\ce{H2 + Cl2 -> 2HCl}$ . Bond energies: H–H = 436, Cl–Cl = 242, H–Cl = 431 kJ/mol. State whether the reaction is exothermic or endothermic.

[4 marks]

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