5 Chemical energetics | IGCSE Chemistry 0620

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🎯What you need to be able to do

State that an exothermic reaction transfers thermal energy to the surroundings (temperature rises), and an endothermic reaction takes it in (temperature falls).
Interpret reaction pathway diagrams for exothermic and endothermic reactions.
Use \( \Delta H \) (negative for exothermic, positive for endothermic) and define activation energy, E_a EXTENDED.
Draw and label reaction pathway diagrams with reactants, products, \( \Delta H \) and E_a EXTENDED.
Explain \( \Delta H \) in terms of bond breaking (endothermic) and bond making (exothermic), and calculate it from bond energies EXTENDED.

📚The chemistry

Exothermic and endothermic

Exothermic: thermal energy is transferred to the surroundings, so their temperature rises. Examples: combustion, neutralisation, most displacement reactions, respiration.
Endothermic: thermal energy is taken in from the surroundings, so their temperature falls. Examples: thermal decomposition, photosynthesis, dissolving ammonium nitrate in water.

EXTENDED The thermal energy transferred is the enthalpy change, \( \Delta H \): negative for exothermic reactions (the chemicals lose energy) and positive for endothermic ones. The activation energy, E_a, is the minimum energy that colliding particles must have to react.

Reaction pathway diagrams

E_a: from the reactants up to the peak. \( \Delta H \): from reactants to products — down (negative) for exothermic, up (positive) for endothermic.

When you draw one: label both axes (energy; progress of reaction), write the formulae of the reactants and products on their levels, and draw the arrows with a single head. The \( \Delta H \) arrow starts at the reactants’ level and ends at the products’ level.

Bond energies EXTENDED

Breaking bonds is endothermic (energy must be put in); making bonds is exothermic (energy is released). A reaction is exothermic when more energy is released making the new bonds than is taken in breaking the old ones:

\[ \Delta H = \text{energy to break bonds} - \text{energy released making bonds} \]

Draw the displayed formulae first so that you count every bond — including how many molecules of each substance the equation shows.

The worked example: 2648 kJ in to break the bonds, 3466 kJ out when the new bonds form, so \( \Delta H = -818 \) kJ/mol.

✏️Worked example EXTENDED

Methane burns: \( \mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O} \). Bond energies in kJ/mol: C–H 413, O=O 498, C=O 805, O–H 464. (a) Calculate the energy needed to break the bonds in the reactants. [1] (b) Calculate the energy released making the bonds in the products. [1] (c) Calculate \( \Delta H \) and state whether the reaction is exothermic or endothermic. [2]

(a) 4 C–H and 2 O=O: \( 4(413) + 2(498) = 1652 + 996 = 2648 \) kJ.

(b) CO₂ has 2 C=O; 2 H₂O have 4 O–H: \( 2(805) + 4(464) = 1610 + 1856 = 3466 \) kJ.

(c) \( \Delta H = 2648 - 3466 = -818 \) kJ/mol. Negative, so exothermic.

Check it. Combustion is always exothermic, so a negative answer is expected ✓. The sign convention is “break minus make”: if you get a positive value for a combustion, the subtraction is the wrong way round.

Counting molecules, not bonds. The two water molecules contain four O–H bonds, and the two oxygen molecules two O=O bonds. Using one of each is the usual slip.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which reaction is endothermic? A: burning methane. B: neutralising an acid with an alkali. C: the thermal decomposition of calcium carbonate. D: magnesium reacting with acid.

C. Thermal decomposition needs a continuous input of heat; the others release energy.

2. (Theory.) When ammonium nitrate dissolves in water the temperature falls from 21 °C to 14 °C. State whether the process is exothermic or endothermic, and explain how you know. [2]

Endothermic: the temperature of the surroundings (the water) falls because thermal energy is taken in.

3. (Theory.) A reaction pathway diagram shows the products at a lower energy level than the reactants. What type of reaction is it? [1]

Exothermic (energy is released to the surroundings).

4. (Theory.) EXTENDED Define activation energy, and state the sign of \( \Delta H \) for an endothermic reaction. [2]

The minimum energy that colliding particles must have to react. \( \Delta H \) is positive.

5. (Theory.) EXTENDED Use bond energies (kJ/mol) N≡N 945, H–H 436, N–H 391 to calculate \( \Delta H \) for \( \mathrm{N_2 + 3H_2 \rightarrow 2NH_3} \). [3]

Broken: \( 945 + 3(436) = 2253 \). Made: 2 NH₃ have 6 N–H: \( 6(391) = 2346 \). \( \Delta H = 2253 - 2346 = -93 \) kJ/mol (exothermic).

6. (Theory.) EXTENDED Explain, in terms of bonds, why the combustion of a fuel is exothermic. [2]

Energy is taken in to break the bonds in the fuel and oxygen, but more energy is released when the bonds in the products (CO₂ and H₂O) form, so there is an overall release of energy.

7. (Theory.) EXTENDED For a reaction, E_a = 150 kJ/mol and \( \Delta H = +60 \) kJ/mol. Sketch the reaction pathway diagram and give the energy of the peak above the products. [3]

Products 60 kJ/mol above the reactants; peak 150 kJ/mol above the reactants, so \( 150 - 60 = 90 \) kJ/mol above the products. Label E_a from the reactants to the peak and \( \Delta H \) (upward arrow) from reactants to products.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Royal Society of Chemistry — temperature-change experiments for exothermic and endothermic reactions
BBC Bitesize — bond energy calculations with worked steps