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Cambridge IGCSE0620

Exothermic and endothermic reactions

Chemistry 0620 Chapter Notes

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Exothermic and endothermic reactions
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In this chapter

  1. 1Exothermic and endothermic reactions
  2. 2Reading a reaction pathway diagram
  3. 3Enthalpy change, ΔH
  4. 4Activation energy, Ea
  5. 5Drawing and labelling a pathway diagram
  6. 6Bond breaking and bond making
  7. 7Calculating ΔH from bond energies
  8. 8Finding an unknown bond energy

1. Exothermic and endothermic reactions

An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases.

An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases.

A rise in temperature means exothermic. A fall means endothermic. Burning ethanol and sodium in water are exothermic. Water boiling and ammonium nitrate in water are endothermic.

Examiner insight

“This question stated that ‘the temperature of the reaction mixture increases’ – however, lots of candidates incorrectly thought that this meant that it was an ‘endothermic’ reaction.”

Worked example 11 mark

Four changes are described.

1 When sodium is added to water, thermal energy is released.

2 When water boils, thermal energy is absorbed.

3 When concentrated sulfuric acid is added to water, the temperature of the surroundings increases.

4 When ammonium nitrate is added to water, the temperature of the surroundings decreases.

Which changes are endothermic?

A 1 and 3 B 1 and 4 C 2 and 3 D 2 and 4

  1. 1

    1 and 3: energy is released or the temperature increases, so exothermic

  2. 2

    2 and 4: energy is absorbed or the temperature decreases, so endothermic

  3. 3

    Endothermic changes are 2 and 4, so the answer is D [1]

2. Reading a reaction pathway diagram

A reaction pathway diagram plots energy against progress of reaction. Compare the energy of the products with the energy of the reactants.

In an exothermic reaction the products are at a lower energy than the reactants. In an endothermic reaction the products are at a higher energy than the reactants.

Examiner insight

“Many candidates could not use the diagram to ‘show that the reaction was exothermic’.” “Many struggled on this question and often just said that ‘heat was given off’ instead of stating that ‘the energy of the reactants is greater than the energy of the product’ or ‘the arrow is pointing downwards’.”

Worked example 11 mark

The reaction pathway diagram for the reaction between hydrogen and iodine to form hydrogen iodide is shown.

Four statements about this reaction are listed.

1 The reaction is exothermic.

2 The reaction is endothermic.

3 The temperature of the mixture increases during the reaction.

4 The temperature of the mixture decreases during the reaction.

Which statements are correct?

A 1 and 3 B 1 and 4 C 2 and 3 D 2 and 4

  1. 1

    The products, 2HI(g), are at a lower energy than the reactants, H₂(g) + I₂(g)

  2. 2

    So thermal energy is transferred to the surroundings: the reaction is exothermic (1)

  3. 3

    The temperature of the mixture increases (3)

  4. 4

    Statements 1 and 3 are correct, so the answer is A [1]

3. Enthalpy change, ΔH

The transfer of thermal energy during a reaction is called the enthalpy change, Δ H, of the reaction.

Δ H is negative for exothermic reactions and positive for endothermic reactions.

Give the sign. A sign is part of the answer.

Δ H < 0 (exothermic)

Δ H > 0 (endothermic)

Examiner insight

“Weaker candidates gave a description of an exothermic reaction (for example, ‘gives out heat’) without mentioning the minus sign.”

Worked example 12 marks

When hydrogen reacts with carbon dioxide in a sealed vessel at 130 °C using a suitable catalyst, methanol is produced.

The equation is: 3H₂(g) + CO₂(g) ⇌ CH₃OH(g) + H₂O(g) Δ H = -40 kJ/mol

The activation energy, E_a, of the forward reaction is +220 kJ / mol.

State how the information shows that this reaction is reversible and exothermic.

  1. 1

    Reversible: the equation uses the reversible arrow, ⇌ [1]

  2. 2

    Exothermic: Δ H is negative, Δ H = -40 kJ/mol [1]

4. Activation energy, Ea

Activation energy, E_a, is the minimum energy that colliding particles must have to react.

On a pathway diagram E_a is the climb from the reactants to the top of the ‘hump’. A catalyst makes E_a smaller. It does not change Δ H.

Examiner insight

“Some candidates confused the activation energy with the overall energy change of reaction, choosing option A.” “Many arrows finished too short of the top of the hump and some candidates simply labelled the top of the ‘hump’ as activation energy.”

Worked example 11 mark

Which row describes the effect of a catalyst on a reaction?

effect on activation energy, E_aeffect on enthalpy change, Δ H
Adecreasesdecreases
Bdecreasesno change
Cincreasesdecreases
Dincreasesno change
  1. 1

    A catalyst makes E_a smaller: E_a decreases

  2. 2

    Δ H is not changed by a catalyst: no change

  3. 3

    Decreases, no change: row B [1]

5. Drawing and labelling a pathway diagram

Write the formulas of the reactants on the left line and of the products on the right line.

Draw E_a as an upward arrow from the energy level of the reactants to the top of the ‘hump’. Draw Δ H as one single-headed arrow from the energy level of the reactants to the energy level of the products: down for exothermic, up for endothermic.

The reverse reaction runs the other way. Its Δ H has the opposite sign. Its E_a climbs from the products to the top of the same ‘hump’, so it is not the forward E_a with a minus sign.

Δ H(reverse) = -Δ H(forward)

E_a(reverse) = E_a(forward) - Δ H(forward)

Examiner insight

“Care should be taken drawing the activation energy arrow from the level of the reactants to the top of the ‘hump’.” “Some candidates incorrectly drew a double headed arrow for the energy change as opposed to the downward pointing arrow showing an exothermic reaction.”

Worked example 12 marks

The equation is: 3H₂(g) + CO₂(g) ⇌ CH₃OH(g) + H₂O(g) Δ H = -40 kJ/mol

The activation energy, E_a, of the forward reaction is +220 kJ / mol.

Figure 4.1 is the completed reaction pathway diagram for the forward reaction.

Use Figure 4.1 and the information given to determine the enthalpy change, in kJ / mol, of the reverse reaction and the activation energy, in kJ / mol, of the reverse reaction.

Include a sign with your answer.

  1. 1

    The reverse reaction has the opposite sign: Δ H = +40 kJ/mol [1]

  2. 2

    The reverse reaction climbs from the products to the top of the ‘hump’: E_a = 220 + 40 = +260 kJ/mol [1]

6. Bond breaking and bond making

Bond breaking is endothermic: it takes in energy. Bond making is exothermic: it releases energy.

A reaction is exothermic when the energy released making bonds is greater than the energy taken in breaking bonds. It is endothermic when the energy taken in breaking bonds is greater.

Say both halves. Bond breaking takes in energy and bond making releases energy.

Cl₂(g) → 2Cl(g) bond breaks: endothermic

H(g) + H(g) → H₂(g) bond forms: exothermic

Examiner insight

“A typical answer such as ‘more energy is released in bond making than breaking’ incorrectly suggests bond breaking also releases energy.” “Similarly, ‘less energy is required to break bonds than make bonds’ suggests bond making requires energy.”

Worked example 11 mark

Which equation represents an endothermic reaction?

A CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

B Cl₂(g) → 2Cl(g)

C H(g) + H(g) → H₂(g)

D 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

  1. 1

    Bond breaking is endothermic. Bond making is exothermic.

  2. 2

    B only breaks a bond: Cl₂(g) → 2Cl(g) is endothermic

  3. 3

    C only makes a bond: H(g) + H(g) → H₂(g) is exothermic

  4. 4

    The answer is B [1]

7. Calculating ΔH from bond energies

Add up the energy needed to break every bond in the reactants. Add up the energy released when every bond in the products forms.

Subtract the second from the first. A negative answer means exothermic. A positive answer means endothermic.

Count each bond in the equation, not in one molecule. Two molecules of H₂O have four O–H bonds.

Δ H = energy to break bonds - energy released making bonds

Examiner insight

“Doing the reverse subtraction to give +120 kJ / mol was a common error.” “This answer has the opposite sign to the correct answer, suggesting that candidates confused the sign of energy changes during bond breaking and bond forming.”

Worked example 11 mark

Hydrogen peroxide, H–O–O–H, decomposes to form water and oxygen.

2H₂O₂(l) → 2H₂O(l) + O₂(g)

The bond energies are in the table. The reaction is exothermic.

What is the enthalpy change for this reaction?

A –346 kJ / mol B –196 kJ / mol C +196 kJ / mol D +346 kJ / mol

bondbond energy in kJ / mol
O–H+460
O–O+150
O=O+496
  1. 1

    Bonds broken: 4 O–H and 2 O–O. Energy in = 4 × 460 + 2 × 150 = 2140 kJ

  2. 2

    Bonds made: 4 O–H and 1 O=O. Energy out = 4 × 460 + 496 = 2336 kJ

  3. 3

    Δ H = 2140 - 2336 = -196 kJ/mol, so the answer is B [1]

8. Finding an unknown bond energy

When Δ H is given, work backwards. Find the energy needed to break the bonds in the reactants. Then find the energy released when the bonds in the products form.

Δ H is the energy to break bonds minus the energy released. So the energy released is the energy to break bonds minus Δ H. For a negative Δ H that means adding its size.

Subtract the known bonds. Divide by how many of the unknown bond there are.

energy released making bonds = energy to break bonds - Δ H

Examiner insight

“The most common error was to subtract the magnitude of the enthalpy change (105 kJ / mol) rather than add it for step 2.” “Some candidates calculated a negative energy value in the last part which would lead to a negative bond energy value.”

Worked example 14 marks

The equation is: 3H₂(g) + CO₂(g) ⇌ CH₃OH(g) + H₂O(g) Δ H = -40 kJ/mol

Carbon dioxide has two C=O bonds. Water has two O–H bonds. Methanol has three C–H bonds, one C–O bond and one O–H bond.

Use the bond energies in the table and the value of Δ H for the forward reaction to calculate the C–H bond energy in kJ / mol.

Use the following steps. Calculate the energy needed to break the bonds in the reactants. Calculate the energy released when the bonds in water form. Calculate the bond energy of the C–H bond.

bondH–HC=OO–HC–O
bond energy in kJ / mol440805460360
  1. 1

    Energy to break the bonds in the reactants = 3 × 440 + 2 × 805 = 2930 kJ [1]

  2. 2

    Energy released when the bonds in water form = 2 × 460 = 920 kJ [1]

  3. 3

    With C–H bond energy x: 2930 - (920 + 360 + 460 + 3x) = -40, so 3x = 2930 - 1740 + 40 = 1230 kJ [1]

  4. 4

    x = 1230 ÷ 3 = 410 kJ/mol [1]

Recap

  • Exothermic: thermal energy transferred to the surroundings, so their temperature increases.
  • Endothermic: thermal energy taken in from the surroundings, so their temperature decreases.
  • Pathway diagram: products lower than reactants is exothermic. Products higher is endothermic.
  • Extended: Δ H is the enthalpy change. It is negative for exothermic and positive for endothermic.
  • Extended: activation energy E_a is the minimum energy that colliding particles must have to react.
  • Extended: draw E_a up from the reactants to the top of the hump. Draw Δ H as one arrow from reactants to products.
  • Extended: bond breaking is endothermic. Bond making is exothermic.
  • Extended: Δ H = energy to break bonds minus energy released making bonds.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what happens to the temperature of the surroundings during an exothermic reaction.1 mark
  2. When ammonium nitrate dissolves in water, the temperature of the water falls. State the type of change and the direction of transfer of thermal energy.2 marks
  3. A student adds three solids to separate samples of water. With solid P the temperature of the water changes from 20.0 °C to 31.0 °C, with solid Q from 22.0 °C to 8.0 °C and with solid R from 25.0 °C to 33.0 °C. (a) State which solids give an exothermic change. (b) Explain how these results show that the change with solid Q is endothermic.2 marks
  4. The reaction pathway diagram for a reaction is drawn. State whether the reaction is exothermic or endothermic and give a reason from the diagram.2 marks
  5. The temperature of a reaction mixture increases. (a) State the type of reaction. (b) State where thermal energy is transferred. (c) State how the energy of the products compares with the energy of the reactants on a reaction pathway diagram.3 marks
  6. State what Δ H represents and give its sign for an endothermic reaction.2 marks
  7. State what is meant by activation energy, E_a. State the effect of a catalyst on E_a and on Δ H.3 marks
  8. A forward reaction has Δ H = -90 kJ/mol and an activation energy of +150 kJ/mol. (a) Give Δ H for the reverse reaction. (b) Give the activation energy of the reverse reaction. (c) State whether the reverse reaction is exothermic or endothermic.3 marks
  9. Hydrazine, N₂H₄, is used as a fuel. It burns in oxygen: N₂H₄(g) + O₂(g) → N₂(g) + 2H₂O(g). A molecule of hydrazine has one N–N bond and four N–H bonds. A molecule of nitrogen has one N≡N bond. (a) Use the bond energies in the table to calculate Δ H for this reaction. (b) Explain, in terms of bond breaking and bond making, why the reaction is exothermic. | bond | N–N | N–H | O=O | N≡N | O–H | | --- | --- | --- | --- | --- | --- | | bond energy in kJ / mol | 160 | 390 | 496 | 945 | 460 |5 marks
  10. Ammonia can decompose into nitrogen and hydrogen: 2NH₃(g) → N₂(g) + 3H₂(g) with Δ H = +93 kJ/mol. A molecule of ammonia has three N–H bonds. The N≡N bond energy is 945 kJ / mol and the H–H bond energy is 436 kJ / mol. The activation energy of the decomposition is +335 kJ / mol. (a) Calculate the N–H bond energy in kJ / mol. (b) Give Δ H and the activation energy of the reverse reaction, each with its sign.6 marks

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