Question 1Multiple choice · 1 markA* challenge
H2 + Br2 → 2HBr
Bond energies in kJ/mol: H–H 436; Br–Br 193; H–Br 366.
What is the energy change for the reaction, in kJ/mol?
- A −1360 kJ/mol
- B −103 kJ/mol
- C 103 kJ/mol
- D 263 kJ/mol
Show answer
Answer: B
Free Exothermic & endothermic reactions questions for IGCSE Chemistry (0620), part of Chemical energetics. Each question has the answer and the marking points an examiner would look for.
Generate a full Exothermic & Endothermic Reactions worksheet →All Chemical energetics questionsH2 + Br2 → 2HBr
Bond energies in kJ/mol: H–H 436; Br–Br 193; H–Br 366.
What is the energy change for the reaction, in kJ/mol?
Answer: B
The energy level diagram is for a reaction. Which statement is correct?
Answer: A
The equation for ammonia formation is
N2 + 3H2 → 2NH3
| bond | N≡N | H–H | N–H |
|---|---|---|---|
| bond energy in kJ/mol | 945 | 436 | 391 |
Calculate the energy needed to break all the bonds in the reactants.
Calculate the energy released when the bonds in the products are formed.
Calculate the energy change, ΔH, for the reaction and state whether it is exothermic or endothermic.
Draw a labelled reaction pathway diagram for this reaction. Show the activation energy and ΔH.
Explain, in terms of bond breaking and bond making, why this reaction is exothermic.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 2253 kJ 1 × 945 + 3 × 436 M1 2253 A1 | 2 |
| (b) | 2346 kJ 6 × 391 M1 2346 A1 | 2 |
| (c) | −93 kJ/mol, exothermic −93 (sign required) B1 exothermic B1 | 2 |
| (d) | See diagram products below reactants, axis labelled energy B1 activation energy shown as arrow from reactants to peak B1 ΔH shown as arrow between reactant and product levels B1 | 3 |
| (e) | More energy is released making bonds in the products than is taken in breaking bonds in the reactants bond breaking is endothermic and bond making is exothermic B1 energy released (making) > energy absorbed (breaking) B1 | 2 |
Reactions can be exothermic or endothermic.
Define an endothermic reaction.
Explain, in terms of bond breaking and bond making, why a reaction is exothermic.
Describe how the reaction pathway diagram for an exothermic reaction shows that it is exothermic.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | takes in thermal energy a reaction that takes in thermal energy from the surroundings B1 | 1 |
| (b) | more energy released making bonds than used breaking bond breaking is endothermic and bond making is exothermic B1 more energy is released making new bonds than is used breaking bonds B1 | 2 |
| (c) | products lower than reactants the products are at a lower energy level than the reactants B1 | 1 |
This question is about methane as a fuel.
Write the equation for the complete combustion of methane.
Bond energies, in kJ/mol, are given in the table.
| bond | C–H | O=O | C=O | O–H |
|---|---|---|---|---|
| bond energy / kJ/mol | 415 | 498 | 799 | 464 |
Calculate the energy change for the complete combustion of one mole of methane.
Use your answer to (b)(i) to calculate the energy released when 32 g of methane is burned. [Mᵣ: CH₄, 16]
Write an equation for the incomplete combustion of methane to form carbon monoxide.
Explain why a gas boiler should be serviced regularly, with reference to carbon monoxide.
Methane is a greenhouse gas. Give one source of methane in the atmosphere other than natural gas leaks.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | CH₄ + 2O₂ → CO₂ + 2H₂O correct formulae B1 balanced B1 | 2 |
| (b)(i) | −798 kJ/mol bonds broken = 4(415) + 2(498) = 2656 M1 bonds formed = 2(799) + 4(464) = 3454 M1 −798 kJ/mol A1 | 3 |
| (ii) | 1600 kJ 32/16 = 2 mol M1 1600 kJ A1 | 2 |
| (c) | 2CH₄ + 3O₂ → 2CO + 4H₂O CO and H₂O as products B1 balanced B1 | 2 |
| (d) | blocked air supply causes incomplete combustion; CO toxic, colourless, odourless a poor air supply causes incomplete combustion B1 carbon monoxide is toxic and colourless / odourless so it cannot be detected B1 | 2 |
| (e) | cattle / decomposition / rice paddies / landfill digestion in animals (cattle) / decomposition of vegetation / landfill B1 | 1 |
A student dissolves 4.0 g of ammonium nitrate, NH₄NO₃, in 100 cm³ of water. The temperature falls by 3.1 °C.
State whether dissolving ammonium nitrate is exothermic or endothermic. Explain your answer.
Calculate the energy change in J. Assume 4.2 J raise the temperature of 1 cm³ of water by 1 °C.
Use your answer to (b) to calculate the energy change in kJ per mole of ammonium nitrate. [Mᵣ: NH₄NO₃, 80]
Draw a labelled reaction pathway diagram for an endothermic reaction. Label the activation energy and the enthalpy change.
Add to your diagram the pathway with a catalyst, and explain its effect on the rate.
Suggest a use for this reaction and explain why polystyrene cups are used in the experiment.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | endothermic; temperature falls, energy taken in from surroundings endothermic B1 the temperature decreases because thermal energy is taken in from the surroundings B1 | 2 |
| (b) | 1300 J 100 × 4.2 × 3.1 M1 1300 J A1 | 2 |
| (c) | +26.0 kJ/mol moles = 4.0/80 = 0.0500 M1 1.30 kJ ÷ 0.0500 M1 +26.0 kJ/mol (positive sign) A1 | 3 |
| (d) | products above reactants; Ea from reactants to peak; ΔH up arrow products higher than reactants B1 activation energy shown from reactants to the top of the curve B1 ΔH shown as an upward arrow from reactants to products B1 | 3 |
| (e) | lower peak; lower activation energy so more successful collisions a lower peak drawn B1 lower activation energy so a greater proportion of collisions are successful B1 | 2 |
| (f) | cold packs; polystyrene is an insulator, reduces energy transfer with surroundings instant cold packs (sports injuries) B1 polystyrene is a poor thermal conductor so less energy is transferred from the surroundings B1 | 2 |
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