Hydrogen gas can be produced by reacting methane with steam at high temperatures. The reaction is endothermic. Explain what the term endothermic reaction means and describe what happens to bonds during an endothermic reaction, including the relationship between energy taken in to break bonds and energy released when bonds are made.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrogen gas is increasingly used as a clean fuel. One method of producing hydrogen involves reacting methane with steam. This reaction requires a continuous supply of heat energy from the surroundings.
Model answer (5 marks)
An endothermic reaction is one that takes in energy from the surroundings. Consequently the temperature of the surroundings falls.
During the reaction bonds in the reactants are broken – this is an endothermic step that requires energy input. Bonds are then formed in the products – this is an exothermic step that releases energy. In an endothermic reaction the energy required to break the bonds in the reactants is greater than the energy released when the new bonds are made in the products, so the net effect is a loss of energy from the surroundings.
During the reaction bonds in the reactants are broken – this is an endothermic step that requires energy input. Bonds are then formed in the products – this is an exothermic step that releases energy. In an endothermic reaction the energy required to break the bonds in the reactants is greater than the energy released when the new bonds are made in the products, so the net effect is a loss of energy from the surroundings.
Examiner tips
- Use the term "endothermic" and state that it takes energy from the surroundings; mention the temperature drop. Explain that bond breaking is endothermic (energy in) and bond making is exothermic (energy out). Show the energy balance: energy in > energy out, giving a net absorption of energy.
Common mistakes
- Confusing endothermic with exothermic; writing that the reaction releases heat. Forgetting to mention the temperature decrease of the surroundings. Saying that bond making requires energy instead of releasing it.
Mark scheme (5 marks)
- An endothermic reaction takes in energy from the surroundings
- The temperature of the surroundings decreases in an endothermic reaction
- Bond breaking is an endothermic process (energy is required/taken in to break bonds)
- Bond making is an exothermic process (energy is released when bonds form in the products)
- In an endothermic reaction the energy needed to break bonds in the reactants is greater than the energy released when bonds are formed in the products
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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