IGCSE Chemistry Chemical energetics topical questions

For each description, name the type of reaction.

1. (a)Thermal energy is transferred from the reaction to the surroundings.[1]
2. (b)Thermal energy is taken in by the reaction from the surroundings.[1]
3. (c)State what happens to the temperature of the surroundings during the reaction in part (b).[1]

A student says that breaking a bond releases energy. Explain why this statement is incorrect.

A student dissolves ammonium nitrate in water in a beaker. After stirring, the outside of the beaker feels noticeably colder.

1. (a)Name the type of energy change taking place.[1]
2. (b)State the direction in which thermal energy is transferred.[1]
3. (c)Explain why the outside of the beaker feels cold.[1]

The reaction between hydrogen and oxygen has a high activation energy even though it releases a large amount of energy overall.

1. (a)Define activation energy.[1]
2. (b)Explain, in terms of collisions, why the reaction does not happen every time two reactant particles meet.[2]
3. (c)The overall reaction is exothermic. State how a reaction pathway diagram for this reaction shows that.[1]

Energy changes in a reaction can be explained using bonds.

1. (a)State what happens to the energy when bonds in the reactants break.[1]
2. (b)State what happens to the energy when bonds in the products form.[1]
3. (c)Use these ideas to explain why some reactions are exothermic.[2]

Zinc powder is added to copper sulfate solution. The temperature of the mixture rises from 19 °C to 26 °C.

1. (a)Name the type of reaction and explain your answer using the temperatures.[1]
2. (b)Describe the reaction pathway diagram for this reaction, including the relative levels of the reactants and products.[2]
3. (c)State what the vertical gap between the reactant level and the product level represents.[1]

Methane burns in oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O. A small flame is needed to start the reaction, but once burning the flame keeps itself going.

1. (a)Explain why energy is needed to start breaking the bonds in the reactant molecules.[2]
2. (b)Explain why the reaction keeps going without further heating.[2]

Bond energies are given in the table. Use them to work out the enthalpy change of the reaction H₂ + Cl₂ → 2HCl.

1. (a)Calculate the energy required to break all the bonds in the reactants.[2]
2. (b)Calculate the enthalpy change of the reaction and state whether it is exothermic or endothermic.[3]

An endothermic reaction has an activation energy of 120 kJ per mol and an overall energy change of +40 kJ per mol.

1. (a)Explain why the temperature of the surroundings falls during this reaction.[2]
2. (b)Describe the shape of the reaction pathway diagram, including where the products lie relative to the reactants.[2]

Steam can be split into hydrogen and oxygen: 2H₂O → 2H₂ + O₂. Bond energies in kJ per mol: H–O 464, H–H 436, O=O 498.

1. (a)Calculate the energy needed to break all the bonds in the steam.[1]
2. (b)Calculate the energy released when the new bonds form in the products.[1]
3. (c)Calculate the overall energy change of the reaction.[2]
4. (d)State whether the reaction is exothermic or endothermic.[1]