Exercises: Exothermic and endothermic reactions

Define an exothermic reaction and an endothermic reaction. State what happens to the temperature of the surroundings in each case.

Classify each of the following as exothermic or endothermic:

1. a) burning natural gas in a boiler

1. b) photosynthesis in a leaf

1. c) a self-heating coffee can

1. d) a cold pack used for sports injuries

When ammonium nitrate dissolves in water in a beaker, the temperature falls from $21\ °\text{C}$ to $14\ °\text{C}$.

1. a) Calculate the temperature change.

1. b) Is the process exothermic or endothermic? Explain.

A student measures the temperature change when different solids dissolve in $100\ \text{cm}^3$ of water.

1. a) Calculate the temperature change for each solid.

1. b) Identify which dissolving is exothermic and which is endothermic.

1. c) Explain what the result for C shows.

An energy profile diagram for a reaction shows reactants at $50\ \text{kJ}$, a peak at $130\ \text{kJ}$ and products at $20\ \text{kJ}$.

1. a) Calculate the activation energy.

1. b) Calculate the energy change of the reaction ($\Delta H$ = products − reactants) and state if it is exothermic or endothermic.

1. c) Describe where the products appear on the diagram compared with the reactants.

Another reaction has reactants at $30\ \text{kJ}$, a peak at $110\ \text{kJ}$ and products at $70\ \text{kJ}$.

1. a) Calculate the activation energy.

1. b) Calculate $\Delta H$ and state whether the reaction is exothermic or endothermic.

1. c) Explain why this reaction needs a continuous supply of energy to keep going, while the burning of a candle does not once started.

Thermal decomposition of calcium carbonate in a lime kiln is endothermic: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$, with $\Delta H = +178\ \text{kJ}$ per mole of $\text{CaCO}_3$.

1. a) Describe the shape of the energy profile for this reaction, stating the relative levels of reactants and products.

1. b) Explain why the kiln must be kept hot continuously.

1. c) Calculate the energy needed to decompose $2.5$ moles of $\text{CaCO}_3$.

1. d) Calculate the mass of $\text{CaCO}_3$ ($M_r = 100$) that requires $890\ \text{kJ}$.

On an energy profile diagram, the uncatalysed reaction has an activation energy of $90\ \text{kJ}$. With a catalyst, the activation energy is $55\ \text{kJ}$. The reactants are at $20\ \text{kJ}$ and the products are at $-30\ \text{kJ}$.

1. a) Calculate $\Delta H$ for the uncatalysed and the catalysed reaction.

1. b) Explain why $\Delta H$ is the same in both cases.

1. c) State the heights of the peaks with and without the catalyst.

1. d) Explain why the catalysed reaction is faster.

A student mixes $4.0\ \text{g}$ of citric acid solution with sodium hydrogencarbonate in $50\ \text{g}$ of water in a polystyrene cup. The temperature falls from $22.0\ °\text{C}$ to $16.0\ °\text{C}$. Assume the specific heat capacity of the mixture is $4.2\ \text{J/g°C}$ and the total mass is $54\ \text{g}$.

1. a) Calculate the energy absorbed using $Q = mc\Delta T$.

1. b) State whether the reaction is endothermic and justify.

1. c) Explain why a polystyrene cup is used instead of a metal can.

1. d) Suggest one reason why the calculated value may differ from the true value.