15 Heat Changes

Which process is exothermic?

• burning petrol in a car engine

• cracking of petroleum

• fractional distillation of petroleum

• melting bitumen for roads

Which of the following is an endothermic process?

• adding sodium to water

• dissolving ammonium nitrate in water

• the oxidation of carbon to carbon dioxide

• the reaction between hydrogen and oxygen

Which statements about endothermic reactions are correct?

1      Energy is released to the surroundings.

2      Energy is absorbed from the surroundings.

3      The temperature of the reactants rises.

4      The temperature of the reactants falls.

• 1 and 3

• 1 and 4

• 2 and 3

• 2 and 4

Solutions of two chemicals are mixed in a beaker.

A reaction occurs and a fall in temperature is observed.

Which statement is correct?

• An endothermic reaction occurs and the reacting chemicals gain energy.

• An endothermic reaction occurs and the reacting chemicals lose energy.

• An exothermic reaction occurs and the reacting chemicals gain energy.

• An exothermic reaction occurs and the reacting chemicals lose energy.

Burning fuels such as methane is an exothermic process.

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

Which statement explains this fact?

• Molecules release energy when they react.

• More bonds are formed than are broken.

• That energy is given out to surroundings and may be detected by an increase in temperature.

• That energy is taken in from the surroundings and may be detected by an increase in temperature.

Four different solids, W, X, Y and Z, are dissolved in equal volumes of water at 20°C.

The table shows the change in temperature when each solid dissolves.

 

W +10°C

X -8°C

Y -5°C

Z +15°C

Which row of the table describes the energy changes when each solid is dissolved in water?

• W dissolves

  endothermic

  Y dissolves

  exothermic

• X dissolves

  exothermic

  W dissolves

  endothermic

• X dissolves

  endothermic

  Z dissolves

  exothermic

• Y dissolves

  exothermic

  Z dissolves

  endothermic

Aqueous sodium hydroxide and dilute hydrochloric acid are reacted together.

The reaction is exothermic.

Both the sodium hydroxide and hydrochloric acid are initially at room temperature.

Which graph shows how the temperature changes when aqueous sodium hydroxide is added to the hydrochloric acid until the alkali is present in excess?

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