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Energy changes for SQA National 5 Chemistry

Energy changes

This page covers the following topics:

1. Thermal energy
2. Exothermic and endothermic reactions

Thermal energy released in a chemical reaction can be obtained by heating a known mass of water in an isolated system and measuring the change in temperature. The energy released is then found by multiplying the mass of water, specific heat capacity of water and the change in temperature.

Thermal energy

In chemical reactions energy is conserved meaning that the energy within a reaction mixture and its surroundings before and after the reaction is equal. An exothermic reaction is a reaction during which it takes less energy to break bonds than to make new ones. Thus, exothermic reactions emit heat to the surroundings. An endothermic reaction is a reaction during which it takes more energy to break bonds than to make new ones. Therefore, endothermic reactions take heat from the surroundings.

To determine if a reaction is endothermic or exothermic, a temperature change throughout the course of a reaction can be measured. If the temperature increases, the reaction is exothermic; if it decreases, the reaction is endothermic. Exothermic reactions are widely used for heating, electricity production, cooking food. Endothermic reactions are involved in cold packs used for cooling down injuries.

Exothermic and endothermic reactions

1

Provide an application of an endothermic reaction.

Endothermic reactions can be involved in the reactions within cold packs used for cooling down injuries.

cold packs

Provide an application of an endothermic reaction.

2

Explain why an exothermic reaction emits heat to the surroundings.

In chemical reactions energy is conserved meaning that the energy within the reaction mixture and the surroundings before and after the reaction is equal. An exothermic reaction is a reaction during which it takes less energy to break bonds than produced when making new ones. Thus, exothermic reactions emit heat to the surroundings.

Less energy is used to break bonds than produced when making new ones. According to the conservation of energy, the energy goes to the surroundings.

Explain why an exothermic reaction emits heat to the surroundings.

3

Darius has mixed two solutions to perform a neutralisation reaction. He has measured a temperature increase of 6.8ยฐC throughout the course of the reaction. Determine whether the reaction is exothermic or endothermic.

To determine if a reaction is endothermic or exothermic, a temperature change throughout the course of a reaction can be measured. If the temperature increases, the reaction is exothermic.

exothermic

Darius has mixed two solutions to perform a neutralisation reaction. He has measured a temperature increase of 6.8ยฐC throughout the course of the reaction. Determine whether the reaction is exothermic or endothermic.

4

Nikesh has carefully placed a small piece of sodium in 200 cmยณ of water. Calculate the temperature change of the water if the energy released during the process is 75 kJ. The specific heat capacity of water is 4180 J/(kgK).

200 cmยณ of water have a mass of 200 g, which is 0.200 kg.
75 kJ = 75000 J

E = mcฮ”T
75000 = 0.200 ร— 4180 ร— ฮ”T
75000 = 836 ร— ฮ”T
ฮ”T = 75000 รท 836 = 89.7 K (3 s. f.)

89.7 K

Nikesh has carefully placed a small piece of sodium in 200 cmยณ of water. Calculate the temperature change of the water if the energy released during the process is 75 kJ. The specific heat capacity of water is 4180 J/(kgK).

5

Define an endothermic reaction.

An endothermic reaction is a reaction during which it takes more energy to break bonds than to make new ones. Therefore, endothermic reactions take heat from the surroundings.

A reaction that takes energy from surroundings.

Define an endothermic reaction.

End of page

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