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Energy changes

Energy changes in reactions

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Explainer Video

Tutor: Lana

Summary

Energy changes in reactions

In a nutshell

The temperature change of a reaction can be measured experimentally and is used to determine if a reaction is endothermic or exothermic. The temperature change of different reactions can be measured for example of: neutralisation reactions, displacement reactions, dissolving salts and precipitation reactions.

Exothermic and endothermic reactions

Chemical reactions can be exothermic or endothermic.

Exothermic reactions increase the temperature of the surroundings. Endothermic reactions decrease the temperature of the surroundings.

The temperature change of a chemical reaction can be used to work out how much energy is released or absorbed by a chemical reaction.

Types of reactions

The temperature change of different reactions can be measured.

Neutralisation reactions

Neutralisation reactions involve an acid and a base reacting to form water and a salt.

Most neutralisation reactions are exothermic, however there are some that are endothermic.

Example

Sodium hydroxide (base) reacts with sulphuric acid to form a sodium sulphate salt and water in an exothermic reaction.

 $sodium\space hydroxide+\space sulphuric \space acid \longrightarrow sodium \space sulphate \space+ water$

$2NaOH(aq)\space +\space H_2SO_4(aq) \longrightarrow Na_2SO_4(s) \space+ 2H_2O(l)$

Displacement reactions

Displacement reactions involve a more reactive element taking the place of a less reactive element in a compound. Displacement reactions are exothermic.

Example

Aluminium reacts with iron oxide and displaces iron, to form aluminium oxide and the element iron.

$aluminium \space +\space iron \space oxide \longrightarrow aluminium \space oxide \space+ iron \\ \ \\$

$2Al(s) \space +\space Fe_2O_3 (aq) \longrightarrow Al_2O_3(aq) \space + \space 2Fe(s)$

Dissolving salt in water

Adding salt to water can be an exothermic or an endothermic process.

Examples

Dissolving potassium chloride and ammonium nitrate in water is an endothermic process which results in a decrease in temperature.

Dissolving calcium chloride in water is an exothermic process which results in an increase in temperature.

Precipitation reaction

Sometimes when two solutions are mixed, precipitates form and settle at the bottom. Precipitation reactions are exothermic.

Example

When potassium chloride solution is mixed with silver nitrate solution, silver chloride forms which is insoluble in water and precipitates out as a white solid.

$silver \space nitrate+\space potassium \space chloride\longrightarrow silver \space chloride \space+ potassium \space nitrate \\ \$

$AgNO_3(aq)\space +\space KCl(aq) \longrightarrow AgCl(s) \space + \space KNO_3(aq)$

Measuring temperature change

The temperature change of a reaction mixture can measured by replicating the following steps:

Procedure

1.	A polystyrene cup is placed into a well insulated beaker. Cotton wool is added to the beaker to insulate it; this reduces energy being lost or gained by the reaction mixture.
2.	A known volume of a reagent is transferred to the cup and the initial temperature of the solution is measured.
3.	A known mass/volume of a second reagent is added to the cup and the reaction mixture is stirred quickly.
4.	A lid is placed on the cup to prevent evaporation and energy loss. The lid must have a hole for a thermometer.
5.	If the reaction is exothermic, the maximum temperature the reaction reaches is recorded. If the reaction is endothermic, the minimum temperature the reaction reaches is recorded.

Chemistry; Rates of reaction and energy changes; KS4 Year 10; Energy changes in reactions

1.	Thermometer
2.	Lid
3.	Polystyrene cup
4.	Cotton wool
5.	Reacting solutions

Learn with Basics

Length:

Unit 1

An introduction to chemical reactions

Unit 2

Exothermic and endothermic reactions and catalysts

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Optional

Unit 3