Exothermic & Endothermic Reactions | Chemistry for Grade 10 PDF Download

Reactions and temperature changes

When a chemical reaction happens, energy is transferred to or from the surroundings. When energy is transferred to the surroundings, this is called an exothermic reaction, and the temperature of the surroundings increases. Examples of exothermic reactions include:

• combustion reactions
• many oxidation reactions
• most neutralisation reactions

Everyday uses of exothermic reactions include self-heating cans and hand warmers.
When energy is taken in from the surroundings, this is called an endothermic reaction and the temperature of the surroundings decreases. Examples of endothermic reactions include:

• thermal decomposition reactions
• the reaction of citric acid and sodium hydrogencarbonate

Everyday uses of endothermic reactions include instant ice packs which can be used to treat sports injuries.
The slideshow describes an exothermic reaction between dilute sodium hydroxide and hydrochloric acid and an endothermic reaction between sodium carbonate and ethanoic acid.

1. Sodium hydroxide solution is poured into a beaker of hydrochloric acid which contains a thermometer showing room temperature

2. The beaker now contains sodium chloride and water, and the thermometer is showing a rise in temperature, so the neutralisation reaction is exothermic

3. Sodium carbonate powder is tipped into a beaker of ethanoic acid which contains a thermometer showing room temperature

4. The beaker now contains sodium ethanoate, water and carbon dioxide, and the thermometer is showing a fall in temperature, so this was an endothermic reaction

In endothermic reactions, energy enters. In exothermic reactions, energy exits.

Required practical - temperature changes in reacting solutions

Aim

To investigate the variables that affect temperature changes in reacting solutions.
Apparatus

• polystyrene cup
• thermometer
• 250 cm3 glass beaker
• measuring cylinder
• top pan balance

Context
You could investigate one or more chemical reactions, for example:

• acids reacting with metals
• acids reacting with metal carbonates
• acids reacting with alkalis
• displacement reactions of metals

Method
Reacting two solutions, eg acid and alkali

• Place the polystyrene cup inside the glass beaker to make it more stable.
• Measure an appropriate volume of each liquid, eg 25 cm3.
• Place one of the liquids in a polystyrene cup.
• Record the temperature of the solution.
• Add the second solution and record the highest or lowest temperature obtained.
• Change your independent variable and repeat the experiment. Your independent variable could be the concentration of one of the reactants, or the type of acid/alkali being used, or the type of metal/metal carbonate being used.

Method
Reacting a solid with a solution, eg metal and acid

• Place the polystyrene cup inside the glass beaker to make it more stable.
• Measure an appropriate volume of the solution, eg 25 cm3.
• Measure an appropriate mass of the solid, or select a suitable sized piece of metal.
• Place the solution in a polystyrene cup.
• Record the temperature of the solution.
• Add the solid and record the highest or lowest temperature obtained.
• Change your independent variable and repeat the experiment. Your independent variable could be the surface area of the solid, or the type of acid being used, or the type of metal being used.

Analysis
The bigger the temperature change in the reaction, the more energy is absorbed or released. Remember that endothermic reactions absorb energy from the surroundings, and exothermic reactions transfer heat into the surroundings.

Evaluation
The biggest source of error in this experiment is unwanted heat transfer. Using a lid can help to reduce this.

Hazards, risks and precautions

Reaction profiles

Exothermic reaction

A downwards arrow shows that energy is given out

Endothermic reaction

The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings.

An upwards arrow shows that energy is taken in

It is usually more helpful to describe how the energy of the chemicals changes during the reaction, so a reaction profile is more useful than an energy level diagram.
A reaction profile includes the activation energy, which is the minimum energy needed by particles when they collide for a reaction to occur. The activation energy is shown as a 'hump' in the line, which:

• starts at the energy of the reactants
• is equal to the difference in energy between the top of the 'hump' and the reactant

The overall change in energy in a reaction is the difference between the energy of the reactants and products.

Exothermic reactions

A reaction profile for an exothermic reaction

Solved Examples

In the profile for an exothermic reaction, the overall change is negative. You can tell this because the products have less energy than the reactants, and the arrow showing the overall change in energy points downwards.

Endothermic reactions

A reaction profile for an endothermic reaction

Example 2: Describe how you can tell from a reaction profile that a reaction is endothermic.

In the profile for an endothermic reaction, the overall change is positive. You can tell this because the products have more energy than the reactants, and the arrow showing the overall change in energy points upwards.

Explaining energy changes in reactions - Higher

Energy is transferred when bonds are broken or are formed.

During a chemical reaction:

• bonds in the reactants are broken
• new bonds are made in the products

The difference between the energy needed to break bonds and the energy released when new bonds are made determines the type of reaction.
A reaction is:

• exothermic if more heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants
• endothermic if less heat energy is released in making bonds in the products than is taken in when breaking bonds in the reactants