# Energy in Chemical Reactions

In this worksheet, students will learn about the energy involved in chemical reactions and how it is used or released by the reaction itself, in endothermic or exothermic reactions, respectively.

Key stage:  KS 3

Curriculum topic:   Chemistry: Energetics

Curriculum subtopic:   Exothermic and Endothermic Reactions

Difficulty level:

### QUESTION 1 of 10

Chemical reactions involve energy changes: during a chemical reaction energy is transferred to or from the surroundings and the temperature changes. For example, when we turn on the gas on our kitchen hob, a chemical reaction, called combustion or simply burning, takes place. Combustion transfers heat to its surroundings, so our food on top of it takes in the heat and cooks. The temperature in our cooking pot rises.

Reactions that transfer energy to their surroundings, like combustion, are exothermic reactions. Neutralisation is another type of exothermic reaction; this is the reaction between acids and alkalis.

The heat energy is released from the atoms bonding to make the products. The reactants actually took in energy in order to break the bonds between them, but this amount of energy was smaller than the energy released when making the products. The temperature of the mixture of an exothermic reaction rises and so does the temperature of the surroundings.

The opposite happens in endothermic reactions. The reactants in endothermic reactions take in more energy to break the bonds between them. This energy is taken from the surroundings. Therefore, the mixture and the surroundings get colder, as the temperature drops. When the bonds in the products form, less energy is released than when reactant bonds break. Electrolysis and thermal decomposition are two types of endothermic reactions.

The following diagram shows the energy changes in the reaction mixtures of exothermic and endothermic reactions.

Study the diagram carefully; this is where it can get tricky. The diagram shows that the reaction energy decreases for exothermic reactions and increases for endothermic. Then how is it possible to have a temperature rise in exothermic reactions and a temperature fall in endothermic? The diagram shows the energy changes on a molecular level. It is true that in exothermic reactions more energy is released when the products form. This energy transfers to the surroundings, so we observe a temperature rise. However, the energy inside the molecules themselves decreases, because it is released to the immediate environment.

Equally, product molecules in endothermic reactions take in a lot of energy when they are formed. This is taken from the surroundings, so a temperature drop is noted.

See if you can use these ideas in this activity (read this through again if it helps).

What is the scientific name for burning?

Complete this sentence.

Exothermic reactions transfer energy _______ their surroundings.

from

to

for

Choose one of the reactions below that is exothermic.

electrolysis

thermal decomposition

neutralisation

Is this statement true or false?

Temperature decrease is observed in exothermic reactions.

true

false

Is this statement true or false?

In exothermic reactions more energy is released when forming products than the energy taken in to break the bonds of the reactants.

true

false

Tick two examples of endothermic reaction from the list below.

combustion

electrolysis

neutralisation

thermal decomposition

Complete this sentence:

In endothermic reactions a temperature ______________ is observed in the mixture and the surroundings.

decrease

increase

Why does the temperature decrease in the mixture and surroundings of endothermic reactions?

When the bonds in the products form, less energy is released than when reactant bonds break.

When the bonds in the products form, more energy is released than when reactant bonds break.

What happens to the internal temperature of the molecules in an exothermic reaction?

The internal temperature decreases.

The internal temperature increases.

Why does the molecular temperature in an endothermic reaction increase if the mixture and surrounding temperature actually decreases?

Product molecules in endothermic reactions take in a lot of energy when they break.

Product molecules in endothermic reactions take in a lot of energy when they are formed.

• Question 1

What is the scientific name for burning?

combustion
EDDIE SAYS
The scientific name for burning is combustion.
• Question 2

Complete this sentence.

Exothermic reactions transfer energy _______ their surroundings.

to
EDDIE SAYS
Exothermic reactions transfer energy to their surroundings. Remember, EXO = "out of it" and THERM = "heat", so exothermic = "heat out of it", i.e. energy transferred TO the surroundings.
• Question 3

Choose one of the reactions below that is exothermic.

neutralisation
EDDIE SAYS
Apart from combustion, neutralisation is another example of exothermic reaction. Acid reacts with alkali and heat energy is released - you can feel the tube or beaker getting warmer.
• Question 4

Is this statement true or false?

Temperature decrease is observed in exothermic reactions.

false
EDDIE SAYS
The statement is false; temperature rise is observed in exothermic reactions.
• Question 5

Is this statement true or false?

In exothermic reactions more energy is released when forming products than the energy taken in to break the bonds of the reactants.

true
EDDIE SAYS
The statement is true. More energy is released when the products form, that's why a temperature rise is observed.
• Question 6

Tick two examples of endothermic reaction from the list below.

electrolysis
thermal decomposition
EDDIE SAYS
Thermal decomposition and electrolysis are two examples of endothermic reaction, taking in more heat energy from their surroundings than they release.
• Question 7

Complete this sentence:

In endothermic reactions a temperature ______________ is observed in the mixture and the surroundings.

decrease
EDDIE SAYS
Remember, in endothermic reactions thermal energy is taken in from the surroundings to break the bonds of the reactants. However, as the products form, less thermal energy is released than absorbed, so overall a temperature decrease is observed.
• Question 8

Why does the temperature decrease in the mixture and surroundings of endothermic reactions?

When the bonds in the products form, less energy is released than when reactant bonds break.
EDDIE SAYS
When the bonds in the products form, less energy is released than when reactant bonds break.
• Question 9

What happens to the internal temperature of the molecules in an exothermic reaction?

The internal temperature decreases.
EDDIE SAYS
The internal temperature of the molecules involved in an exothermic reaction actually decreases as more thermal energy is transferred to the surroundings and so the energy state of the molecules themselves is lower at the end of the reaction than the beginning.
• Question 10

Why does the molecular temperature in an endothermic reaction increase if the mixture and surrounding temperature actually decreases?

Product molecules in endothermic reactions take in a lot of energy when they are formed.
EDDIE SAYS
Product molecules in endothermic reactions take in a lot of energy when they are formed. This is taken from the surroundings, so a temperature drop is observed.
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