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Understand Exothermic and Endothermic Reactions

Worksheet Overview

QUESTION 1 of 10

During chemical reactions energy transfer takes place to and from the surroundings. This transfer is in the form of thermal energy, i.e. heat. A chemical reaction can be either exothermic or endothermic.

Exothermic Reactions

Exo- means 'out; think about words like "exit". Exothermic reactions release heat to their surroundings. The picture shows an example of an exothermic reaction:

 

 

Clearly, this looks like an explosion. Explosions are exothermic reactions, but a reaction can be exothermic without exploding. Any increase in temperature indicates an exothermic reaction. If you were to hold a container where an exothermic reaction takes place, you would feel the heat and a thermometer inside the container would show an increase in temperature. Examples of exothermic reactions are combustion (burning) and neutralisation. Hand warming pads contain chemicals which undergo an exothermic reaction when pressed together.

Endothermic Reactions  

Endo- means 'inside'; an endoscope is a camera a doctor puts inside a patient's body. Endothermic reactions take in energy from their surroundings. If you were to hold a container where an endothermic reaction takes place, you would feel your hands getting cold, as the reaction would take heat from the container and even your hands if you kept holding it! A thermometer in the container would show a decrease in temperature.

The energy of the products is higher than the energy of the reactants. Examples of endothermic reactions are electrolysis, photosynthesis and thermal decomposition. In both these reactions, we need to keep putting energy into the reaction for it to keep going. In electrolysis the energy source is the battery, in photosynthesis the energy is light,  and in thermal decomposition the energy source is a heat source (like a flame).  Cold pads to relieve injuries can use chemicals which undergo an endothermic reaction.

Reaction profiles

An reaction profile is a sort of graph, comparing the energy of a set of chemicals before, during and after a chemical reaction. Before the reaction is on the left, and after the reaction is on the right. Energy is on the vertical axis; high energy is high up and low energy is low down.

In an exothermic reaction (on the left), the energy of the chemicals starts high and ends lower. The energy difference is released into the surroundings.

In an endothermic reaction (on the right), the energy of the chemicals starts low and ends higher. The energy difference is taken from the surroundings.

Both reaction profiles have a peak in the middle, where the energy of the chemicals is highest of all. This is because nearly all chemical reactions need some energy input so they can begin to happen. The energy needed to start a chemical reaction is called the activation energy- it's the energy needed to make the reaction active.

Reaction profiles help us visualise the difference between exothermic and endothermic reactions, and to compare reactions involving different amounts of energy; for example, an explosion releases a large amount of energy in a short time.

 

 

 

What type of reactions release thermal energy into the surroundings?

endothermic

exothermic

Choose two examples of exothermic reactions from the options below:

neutralisation

thermal decomposition

combustion

electrolysis

What would you see if you place a thermometer in a container where an endothermic reaction takes place?

The temperature would increase.

The temperature would decrease.

The temperature would remain stable.

Compare the energy of the reactants and the energy of the products in an endothermic reaction. 

The energy of the reactants and products in an endothermic reaction is equal.

The reactants in an endothermic reaction have higher energy than the products.

The products in an endothermic reaction have higher energy than the reactants.

A group of students perform a chemical reaction. The thermometer shows an increase in temperature.

What type of reaction was that?

Which of these devices would need an exothermic reaction to make them work?

hand warmer

gas cooker

electric cooker

battery

hand cooler

Match these endothermic reactions with their energy source.

Column A

Column B

electrolysis
light
thermal decomposition
electricity
photosynthesis
heat
combustion
is not endothermic

What does activation energy mean?

The energy needed to make atoms start to move.

The energy released by an active reaction.

The energy needed to start a chemical reaction

When you burn gas with a Bunsen burner, you need energy from a match or lit splint to make the gas burn. What does this tell us about burning as a chemical reaction?

Burning is endothermic, because we need to put energy into the system.

Burning is exothermic, because we need to put energy into the system.

We need to supply activation energy with the match, but the reaction then releases energy, so it's exothermic.

Electrolysis stops whenever the electrical supply is switched off. What does this tell us about electrolysis as a chemical reaction?

Electrolysis is endothermic, because we need to keep putting energy into the system.

Electrolysis is exothermic, because we need to put energy into the system.

We need to supply activation energy with electricity, but the reaction then releases energy, so it's exothermic.

  • Question 1

What type of reactions release thermal energy into the surroundings?

CORRECT ANSWER
exothermic
EDDIE SAYS
Exothermic reactions release thermal energy into their surroundings; exo is like exit.
  • Question 2

Choose two examples of exothermic reactions from the options below:

CORRECT ANSWER
neutralisation
combustion
EDDIE SAYS
Neutralisation and combustion are exothermic reactions, because they release heat into the surroundings.
  • Question 3

What would you see if you place a thermometer in a container where an endothermic reaction takes place?

CORRECT ANSWER
The temperature would decrease.
EDDIE SAYS
A thermometer in a container where an endothermic reaction takes place would show a decrease in temperature.
  • Question 4

Compare the energy of the reactants and the energy of the products in an endothermic reaction. 

CORRECT ANSWER
The products in an endothermic reaction have higher energy than the reactants.
EDDIE SAYS
The energy levels in the products of an endothermic reaction are higher than the energy in the reactants, because energy is taken in while the reaction is taking place.
  • Question 5

A group of students perform a chemical reaction. The thermometer shows an increase in temperature.

What type of reaction was that?

CORRECT ANSWER
exothermic
EDDIE SAYS
An increase in temperature is shown in exothermic reactions.
  • Question 6

Which of these devices would need an exothermic reaction to make them work?

CORRECT ANSWER
hand warmer
gas cooker
battery
EDDIE SAYS
Exothermic reactions release energy into their surroundings. We normally see that as heat (e.g. hand warmers, or burning methane in a gas cooker), but a battery transferring energy by electricity also depends on an exothermic reaction.
  • Question 7

Match these endothermic reactions with their energy source.

CORRECT ANSWER

Column A

Column B

electrolysis
electricity
thermal decomposition
heat
photosynthesis
light
combustion
is not endothermic
EDDIE SAYS
Endothermic reactions take energy from their surroundings. Often this involves heat, but other types of energy transfer can be used as well.
  • Question 8

What does activation energy mean?

CORRECT ANSWER
The energy needed to start a chemical reaction
EDDIE SAYS
Activation energy is the energy needed to make a reaction begin to happen. This is linked to the energy needed to start breaking atoms apart in the structures of the reactants.
  • Question 9

When you burn gas with a Bunsen burner, you need energy from a match or lit splint to make the gas burn. What does this tell us about burning as a chemical reaction?

CORRECT ANSWER
We need to supply activation energy with the match, but the reaction then releases energy, so it's exothermic.
EDDIE SAYS
The match only supplies energy at the very beginning, which is what we expect for activation energy. If burning methane was endothermic, we would have to keep using a match (or similar) to keep the reaction going.
  • Question 10

Electrolysis stops whenever the electrical supply is switched off. What does this tell us about electrolysis as a chemical reaction?

CORRECT ANSWER
Electrolysis is endothermic, because we need to keep putting energy into the system.
EDDIE SAYS
The key for recognising an endothermic reaction is that it stops if we stop putting energy in the system.
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