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Understand Exothermic and Endothermic Reactions and the Haber Process

Worksheet Overview

QUESTION 1 of 10

During chemical reactions, energy is transferred 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', so exothermic reactions release heat to their surroundings. The picture shows an example of an exothermic reaction:

 

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 when an exothermic reaction is taking 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.

 

Endothermic Reactions  

Endo means 'inside', so endothermic reactions take in energy from their surroundings. Be careful because this can be confusing: if you were to hold a container when an endothermic reaction is taking 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 graph below shows the difference in energy levels of reactants and products in an endothermic reaction:

 

Graph to show energy levels in an endothermic reaction. 

 

As you can see, the energy of the products is far higher than the energy of the reactants. Examples of endothermic reactions are electrolysis and thermal decomposition.  

 

The Haber Process  

Some reactions are reversible, which means the products can react to produce the reactants again! The Haber Process is an example -  it is used in industry to produce ammonia -  a compound of nitrogen and hydrogen. Ammonia is a colourless gas used in the production of fertilisers, nylon, dyes, cleaning products and explosives.

The chemical equation below shows the Haber Process (note that a double arrow is used to show reversibility):

Equation for Haber Process 

The forward reaction in the Haber Process is exothermic, i.e. when ammonia is produced, heat is released. This means an increase in temperature shifts the equilibrium to the reverse reaction, so less ammonia is produced. However, if the temperature is too low, not enough ammonia will be produced.

 

The Haber Process is illustrated in the diagram below:

Diagram to show Haber Process.

 

Time for some questions now.

What type of reactions release thermal energy into the surroundings?

Endothermic

Exothermic

Which of the options below are examples of exothermic reactions?

Neutralisation

Thermal decomposition

Combustion

Electrolysis

What would you see if you placed a thermometer in a container where an endothermic reaction is taking 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 carry out a chemical reaction. The thermometer shows an increase in temperature.

 

What type of reaction was it?

What type of a reaction is the Haber Process?

Reversible

Irreversible

What type of energy changes occur in a reversible reaction?

The energy of the reactants is higher than the energy of the products

The reverse reaction is always the endothermic one

One direction is exothermic and the other is endothermic, but that differs according to the reaction

How would you describe ammonia?

A white solid

A colourless gas

A blue liquid

What would be a potential problem if the temperature during the Haber Process was kept too low?

Ammonia would be produced uncontrollably

Not enough ammonia would be produced

What type of catalyst is used in the Haber Process?

Iron

Mercury

Magnesium

  • Question 1

What type of reactions release thermal energy into the surroundings?

CORRECT ANSWER
Exothermic
EDDIE SAYS
How did you do in this first question? If you remembered that the prefix 'exo' means 'out' (just as an exit is a way out), this question would have been easier. This is a helpful hint for many science questions - even if you're not sure of the correct word, sometimes you can work it out from breaking the word down into recognisable parts. Exothermic reactions let energy out - they release thermal energy into their surroundings.
  • Question 2

Which of the options below are examples of exothermic reactions?

CORRECT ANSWER
Neutralisation
Combustion
EDDIE SAYS
There were two correct options here - did you get them both? Neutralisation and combustion are exothermic reactions because they release heat into the surroundings.
  • Question 3

What would you see if you placed a thermometer in a container where an endothermic reaction is taking place?

CORRECT ANSWER
The temperature would decrease
EDDIE SAYS
A thermometer in a container where an endothermic reaction is taking place would show a decrease in temperature. This is because endothermic reactions take in energy from their surroundings and so make the container and thermometer colder.
  • 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
Don't worry if you found this one a bit harder - it is quite a tricky concept to imagine. It's worth having another look at the diagram in the Introduction if you're still muddled. The energy levels in the products of an endothermic reaction are higher than the energy in the reactants because heat energy is taken in while the reaction is taking place.
  • Question 5

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

 

What type of reaction was it?

CORRECT ANSWER
exothermic
EDDIE SAYS
Don't get muddled between your exos and endos! An increase in temperature is seen in exothermic reactions.
  • Question 6

What type of a reaction is the Haber Process?

CORRECT ANSWER
Reversible
EDDIE SAYS
The Haber Process is a reversible reaction which means that the products can react to produce the reactants once more. All those products and reactors can be muddling so it's worth studying the diagram in the Introduction to help get this sorted.
  • Question 7

What type of energy changes occur in a reversible reaction?

CORRECT ANSWER
One direction is exothermic and the other is endothermic, but that differs according to the reaction
EDDIE SAYS
This was a tricky one, so don't worry if you found it hard. The third option was the correct one - one direction is exothermic and the other is endothermic, but that differs according to the reaction. This means that, while in the Haber Process the forward reaction is exothermic, that is not always the case with all reversible reactions. In some others, it could be the other way round.
  • Question 8

How would you describe ammonia?

CORRECT ANSWER
A colourless gas
EDDIE SAYS
It is important to know your chemicals! Ammonia is a colourless gas which is a compound of nitrogen and hydrogen.
  • Question 9

What would be a potential problem if the temperature during the Haber Process was kept too low?

CORRECT ANSWER
Not enough ammonia would be produced
EDDIE SAYS
This is a difficult concept to grasp, so make sure you read the Introduction again if you're finding this hard. The forward reaction in the Haber Process is exothermic, i.e. when ammonia is produced, heat is released. This means that an increase in temperature would shift the equilibrium to the reverse reaction, so that less ammonia would be produced. However, if the temperature is too low, not enough ammonia will be produced.
  • Question 10

What type of catalyst is used in the Haber Process?

CORRECT ANSWER
Iron
EDDIE SAYS
An iron catalyst is used in the Haber Process. Nitrogen and hydrogen are passed over iron during the reaction, which speeds up the production of ammonia. Some science concepts are rather challenging, aren't they, and this topic can be a bit confusing, so well done for reaching the end of this activity.
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