Module 3

Lesson 1 - Enthalpy Changes in Chemistry

Starter

• When a chemical reaction takes place heat may be given out or taken in.

1. Can you remember the word we use when heat is given out?

2. Can you remember the word we use when heat is taken in?

AfL - Energy level diagrams

Which diagram do you think is endothermic and which is exothermic?

Heat taken inHeat given out

Change in energy

• Chemical reactants have a certain amount of chemical energy stored within them.

• When the reaction has taken place they have either more or less energy stored within them than before.

Objectives

MustRecall and use the terms ENDOTHERMIC and EXOTHERMICShouldDescribe examples of ENDOTHERMIC and EXOTHERMIC reactions.CouldExplain ENDOTHERMIC and EXOTHERMIC reactions using energy level diagrams.

Reactants Description of reaction Initial temp. / °C

Final temp. /°C

Change in temp. ΔT / °C

Exo or endo?

Sodium and chlorine gas

N/A (demo)

N/A (demo)

N/A (demo)

Sugar and sulphuric acid

N/A (demo)

N/A (demo)

N/A (demo)

Dissolving ammonium nitrate

N/A (demo)

N/A (demo)

N/A (demo)

Barium hydroxide and ammonium nitrate

N/A (demo)

N/A (demo)

N/A (demo)

Exothermic Demos

• Chlorine gas reacting with sodium metal (video)

• Concentrated sulphuric acid and sugar (demo)

Definitions

• In your notes write down a definition of an exothermic reaction;

AND• Describe what happened in the two

experiments you have just seen.

BE PREPARED TO SHARE YOUR ANSWERS – I COULD ASK YOU!

Exothermic Reactions

Correct definition:

An exothermic chemical reaction where the products have less energy than the reactants. Energy is given out to the surroundings (temperature goes up).

Endothermic Demos

• Dissolving ammonium nitrate• Reaction between ammonium nitrate and

barium hydroxide.

Definitions

• In your notes write down a definition of an endothermic reaction;

AND• Describe what happened in the two

experiments you have just seen.

BE PREPARED TO SHARE YOUR ANSWERS – I COULD ASK YOU!

Endothermic Reactions

Correct definition:

An endothermic chemical reaction is where the products have more energy than the reactants. Energy is taken in from the surroundings (temperature goes down).

AfL - Which of the following statements is correct?

1. The products of a chemical reaction have more energy than the reactants so an exothermic reaction must have taken place.

2. The products of a chemical reaction have less energy than the reactants so an endothermic reaction must have taken place.

3. The products of a chemical reaction have less energy than the reactants so an exothermic reaction must have taken place.

Which of the following statements is correct?

The products of a chemical reaction have less energy than the reactants so an exothermic reaction must have taken place.

Q - How can we illustrate this?

Energy level diagrams – copy into your notes

Endothermic Exothermic

Heat taken in Heat given out

Energy level of products is higher than reactants so heat taken in.

Energy level of products is lower than reactants so heat given out.

Question

Answer

Plenary – Can you recall?

• Draw an energy profile for an exothermic reaction marking clearly the activation energy.

Homework

• Question booklet.