Lesson 1 - Enthalpy Changes in Chemistry

Starter

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

1. Exothermic: when heat is given out

2. Endothermic: when heat is taken in

Energy level diagrams

Which diagram do you think is and which is ?

Heat taken in Heat given out

Change in energy

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

• When the reaction has taken place they have either within them than before.

Reactants Description of reaction Initial temp. / °C

Final temp. /°C

Change in temp. ΔT / °C

Exo or endo?

Sodium and chlorine gas

https://www.youtube.com/watch?v=Mx5JJWI2aaw

N/A (demo)

N/A (demo)

N/A (demo)

Sugar and sulfuric acid

https://www.youtube.com/watch?v=nqDHwd9rG0s

N/A (demo)

N/A (demo)

N/A (demo)

Dissolving ammonium nitrate

https://www.youtube.com/watch?v=G3S4YvhhAic

N/A (demo)

N/A (demo)

N/A (demo)

Barium hydroxide and ammonium nitrate

https://www.youtube.com/watch?v=GQkJI-Nq3Os

N/A (demo)

N/A (demo)

N/A (demo)

Exothermic Demos

• Chlorine gas reacting with sodium metal (video)

• Concentrated sulfuric acid and sugar (demo)

Definitions

• In your notes write down a ;

AND

what happened in the two experiments you have just seen.

Exothermic Reactions

Correct definition:

An chemical reaction where the products have less energy than the reactants.

.

Endothermic Demos

• Dissolving ammonium nitrate

• Reaction between ammonium nitrate and barium hydroxide.

Definitions

• In your notes write down a ;

AND

what happened in the two experiments you have just seen.

Endothermic Reactions

Correct definition:

An chemical reaction is where the products have more energy than the reactants.

.

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

Wrap up – Can you recall?

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