Bonds and Energy

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Slide 1

Understanding Chemical Reactions

Lesson: Bonds & Energy

Slide 2

Basic Revision

For a chemical reaction to occur, bonds must be both formed and broken

Fe + CuSO4 Cu + FeSO4

Slide 3

Basic Revision

Chemical reactions always involve energy changes.

Making and breaking bonds involves energy changes

Slide 4

Energy and chemical reactions

Exothermic reactions

These reactions give out heat energy.

Combustion is an exothermic reaction.

Slide 5

ENERGY AND CHEMICAL REACTIONS

Endothermic reactions

These reactions take in heat energy from their surroundings.

These cause temperatures to fall.

Slide 6

Slide 7

ENERGY AND CHEMICAL REACTIONS

Use the apparatus as shown.

Work with 20ml of each substance in turn.

Copy the results table before you start

Slide 8

Risk assessment

You will be using acids so wear safety goggles.

Wipe up spills immediately

Replace stoppers

Do not put solids down the sink – use the bowl at the front!

Slide 9

ENERGY AND CHEMICAL REACTIONS

Slide 10

Energy level diagrams

We can show the energy transfers in reactions on an energy level diagram.

These show us the energy stored in the reactants compared to the energy stored in the products.

Slide 11

Exothermic energy level diagram

Slide 12

Exothermic energy level diagram

∆H (‘delta H’) is the symbol for the ‘change in energy’.

In an exothermic reaction the products have less energy than the reactants.

∆H is negative for an exothermic reaction.

Slide 13

Endothermic energy level diagram

Slide 14

Endoothermic energy level diagram

In an endothermic reaction the products have more energy than the reactants.