Understanding Chemical Reactions

Understanding Chemical Reactions

Basic Revision
For a chemical reaction to occur, bonds must be both formed and broken
Fe  +  CuSO4               Cu    +  FeSO4

Basic Revision

Chemical reactions always involve energy changes.
Making and breaking bonds involves energy changes

ENERGY AND CHEMICAL REACTIONS

Exothermic reactions
These reactions give out heat energy.
Combustion is an exothermic reaction.

ENERGY AND CHEMICAL REACTIONS

Endothermic reactions
These reactions take in heat energy from their surroundings.
These cause temperatures to fall.

ENERGY AND CHEMICAL REACTIONS

Work with 20ml of each substance in turn.
Copy the results table before you start

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!

ENERGY AND CHEMICAL REACTIONS

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.

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.

Endoothermic energy level diagram

In an endothermic reaction the products have more energy than the reactants.
∆H is positive for an endothermic reaction.

Making and breaking bonds

We have seen how bonds are formed (both ionic and covalent)
Do you think energy is needed to break bonds?
Breaking bonds requires energy. It is endothermic.
Breaking bonds requires energy. It is endothermic.
Making new bonds gives out energy. It is exothermic.

 Thank You