Understanding Chemical Understanding Chemical ReactionsReactions

Bonds & EnergyBonds & Energy

Basic RevisionBasic Revision

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

Fe + CuSOFe + CuSO4 4 Cu + FeSOCu + FeSO44

Basic RevisionBasic Revision

Chemical reactions always involve Chemical reactions always involve energy changes.energy changes.

Making and breaking bonds involves Making and breaking bonds involves energy changesenergy changes

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

Exothermic reactionsExothermic reactions These reactions give out heat These reactions give out heat

energy.energy. Combustion is an exothermic Combustion is an exothermic

reaction.reaction.

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

Endothermic reactionsEndothermic reactions These reactions take in heat energy These reactions take in heat energy

from their surroundings.from their surroundings. These cause temperatures to fall.These cause temperatures to fall.

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

Use the apparatus Use the apparatus as shown.as shown.

Work with 20ml of Work with 20ml of each substance in each substance in turn.turn.

Copy the results Copy the results table before you table before you startstart

RISK ASSESSMENTRISK ASSESSMENT

You will be using acids so wear You will be using acids so wear safety goggles.safety goggles.

Wipe up spills immediatelyWipe up spills immediately Replace stoppersReplace stoppers Do not put solids down the sink – use Do not put solids down the sink – use

the bowl at the front!the bowl at the front!

ENERGY AND CHEMICAL ENERGY AND CHEMICAL REACTIONSREACTIONS

ReactionReaction temp. temp. before before mixing mixing ((00C)C)

temp. temp. after after mixing mixing ((00C)C)

Exothermic or Exothermic or Endothermic?Endothermic?

sodium hydroxide sol’n + sodium hydroxide sol’n + hydrochloric acid hydrochloric acid

sodium hydrogen sodium hydrogen carbonate sol’n + citric carbonate sol’n + citric acidacid

copper sulphate sol’n + copper sulphate sol’n + magnesium powdermagnesium powder

sulphuric acid + sulphuric acid + magnesium ribbonmagnesium ribbon

barium chloride + sodium barium chloride + sodium sulphatesulphate

ENERGY LEVEL DIAGRAMSENERGY LEVEL DIAGRAMS

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

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

Exothermic energy level diagramExothermic energy level diagram

Exothermic energy level diagramExothermic energy level diagram

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

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

∆∆H is negative for an H is negative for an exothermic reaction.exothermic reaction.

Endothermic energy level diagramEndothermic energy level diagram

Endoothermic energy level diagramEndoothermic energy level diagram

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

∆∆H is positive for H is positive for an endothermic an endothermic reaction.reaction.

Making and breaking bondsMaking and breaking bonds

We have seen how bonds are formed We have seen how bonds are formed (both ionic and covalent)(both ionic and covalent)

Do you think energy is needed to Do you think energy is needed to break bonds?break bonds?

Breaking bonds requires energy. Breaking bonds requires energy. It is endothermic.It is endothermic.

Breaking bonds requires energy. Breaking bonds requires energy. It is endothermic.It is endothermic.

Making new bonds gives out Making new bonds gives out energy. It is exothermic.energy. It is exothermic.