What is a chemical reaction?
• Can you define a ‘chemical reaction’?
• What do the following words mean:
– ENDOTHERMIC – EXOTHERMIC
• Can you define a ‘chemical reaction’?
• A chemical reaction is a process that leads to the transformation of one set of chemical
substances to another.
• What do the following words mean:
– ENDOTHERMIC – takes in heat – EXOTHERMIC – gives out heat
Examples
• Endothermic reaction
citric acid + sodium bicarbonate
sodium citrate + carbon dioxide + water
• Exothermic reaction
magnesium + hydrochloric acid
magnesium chloride + hydrogen
Collision theory
• For a chemical reaction to occur,
the reactant particles must collide. But collisions with too little energy do not produce a reaction.
• The particles must have enough energy for the collision to be successful in producing a reaction.
• The rate of reaction depends on the rate of
successful collisions between reactant particles.
The more successful collisions there are, the faster the rate of reaction.
Chemical Reactions – what needs to happen?
• First of all chemical bonds need to be broken.
– Breaking bonds is endothermic – the process needs energy.
• Then new bonds are formed
– Making bonds is exothermic – the process gives out energy.
Combustion Reactions
When methane reacts with
oxygen the total energy given out is greater than the total energy taken in.
So overal the reaction gives out energy. It is an exothermic
reaction.
The bonds in the products are stronger than the bonds in the reactants.
The products are more stable.
Example of Endothermic Reactions
• Endothermic reactions are less common.
• Energy is absorbed from the surroundings.
• The reaction of nitrogen and oxygen is endothermic.
• The bonding of the products is weaker than that of the reactants.
• Overal energy is taken in.
• Photosynthesis is also endothermic.
Exothermic reactions
• Most reactions require some energy to get started (activation energy);
• Exothermic reactions give out more energy than is put in:
Endothermic reactions
• Endothermic reactions take in more energy than they give out:
Heat of Reaction
• The energy change in going from the reactants to the products in a chemical reaction is
known as the heat of reaction ∆H (kJ) 1 kilojoule (1KJ) = 1000 joules (1000J)
The ∆H of a reaction is usually calcualted per mole (KJ/mol)
Heat of Reaction ∆H
• For exothermic reactions ∆H is negative, heat is given out.
• For endothermic reactions ∆H is positive, heat is taken in.
Bond Energy
• Experiments have been carried out to find out how much energy is needed to break various bonds. Bond Energy
Using Bond Energy to calculate ∆H
Bond Breaking
4 C-H bonds 4 x 435 = 1740 2 O=O bonds 2 x 498 = 996 Total = 2736 KJ/mol
Bond Making
2 C=O bonds 2 x 803 = 1606 4 O-H bonds 4 x 464 = 1856 Total = 3462 KJ/mol
∆H = energy difference
∆H = energy to break bonds – energy to make bonds
∆H = 2736 – 3462
∆H = -726 KJ/mol
Activation Energy E
A• Even though most reactions are exothermic only a few are
spontanious.
– Eg. Na or K reaction with water
• Most reactions need some energy to get them going.
– Fuels burning need a spark to ignite them.
– It is required as some bonds need to be broken before the reaction can take place.
– This is the activation energy EA
Task:
1. The reaction between oxygen and hydrogen is extremely exothermic.
a) Write the balanced symbol equation for the reaction.
b) Why isn’t there an explosion when the 2 gases are mixed?
2. In Wales it is cold and wet; in Borneo it is hot and wet.
Where do you think a car will rust faster, Wales or Borneo – explain your answer.
3. Do questions on page 204
