Endothermic and Exothermic reactions

Exothermic reactions transfer energy to the surroundings.

The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter.

Examples:

• Combustion (burning)
• Many oxidation reactions, for example rusting
• Neutralising reactions betweeen acids and alkalis

These are reactions that take in energy from the surroundings.

The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. 

We usually use these in cold packs

Some examples of endothermic reactions are:

• Electrolysis
• The reaction between ethanoic acid and sodium carbonate
• The thermal decomposition of calcium carbonate in a blast furnace

In reversible reactions, the reaction in one direction will be exothermic and the reaction in the other direction will be endothermic.

The reaction between anhydrous copper sulfate and water is reversible:

hydrated copper sulfate (blue)anhydrous copper sulfate (white) + water

Water is driven off from hydrated copper sulfate when it is heated, so the forward reaction is endothermic - energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic - energy is transferred to the surroundings when it happens. This is easily observed. When water is added to anhydrous copper sulfate, enough heat is released to make the water bubble and boil.

The products is less than the reactants, That is because the energy is transferred from the reactants to the surroundings. 

The difference between the energy of the products and the energy of the reactants tells us the energy that is released to the surroundings. 

Products have more energy than the reactants, that is because energy has been taken in from the surroundings

The difference between the reactants and the products show the amount of energy that has been taken in

Reactions that can only occur when particles collied with each other and with efficient energy.

The minimum amount of enrgy that particles must have in order to react is called activation energy.

Chemical cells use chemical reactions to transfer energy by electricity.

The voltage produced by a cell depends upon a number of factors including the type of electrode and electrolyte

In non-rechargeable cells, eg alkaline cells, a voltage is produced until one of the reactants is used up.

In rechargeable cells and batteries, like the one used to power your mobile phone, the chemical reactions can be reversed when an external circuit is supplied.

The biggest voltage occurs when the difference in the reactivity of the two metals is the largest. For example magnesium and silver

Fuel cells work in a different way than chemical cells. Fuel cells produce a voltage continuously, as long as they are supplied with:ma constant supply of a suitable fuel, oxygen, eg from the air

In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage. Water is the only product.

• Anode - the positive electrode
• Cathode - the negative electrode

Type of cell:     Pros        Cons 

Alkaline cell  :  Cheaper to manufacture, May end up in landfill sites once fully discharged; recyclable though it is expensive

Rechargeable cell  : Can be recharged many times before being recycled, reducing the use of resources , Costs more to manufacture

Hydrogen fuel cell  : Easy to maintain as there are no moving parts; small size; water is the only chemical product,   Very expensive to manufacture; need a constant supply of hydrogen fuel, which is a flammable gas

Concentration in mol/dm³ = amount in mol ÷ volume in dm³

no. of moles = concentration x volume

Here are the steps needed to calculate the energy change:

1. Draw out the bonds in each of the reactants and products.

2. Calculate the energy absorbed when all of the reactant bonds are broken.

3. Calculate the energy released when all of the new bonds in the products are made.

4. Subtract the total for the new bonds from the total for the old bonds. A negative final answer means that the reaction is exothermic.