Chapter 5 - energy changes

Energy cannot be created or destroyed

The amount of energy in the universe at the end of the chemical reaction is the same as before the reaction takes place

It means that the reaction took in the difference in energy between the products and the reactants from the surroundings during the reaction (the amount of energy absorbed)

An exothermic reaction is one which transfers energy to the surroundings

The temperature of the surroundings increases

• Combustion reactions (burning fuels)

• Neutralisation reactions (acid + alkali)

• Oxidation reactions

• hand warmers

• self heating cans

An endothermic reaction is one that takes in energy from the surroundings

The temperature of the surroundings decreases

• thermal decomposition

• reaction between citric acid and sodium hydrogencarbonate

The reactions in sports injury packs

Reaction profiles show…

• the relative energies of the reactants and products

• the activation energy

• the overall energy change of a reactant

The difference between the reactants and the products is the overall energy change

• could be the amount of the energy taken in by the reaction

• or the amount of energy released by a reaction

The energy rise from the reactants to the peak of the curve shows the ACTIVATION ENERGY

Reactions can only occur when…

• The reactant particles collide with each other…

• …with a sufficient amount of energy

Activation energy is the minimum amount of energy that particles must have in order to react

In a chemical reaction, old bonds and broken and new bonds are formed

Because energy must be supplied from the surroundings to break existing bonds in the reactants

Because energy is released when new bonds are formed in the products

Bond energies help us calculate the energy needed to break bonds and the energy released when bonds are formed

In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds

In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds

An electrochemical cell is a basic system made up of two different electrodes in contact with an electrolyte used to produce electricity

The two electrodes have to be metals so that the cell is able to conduct electricity

An electrolyte is a solution that contains ions which react with the electrodes to produce electricity

You would see a potential difference of a voltage between the two electrodes and an electric current would flow through the wire

The two factors that will affect the voltage of a cell are

• the type of electrodes

• the electrolyte used

The electrodes used will affect the voltage of a cell as different metals have different reactivities (use reactivity series)

The greater the difference between the reactivity of the metal electrodes, the greater the potential difference produced by the cell

A battery consists of two or more cells connected together in series to produce a greater voltage

Chemical reactions eventually stop in non-rechargeable batteries and cells when the reactants are used up

• electricity will no longer be produced

Alkaline batteries are non-rechargeable batteries

Alkaline batteries contain cells which use irreversible reactions so they need to be replaced once the reactant is used up

Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when we apply an external electric current to it

A cell in which a fuel, such as hydrogen, is reacted with pure oxygen or air, to produce an electric current

The fuel enters the fuel cell and it becomes oxidised electrochemically to produce a potential difference within the cell

The hydrogen fuel is being oxidised to produce a waste product of water

The electrolyte is usually an acid such as phosphoric acid

The hydrogen goes into the negatve electrode (anode)

The oxygen goes into the positive electrode (cathode)

Oxidation is happening as hydrogen loses electrons to produce H+ ions

The H+ ions in the electrolyte then move to the positive electrode (cathode)

Reduction is happening as oxygen gains electrons from the cathode and reacts with the H+ ions in the electrolyte to form water

The electrons are flowing from the anode to the cathode through an external circuit to produce an electric current

2H₂ + 4OH^- → 4H₂O + 4e^-

O₂ + 2H₂O + 4e^- → 4OH^-

2H₂ + O₂ → 2H₂O

• A hydrogen fuel cell will produce electricity as long as you provide hydrogen

• they do not get less efficient the longer they run

• no harmful waste product

• no pollutants produced

• can be many different sizes for many different uses

• run on hydrogen gas which is highly flammable and explosive

• hydrogen is difficult to store safely

• sometimes the hydrogen is produced by non renewable methods e.g. from hydrocarbons in fossil fuels

• can be recharged many times before being recycled

• takes up less space to store

• no dangerous fuels required

• produces a greater potential difference than a hydrogen fuel cell

• costs more to manufacture

• they will run out and need to be recharged

• they store less energy the more charging cycles they go through so they are less efficient than hydrogen fuel cells