AQA Chemistry 5: Energy changes

What happens to the amount of energy in a chemical reaction?

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

What is an exothermic reaction?

-energy is transferred to the surroundings

-temperature of the surroundings increases

-eg combustion, many oxidisation reactions, neutralisation

-eg self-heating cans, hand warmers

What is an endothermic reaction?

-energy is taken in from the surroundings

-temperature of the surroundings decreases

-eg thermal decompositions, reaction of citric acid and sodium hydrogencarbonate

-eg sports injury packs

When do chemical reactions occur?

When reacting particles collide with each other with sufficient energy

What do reaction profiles show?

-relative energies of the reactants and products in a reaction

-how energy changes over the course of the reaction

What does an exothermic reaction profile look like?

What does an endothermic reaction profile look like?

[HT only] What happens to bonds during chemical reactions?

• energy must be supplied to break bonds in the reactants

• energy is released when bonds in the products are formed.

[HT only] How do you calculate bond energy?

1. Add together the bond energies for all the bonds in the reactants - this is the 'energy in'.

2. Add together the bond energies for all the bonds in the products - this is the 'energy out'.

3. Calculate the energy change = energy in - energy out.

[HT only] What is the overall energy change of a reaction?

The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed

[HT only] In an exothermic reaction, is bond energy positive or negative?

-positive

-the energy released from forming new bonds is greater than the energy needed to break existing bonds

[HT only] In an endothermic reaction, is bond energy positive or negative?

-negative

-the energy needed to break existing bonds is greater than the energy released from forming new bonds

[Triple only] What is a cell?

Cells contain chemicals which react to produce electricity

[Triple only] What does the voltage produced by a cell depend on?

-type of electrode

-type of electrolyte

[Triple only] How can a simple cell be made?

A simple cell can be made by connecting two different metals in contact with an electrolyte.

[Triple only] What is a battery?

Batteries consist of two or more cells connected together in series to provide a greater voltage

[Triple only] What is the difference between non-rechargeable and rechargeable cells and batteries?

non-rechargeable cells and batteries:

-the chemical reactions stop when one of the reactants has been used up.

-Alkaline batteries are non-rechargeable.

-rechargeable cells and batteries:

-can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

[Triple only] What is a fuel cell?

Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air.

[Triple only] How does fuel in a fuel cell produce a potential difference?

The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

[Triple only] What does the reaction in a hydrogen fuel cell involve?

the oxidation of hydrogen to produce water.

[Triple only] What are the pros and cons of using hydrogen fuel cells?

Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.

+ easy to maintain, no moving parts

+ small size

+ water is only chemical product

- expensive to manufacture

- need a constant supply of hydrogen gas

- hydrogen gas is flammable

[Triple only] What are the pros and cons of using alkaline cells (batteries)?

+ cheap to manufacture

- may end up in landfill once used

- expensive to recycle

[Triple only] What are the pros and cons of using rechargeable cells?

+ can be recharged many times before being recycled, reduces use of resources

- more expensive than alkaline cells to manufacture