FULL TOPIC 5: ENERGY CHANGES

What is a fuel cell?

An electrical cell that uses a fuel and oxygen to produce electrical energy.

How does a fuel cell produce electrical energy?

By using energy from the reaction between fuel and oxygen.

What is the efficiency of energy production in a fuel cell?

Fuel cells produce electrical energy efficiently.

What does fuel cell + hydrogen and oxygen =

Nice clean water and release energy

Fuel cell structure

Left:
Electrode NEGATIVE CATHODE
OUTSIDE - Anode compartment
Middle:
Electrolyte
Right:
Electrode POSITIVE CATHODE
OUTSIDE - Cathode compartment

What are the electrodes made up of in a fuel cell?

Porous Carbon (tiny holes) + catalyst

How do fuel cells fuse hydrogen + oxygen?

1. Hydrogen enter through left inlet
2. Oxidised by anode
3. Electrons then pass around wire to cathode
4. Hydrogen atoms move through the electrolytes to the cathode
5. Electrons and hydrogen atoms can react to the oxygen that come in from the right inlet
6. All combine to make water
7. 1 Oxygen + 4 Hydrogen + 4 Electrons = 2 molecules of water

How do you get electrical energy from fuel cells?

Movement of electrons

How do fuel cells work? (EXAMS)

• Fuel enters the cell
• Becomes oxidised (hydrogen atoms lose electrons)
• Sets up potential difference across the cell
  This drives the electrons around the circuit

Pros of fuel cells

• Only requires hydrogen + oxygen
• Doesn't produce any carbon dioxide or pollutants
• Simple devices, last longer than batteries, less polluting to dispose of

Cons of fuel cells

• Hydrogen is a gas
  More space to store than fossil fuels or batteries
• Explosive when mixed with air
• Dangerous to store
• Making hydrogen fuel requires making energy

What is an electrochemical cell made up of?

two different electrodes in contact with an electrolyte

Electrolyte

A liquid that contains ions which react with electrodes

What happens in the chemical reactions between the electrodes and the electrolytes?

They set up a charge difference between the electrodes

How is electricity produced via electrodes?

If the electrodes are then connected by a wire, the charge is able to flow - electricity produced

What does the voltage of the cell depend upon?

• Different metals
• Type of electrodes

How do metals change the voltage of the cell?

Different metals react differently with the same electrolyte - causes the charge difference, or the voltage of the cell. The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell

How do you make a battery?

Connecting two or more cells together in series

What do non - reachable batteries contain?

Cells which use irreversible reactions, once one of the reactants is used up, they don't produce anymore charge and you have to replace them

How do you reverse a reaction in a reachable cell?

By connecting it to an external electric current

Is bond breaking endothermic or exothermic?

Endothermic, energy must be supplied to break existing bonds

Is bond formation endothermic and exothermic?

Exothermic, energy is released when new bonds are formed

Bond energy calculations

Overall energy change = energy required to break bonds - energy released by forming bonds

What is MexoBendo?

Making bonds = exothermic, Breaking bonds = endothermic

How much energy is stored in Chemical Reactions?

Chemicals store a certain amount of energy - and different chemicals store different amounts

What happens if the product of a reaction store has more energy than the original reactants?

They must have taken in the difference in energy between the products and reactants from the surroundings during the reaction

What happens if the product of a reaction store has less energy than the original reactants?

The excess energy was transferred to the surroundings during the reaction

Law of conservation

Energy cannot be created or destroyed only moved around

Exothermic

Releases energy

Where does exothermic reactions release energy to?

To the surroundings

How are exothermic reactions shown?

A rise in temperature

An example of an exothermic reaction

Combustion

Everyday uses for exothermic reactions

Hand warmers, self heating cans

Endothermic

Absorbs energy

Where do endothermic reactions take in energy from?

The surroundings

How are endothermic reactions shown?

A fall in temperature

Examples of endothermic reactions

Thermal decomposition, photosynthesis

Everyday uses of endothermic reactions

Sports injury packs

How do you measure the amount of energy released by a chemical reaction?

Taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction

What are the problems with measuring energy transfer?

There may be energy lost to the surroundings

How do you reduce lost energy to the surroundings when measuring energy transferred?

Reduction by putting the polystyrene into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation

What are reaction profiles?

Diagrams that show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction

Exothermic reaction profile

Products are at a lower energy than the reactants

Endothermic reaction profile

Products are at a higher energy than the reactants