Chemistry: Energy changes

What is an Exothermic reaction?

When energy is released to the surroundings in the form of heat, therefore the temperature of the surroundings increases.

What is an endothermic reaction?

Endothermic is when heat energy is taken in from the surroundings therefore the temperature of the surroundings decreases as energy has been absorbed.

Is it an exothermic reaction or an endothermic reaction when bonds are being made?

Exothermic

Why is bond making an exothermic reaction?

Bond making is exothermic as when you make the bonds energy is released to the surroundings.

Is Bond breaking exothermic or endothermic?

Endothermic

Why is bond breaking endothermic?

Bond breaking is endothermic as it requires energy taken in from the surroundings to break the bonds.

How do you know from Bond energy calculations if a reaction is exothermic?

As a reaction is exothermic if more energy is used in making bonds and you will get a negative value

How do you know in bond energy calculations if a reaction is endothermic?

As more energy is used in breaking bonds and you will get a positive value

What type of reaction is this and how do you know?

It is an endothermic reaction and we know this because the products are higher than the reactants as energy has been taken in from surroundings.

What type of reaction is this? And how do you know?

This is an exothermic reaction and we know this as the products are lower than the reactant because energy has been released to the surroundings.

What is a cell?

A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.

What is a battery?

Two or more cells connected in series

What factors affect the voltage produced by the cell?

Type of electrode: Rita, the difference in reactivity between the two electrodes the greater the potential difference of the cell.

Type of electrolyte: Different electrolytes contain different ions, which react differently with the electrodes and change their voltage

Describe rechargeable cells

A rechargeable cell has a reaction that can be reversed by connecting the cell to an external electrical current. This turns the product back into reactants so the cell can produce voltage again.

Describe non-rechargeable cells

Non-rechargeable cells have a reaction that cannot be reversed once one reactant is used up the reaction stops and no more electricity is produced. Recharging alkaline batteries risk leak leakage.

What are the advantages of non-rechargeable cells?

Cheap upfront cost

Good for low-power devices

What are the disadvantages of non-rechargeable cells?

Creates waste when discarded

Reactants are fully exhausted

What are the advantages of rechargeable cells?

Reversible reactions allow reuse

Cost-effective over long-term

What are the disadvantages of rechargeable cells?

Higher initial cost

Needs mains power to replenish

What is a fuel cell?

A fuel cell producers electricity directly from a chemical reaction between a fuel and oxygen

What happens to the fuel inside a fuel cell to produce an electric current?

The fuel is oxidised, which sets of a potential difference (voltage) that drives an electric current through the circuit.

In a hydrogen fuel cell, what happens to the electrons and hydrogen ions produced at the anode?

Electrons - Travel through the external circuit to the cathode, creating an electrical current

Hydrogen ions - Move through the electrolyte to the cathode.

Explain how a hydrogen fuel cell generates electricity.

1) Hydrogen enters the anode and is oxidised, releasing electrons and forming hydrogen ions.

2) Electrons flow through the external circuit to the cathode, generating an electric current.

3) Hydrogen ions move through the electrolyte to the cathode.

4) Oxygen reacts with hydrogen ions and electrons at the cathode to form water.

5) Water and heat are produced.

What are the advantages of using hydrogen fuel cells?

They produce no pollutants only water

Maintains capacity - does not lose ability to hold charge over time

Quick to refuel - Takes minutes compared to hours for recharging batteries

What are the disadvantages of using hydrogen fuel cells?

Hydrogen is highly flammable and explosive

Hydrogen is produced by non-renewable sources

Hydrogen is difficult to store