Topic 5 - Energy Changes

What is an exothermic reaction?

An exothermic reaction is one which transfers energy to the surroundings. This results in a rise of temperature.

What are some examples of exothermic reactions?

Combustion - Burning fuels

Neutralisation - Acid + Alkali

What is the reaction profile for an exothermic reaction?

See Image:

What is an endothermic reaction?

An endothermic reaction is one which takes in energy from the surroundings. This results in a fall of temperature.

What are some examples of endothermic reactions?

Citric acid + Sodium hydrogencarbonate

Thermal Decomposition

What is the reaction profile for an endothermic reaction?

See Image:

Required Practical: How can energy transfer be measured?

Measure the amount of energy released in a neutralisation reaction:

1. Put 25cm³ of chosen acid in a polystyrene cup.

2. Place a thermometer in the cup.

3. Measure the start temperature.

4. Get a chosen metal or carbonate.

5. Add the metal/carbonate and stir.

6. Take the highest/lowest temperature in the reaction.

7. Repeat steps with other metals/carbonates.

What reaction is bond breaking and bond forming?

Bond Breaking - Endothermic

Bond Forming - Exothermic

What is the format of a bond energy calculation?

You are given a reaction: H-H + Cl-Cl → H-Cl H-Cl

You are given the bond energies: H-H: 436 Cl-Cl: 242 H-Cl: 431

First find the energy needed to break the bonds:

(1 x H-H) + (1 x Cl-Cl) = 436 + 242 = 678

Then find the energy released by forming new bonds:

(2 x H-Cl) = 2 × 431 = 862

Then use (Overall = energy required - energy released) to find the overall energy change:

678 - 862 = -184 kJ/mol

If it is negative it is exothermic.

If it is positive it is endothermic.

Explain chemical reactions in a cell?

It is a system made up of two different electrodes in contact with an electrolyte. The two electrodes are metals that conduct and the electrolyte is a liquid that contains ions which react with the electrodes.

The chemical reaction between the electrodes and the electrolyte set up a charge difference between the electrodes. If the electrodes are then connected by a wire, the charge s able to flow and electricity is produced.

The type of electrodes affect the voltage. A higher difference in reactivity creates a higher voltage.

A battery is formed by connecting two or more cells together in series.

Explain a non-rechargeable cell?

Explained: The chemicals are used up and are irreversible. Once one reactant is used up no electricity is produced.

Examples: Alkaline Batteries.

Advantages: Cheap.

Disadvantages: They need replacing, They need to be recycled.

Explain a rechargeable cell?

Explained: They produce electricity in the same way as non-rechargeable cells. The reaction is reversed when connected to an external electric current.

Examples: Lithium-Ion Cells.

Advantages: The reaction can be reversed to produce electricity again, Used multiple times.

Disadvantages: They need to take time to be recharged, They lose electricity after each recharge cycle.

Explain a fuel cell?

Explained: A cell that uses fuel, oxygen and energy from the reaction to produce electrical charge. It uses a redox reaction to produce a product.

Examples: Hydrogen-Oxygen Fuel Cells.

Advantages: They produce electricity for as long as you provide hydrogen, They stay the same efficiency over time.

Disadvantages: Hydrogen is explosive and hard to store, It produces a relatively low amount of electricity.