Topic 5 - Energy changes

Question 1

Compare an exothermic and endothermic reaction. Give examples of both.

Answer 1

Exothermic:
— Transfers energy to the surroundings which is shown by an increase in temperature of the surroundings. Examples of exothermic reactions include combustion, oxidation reactions and neutralisation. Some everyday uses are hand warmers and self-heating cans.

Endothermic:
— Takes in energy from the surroundings which is shown by a decrease in temperature of the surroundings. Examples of endothermic reactions include thermal decomposition and the reaction between citric acid and sodium hydrogen carbonate. Everyday uses are sports injury packs.

Question 2

What is the definition of activation energy?

Answer 2

Activation energy:

— The minimum amount of energy the reactant particles require to collide with each other and react.

Question 3

Compare the reaction profiles for an exothermic and endothermic reaction.

Answer 3

In an exothermic reaction, the products are at a lower energy level than the reactants due to the fact that energy is released into the surroundings. They also require less activation energy than endothermic reactions. This is vise versa for an endothermic reaction as the products are at a higher energy level than the reactants due to the fact that they take in energy from the surroundings.

Question 4

Compare bond breaking and bond formation. Link this to bond energy calculations

Answer 4

Bond breaking is an endothermic reaction because energy must be supplied to break the bonds and bond formation is an exothermic process as energy is released into the surroundings.

In an exothermic reaction, the energy released from forming the bonds is greater than the energy used to break the bonds. In an endothermic reaction, the energy used to break bonds is greater than the energy used to form the bonds.

If the answer to a bond energies calculation is negative then that means the reaction is exothermic and if it is positive then that means the reaction is endothermic.

Question 5

What is an electrochemical cell and what is a battery?

Answer 5

Electrochemical cell:
— A basic system made by connecting two or more metal electrodes in contact with an electrolyte.

Battery:
— Two or more cells connected in series to provide a greater voltage.

Question 6

What factors affect the voltage of a cell?

Answer 6

• – The difference in reactivity of the electrodes so the bigger the difference in reactivity, the greater voltage produced
• – The electrolyte used as different ions react differently to different metals
• – The type of electrode used

Question 7

What is the difference between a non-rechargeable battery and rechargeable battery?

Answer 7

In a non-rechargeable cell or battery, the chemical reactions eventually stop because one of the reactants get used up whereas in a rechargeable cell or battery, the chemical reactions can be reversed when connected to an external electrical current. An example of a non-rechargeable battery are alkaline batteries.

Question 8

What is a fuel cell?

Answer 8

Fuel cell:
— An electrical cell supplied with an external source of fuel and oxygen (or air). The fuel is oxidised within the cell which produces a potential difference.

Question 9

How do fuel cells work?

Answer 9

NOTE: in fuel cells, the negative electrode is the anode and the positive electrode is the cathode whereas in electrolysis, it is the other way around.

• – Hydrogen enters the fuel cell in the anode compartment while oxygen enters in the cathode compartment.
• – At the anode, hydrogen loses electrons to form hydrogen ions. This is oxidation.
• – The hydrogen ions move towards the cathode.
• – At the cathode, oxygen gains electrons and reacts with the H+ ions to make water. This is reduction.

REFER BACK TO THE TEXTBOOK FOR THE DIAGRAM AND HALH EQUATIONS