C5 - Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

When energy is released and transferred to the surroundings.

Question 2

What are some examples of exothermic reactions?

Answer 2

• Combustion.
• Neutralisation reactions.
• Many oxidation reactions.

Question 3

What are some everyday uses of exothermic reactions?

Answer 3

• Hand warmers use the exothermic oxidation of iron in air to release energy.
• Self heating cans use exothermic reactions between chemicals in their bases.

Question 4

What is an endothermic reaction?

Answer 4

Energy is taken in from the surroundings.

Question 5

What are some examples of endothermic reactions?

Answer 5

• The reaction between citric acid and sodium hydrogencarbonate.
• Thermal decomposition.

Question 6

What is an everyday use of endothermic reactions?

Answer 6

Sports injury packs use the chemical reaction to become instantly cooler without having to put it in the freezer.

Question 7

Describe an investigation the amount of energy released in a reaction.

Answer 7

1) Take the initial temperature of the reagents, mix them in a polystyrene cup and measure the temperature of the solution at the end of the reaction.
2) However, lots of energy is lost to the surroundings, so it is not accurate.
3) To reduce the amount of energy transfer, place the polystyrene cup into a beaker of cotton wool for more insulation.
4) Place a lid on top of the cup to reduce energy lost by evaporation.
5) This method can also be used to investigate the effect mass or concentration of the reactants used has on the amount of energy released in a reaction.

Question 8

Describe an investigation which tests the effect of acid concentration on the amount of energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide.

Answer 8

1) Put a known volume of a known concentration of hydrochloric acid and sodium hydroxide in separate beakers.
2) Place both beakers in a water bath set to 25 degrees C until both beakers reach the same temperature (25 degrees C).
3) Add the hydrochloric acid followed by the sodium hydroxide to a polystyrene cup that is placed in a beaker of cotton wool. Place a lid on top of the cup after finished pouring the substances in the cup.
4) Take the temperature of the mixture every 30 seconds and record the highest temperature.
5) Repeat steps 1-4 using different concentrations of hydrochloric acid, keeping the volume the same. this

Question 9

What do reaction profiles show?

Answer 9

How energy changes over the course of the reaction.

Question 10

How does a reaction profile show an exothermic or endothermic reaction?

Answer 10

Exothermic: The products are at a lower energy than the reactants.
Endothermic: The products are at a higher energy than the reactants.

Question 11

What does each feature on a reaction profile show?

Answer 11

• Difference in height of products and reactants: The overall energy change in the reaction.
• The initial rise in energy: Activation energy.

Question 12

What is activation energy?

Answer 12

The minimum amount of energy the reactants need to collide with each other to start a reaction.The greater the activation energy, the more energy needed to start the reaction.

Question 13

What happens during a chemical reaction in terms of bonds?

Answer 13

Old bonds are broken and new bonds are formed.

Question 14

What happens when bonds are broken and formed?

Answer 14

Broken: Energy must be supplied to break existing bonds, so bond breaking is an endothermic process.
Formed: Energy is released when new bonds are formed, so bond formation is an exothermic process.

Question 15

What is a bond energy?

Answer 15

The amount of energy a chemical bond has. This can vary slightly depending on the compound the bond occurs in.

Question 16

How do you calculate the overall energy change for a reaction?

Answer 16

Sun of the energies needed to break bonds minus the sun of the energy released when new bonds are formed.

Question 17

What is an electrochemical cell?

Answer 17

A basic system made up of two different electrodes in contact with an electrolyte.

Question 18

How does a cell work?

Answer 18

The chemical reactions between the electrodes and electrolyte set up a charge difference between the electrodes. If a wire is connected to the’, the charge is able to flow and electricity is produced.

Question 19

What does the voltage of a cell depend on?

Answer 19

• Type of electrodes: the bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell.
• Electrolyte used: Different ions in the solution will react differently with the metal electrodes used.

Question 20

What is a battery?

Answer 20

Two or more cells together in series.

Question 21

What has a bigger voltage? A cell or a battery?

Answer 21

A battery because the voltages of the cells in the battery are combined, so there is a bigger voltage overall.

Question 22

What happens in non-rechargeable batteries?

Answer 22

• In some cells, the chemical reactions that happen at the electrodes are irreversible.
• Overtime, the ions in the electrolyte and the metal ions on the electrode get used up and turned into the products of the reaction.
• Once any of the reactants are used up, the reaction can’t happen, so no electricity is produced.
• The products can’t be turned back into the reactants, so the cell can’t be recharged.

Question 23

How can a rechargeable cell be recharged?

Answer 23

The reaction can be reversed by connecting it to an external electric current.

Question 24

What is a fuel cell?

Answer 24

An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently.

Question 25

How do fuel cells produce electrical energy?

Answer 25

When the fuel enters the cell, it becomes oxidised and sets up a potential different within the cell.

Question 26

What happens in a hydrogen-oxygen fuel fuel cell?

Answer 26

• The electrolyte is often an acid whereas the electrodes are often porous carbon with a catalyst.
• Hydrogen goes into the negative anode compartment and oxygen goes to the positive cathode compartment.
• At the negative anode, hydrogen is oxidised and loses electrons to produce H+ ions.
• The H+ ions in the electrolyte move to the positive cathode.
• At the positive cathode, oxygen is reduced and gains electrons from the cathode. The oxygen reacts with H+ ions to make water.
• The electrons flow through an external circuit from the anode to the cathode. This is the electric current.

Question 27

What are the advantages and disadvantages of using hydrogen-oxygen fuel cells in vehicles?

Answer 27

+ Don’t produce as many pollutants as other fuels as the only by-products are water and heat.
+ Batteries in electric vehicles are rechargeable, but there’s a limit to how many times they can recharged before they need replacing. Batteries are more expensive to make them fuel cells.
+ Fuel cells store more energy, so it wouldn’t have to be recharged as often as batteries.

• Hydrogen is a gas, so it takes up lots more space to store than a rechargeable battery.
• Hydrogen is explosive when mixed with air so it’s hard to store safely.
• The hydrogen fuel is often made either from hydrocarbons or by electrolysis of water, which uses electricity. Both methods require burning fossil fuels, which releases lots of greenhouse gases.