C7

Question 1

What is a exothermic reaction?

Answer 1

An exothermic reaction is one which transfers energy to the surroundings, usually by heating. This is shown by a rise in temperature.

Question 2

Give 3 examples of exothermic reactions

Answer 2

• burning fuels - Combustion - this gives out a lot of energy - it’s very exothermic
• Neutralisation reactions ( acid + alkali )
• Many Oxidation reactions are exothermic - for e.g. adding sodium to water releases energy, so it must be exothermic

Question 3

Give some uses of exothermic reactions

Answer 3

• Some hand warmers use the exothermic reaction of iron in air to release energy
• Self heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals in their bases

Question 4

What is an endothermic reaction?

Answer 4

An Endothermic reaction is one which takes in energy from the surroundings. This is shown by a fall in temperature

Question 5

List some endothermic reactions

Answer 5

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

Question 6

Give a use of an endothermic reaction

Answer 6

Endothermic reactions are used in some sports injury packs - the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer.

Question 7

How can you measure the amount of energy transfer ( rp ) ?

Answer 7

1) Put 25 cm3 of 0.25 mol/dm3 of hydrochloric acid and sodium hydroxide in separate beakers.

2) Place the beakers in a water bath set to 25 °C until they are both at the same temperature (25
°C).
3) Add the HCI followed by the NaOH to a polystyrene cup with a lid and you could add cotton buds to help insultate so that energy doesn’t get lost to the surroundings

4) Take the temperature of the mixture every 30 seconds, and record the highest temperature.

5) Repeat steps 1-4 using 0.5 mol/dm3 and then 1 mol/dm3 of hydrochloric acid.

Question 8

What do reaction profiles show?

Answer 8

Energy change

Question 9

What happens in a exothermic reaction profile?

Answer 9

The products are at a lower energy than the reactants

Question 10

What does the difference in height show in a reaction profile?

Answer 10

It represents the overall energy change in the reaction

Question 11

Whatis the activation energy?

Answer 11

The activation energy is the minimum amount of energy the reactants meed to collide with each other and react

Question 12

What happens in a endothermic reaction profile?

Answer 12

The products are at a higher energy than the reactants

Question 13

Energy must be supplied to break…..

Answer 13

Bonds

Question 14

Energy must be supplied to break bonds so bond breaking is a …….. process

Answer 14

Endothermic

Question 15

Energy is released when new bonds are formed so bond formation is a …….. process

Answer 15

Exothermic

Question 16

In exothermic reactions the energy released by forming bonds is …….. than the energy used to break them

Answer 16

Greater

Question 17

In endothermic reactions the energy used to break bonds is …….. than the energy released by forming them

Answer 17

Greater

Question 18

What is a electrochemical cell?

Answer 18

Its a basic system made up of 2 different electrodes in contact with an electrolyte

Question 19

What is an electrolyte ?

Answer 19

It is a liquid that contains ions which react with the electrodes

Question 20

Chemical reaction in a cell produce…….

Answer 20

Electricity

Question 21

What can affect the voltage of a cell?

Answer 21

• The types of electrodes used will affect the voltage of the cell
• The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell
• The electrolyte used in a cell will also effect the size of voltage

Question 22

How is a battery formed?

Answer 22

A battery is formed by connecting 2 or more cells together in a series. The voltages of the cells in the battery are combined so there is a bigger voltage overall.

Question 23

What is a non-rechargeable battery?

Answer 23

It is where the reactants get used up

A non-rechargeable battery contains cells which use irreversible reactions. Once one of the reactants is used up, they wont produce any more charge and you have to replace them

Question 24

How do fuel cells produce electrical energy?

Answer 24

Fuel cells use fuel and oxygen to produce electrical energy

Question 25

What happens when fuel enters the cell in a fuel cell?

Answer 25

When fuel enters the cell it becomes oxidised (loses electron) and sets up a potential difference within the cell

Question 26

What is the hydrogen-oxygen fuel cell?

Answer 26

It is a fuel cell that combines hydrogen and oxygen to produce nice clean water and release energy

Question 27

How does the hydrogen-oxygen fuel cell work?

Answer 27

1) Hydrogen goes into the negative electrode compartament ( anode ) and oxygen goes into the positive compartament ( cathode )

2) At the negative electeode ( anode ), hydrogen loses electrons to produce H+ ions. This is oxidation. H2 —> 2H+ + 2e-

3) H+ ions in the electrolyte then move to the cathode

4) At the positive electrode ( cathode ), oxygen gains electrons from the cathode and reacts with the H+ ions ( from the electrolyte ) to make water. This is reduction
O2 + 4H+ +4e- —–> 2H2O

5) The overall reactions is hydrogen plus oxygen which gives water. 2H2 + O2 —–> 2H20

6) Then the H2O and heat goes out and the reaction starts again

Question 28

What are the advantages of fuel cells?

Answer 28

• They are very effiecient
• Environmetly friendly only by products is water and heat
• Fuel cells are cheaper than batteries in cars
• Batteries also store less energy than fuel cells

Question 29

What are the disadvantages of fuel cells?

Answer 29

-Hydrogen is a gas so it takes up loads 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 from hydrocarbons or by electrolisis of water which uses electricity and that has to be generated somehow usually fossil fuel