Energy changes - chemsistry

Question 1

What does it mean if the products of a reaction store more energy than the original reactants

Answer 1

They must have taken in the difference in energy between the products and reactants from the surroundings during this reaction

Question 2

What does it mean if the products of a reaction store less energy than the original reactants

Answer 2

The excess energy was transferred from the surroundings during the reaction

Question 3

Why does overall amount of energy in a reaction not change

Answer 3

Because energy is conserved in reactions - it can’t be created or destroyed, only moved around.

Question 4

Exothermic reaction

Answer 4

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

Question 5

Example of exothermic reaction

Answer 5

• Combustion
• Neutralisation reactions
• Oxidation reactions

Question 6

Every day uses of exothermic reactions

Answer 6

• Hand warmers use the exothermic oxidation 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 7

Endothermic reaction

Answer 7

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

Question 8

Example of endothermic reactions

Answer 8

• Citric acid and sodium hydrogen carbonate
• Thermal decoposition

Question 9

Every day uses of endothermic reactions

Answer 9

Used in some sport injury packs

Question 10

Practical to test the effect of acid concentration on the energy released in a neutralisation reaction

Answer 10

Put 25cm cubed of 0.25 mol/dm cubed of hydrochloric acid and sodium hydroxide in separate beakers
Place the beakers in a water bath set to 25 degrees celcius until they are both at the same temperature (25 degrees)
Add the HCl followed by the NaOH to a polystyrene cup with a lid
Take the temp of the mixture every 30 seconds, and record the highest temperature
Repeat steps using a higher concentration of HCl

Question 11

Problem with energy measurements and how to reduce it

Answer 11

You cannot control the amount of energy lost to the surroundings so to reduce this problem you can put a polystyrene cup into a beaker of cotton wool to give it more insulation and putting a lid on the cup to reduce energy lost by evaporation

Question 12

What must be supplied to break bonds

Answer 12

Energy

Question 13

What process is the breaking of bonds

Answer 13

Endothermic process

Question 14

What process is the formation

Answer 14

Exothermic process

Question 15

In exothermic reactions the energy released by forming bonds is

Answer 15

Greater than the energy used to break them

Question 16

In endothermic reactions the energy used to break bonds

Answer 16

Is greater than the energy released by forming them

Question 17

Electrochemical cell

Answer 17

A basic system made up of two different electrodes in contact with an electrolyte. The two electrodes must be able to conduct electricity and so are usually metals

Question 18

Electrolyte in electrochemical cells

Answer 18

A liquid that contains ions which react with the electrodes . The chemical reaction between the electrodes and electrolyte set up a charge difference between the electrodes.

Question 19

Why are electrodes connected by a wire in electrochemical cells

Answer 19

The charge set up by the chemical reaction between the electrolyte and the electrode is able to flow and electricity is produced. A voltmeter can also be connected to the circuit to measure the voltage of the cell.

Question 20

What factors does the voltage of a cell depend on

Answer 20

• Different metals will react different.y with the same electrolyte so the type of electrodes used will affect the voltage of a cell
• The bugger the difference in reactivity of the electrodes, the bigger the voltage of the cell
• The electrolyte used in a cell since different ions i solution will react differently with the metal electrode
• More cells the bigger the overall voltage

Question 21

How do reactants in non-rechargeable batteries get used up

Answer 21

The chemical reaction is irreversible sp over time the reacting particles (the ions in the electrolyte and the metal ions in the electrode) get used up and turn into the products of the reaction. Once any one of the reactants is used up the reactuon can’t happen and so no electricty ios produced

Question 22

Why are rechargeable cells rechargeable

Answer 22

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

Question 23

Fuel cell

Answer 23

A fuel cell is an electrical cell that’s supplied with fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently

Question 24

How do fuel cells work

Answer 24

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

Question 25

What do hydrogen fuel cells produce

Answer 25

Clean water and release energy

Question 26

What reaction do hydrogen fuel cells involve

Answer 26

A redox reaction

Question 27

What electrolyte and electrodes are often used in hydrogen-oxygen fuel cells

Answer 27

Often an acid, such as phosphoric acid. The electrodes are often porous carbon with catalyst

Question 28

Where do hydrogen and oxygen enter in fuel cells

Answer 28

Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment

Question 29

In fuel cells what is the positive electrode and what is the negative electrode

Answer 29

Cathode - positive
Anode - negative

Question 30

What happens at the anode in fuel cells

Answer 30

At the anode hydrogen loses electrons to produce H+ ions. This is oxidation. H+ ions in the electrolyte move to the cathode

Question 31

What happens at the cathode in fuel cells

Answer 31

At the cathode oxygen gains electrons from the cathode and reacts with H+ to make water. This is reduction

Question 32

Where do electrons flow in a hydrogen cell

Answer 32

Through an external circuit from the anode to the cathode - this is the electric current

Question 33

Half eauation for anode in hydrogen-oxygen fuel cells

Answer 33

H2 ————> 2H+ + 2e-

Question 34

Half equation for cathode in hydrogen-oxygen fuel cells

Answer 34

2H2 + 02 ——–> 2H20

Question 35

Advantages of hydrogen-fuel cells

Answer 35

• don’t produce as many pollutants as other fuels - no greenhouse gases, nitrogen oxides ect.
• Batteries are more expensive
• Batteries store less energy than fuel cells and so would need to be charged more often

Question 36

Disadvantages of hydrogen fuel cells

Answer 36

• hydrogen is a gas so it takes up loads more space to store
• 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