Topic 5 Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

• in an exothermic reaction - heat is given out
• transfers energy to the surroundings - usually by heating - shown by a rise in temperature

Question 2

Examples of exothermic reactions?

Answer 2

• burning fuels - combustion.
• neutralisation reactions.
• many oxidation reactions.

Question 3

Every-day uses of exothermic reactions?

Answer 3

• some hand warmers use exothermic reactions of iron in air
• self heating cans also rely on exothermic reactions

Question 4

What is an endothermic reaction?

Answer 4

• in an endothermic reaction - heat is taken in
• takes in energy from the surroundings - shown by a fall in temperature

Question 5

Which reaction is more common?

Answer 5

exothermic reactions are much more common than endothermic reactions

Question 6

Examples of endothermic reactions?

Answer 6

• reaction between citric acid and sodium hydrogencarbonate
• thermal decomposition. E.g. heating calcium carbonate causes it to decompose into calcium oxide (quicklime) and carbon dioxide

Question 7

Every-day uses of endothermic reactions?

Answer 7

sports injury packs

Question 8

Method to measure energy transfer based on acid concentration?

Answer 8

1. Put 25cm of 0.25mol/dm of HCL and sodium hydroxide in separate beakers.
2. Place the beakers in a water bath set to 25 degrees celcius until they are both the same temperature.
3. Add the HCL followed by the NaOH to a polystyrene with a lid.
4. Take the temperature of the mixture every 30 seconds, and record the highest temperature.
5. Repeat steps 1-4 using 0.5 mol/dm and then 1 mol/dm of HCL.

Question 9

How would you improve the energy transfer method?

Answer 9

• reduce energy lost to surroundings by putting polystyrene cup into beaker of cotton wool - more insulation
• place a lid on top to reduce energy lost by evaporation

Question 10

Describe reaction profile of an exothermic reaction?

Answer 10

• products are at a lower energy than the reactants

Question 11

Describe reaction profile of an endothermic reaction?

Answer 11

• products are at a higher energy than the reactants

Question 12

What is activation energy?

Answer 12

The minimum amount of energy the reactants need to collide with each other and react

Question 13

Is bond breaking exothermic or endothermic?

Answer 13

• energy must be supplied to break bonds - bond breaking is an endothermic process

Question 14

Is bond forming exothermic or endothermic?

Answer 14

• energy is released when new bonds are formed - bond formation is an exothermic process

Question 15

Describe exothermic reactions in terms of bonds?

Answer 15

• in exothermic reactions energy released by forming bonds is greater than energy used to break them

Question 16

Describe endothermic reactions in terms of bonds?

Answer 16

• in endothermic reactions energy used to break bonds is greater than energy released by forming them

Question 17

What is an electrochemical cell?

Answer 17

• basic system made up of two different electrodes in contact with an electrolyte
• generates electrical energy from the chemical reactions occuring within in it

Question 18

How does an electrochemical cell work?

Answer 18

• formed from two electrodes in contact with each other via a wire and an electrolyte
• electrolyte is a liquid that contains ions which react with the electrodes
• chemical reactions between electrodes and electrolyte set up charge difference between the electrodes
• wire allows the charge to flow and electricity is produced

Question 19

What affects the voltage of a cell?

Answer 19

• type of electrodes used - bigger the difference in reactivity between the electrodes - bigger voltage of cell
• electrolyte used in a cell - different ions in solultion react differently with the metal electrodes

Question 20

If the metal used for electrode A is less reactive than the other metal electrode, will the voltage be positive or negative?

Answer 20

Positive

Question 21

If the metal used for electrode A is more reactive than the other metal electrode, will the voltage be positive or negative?

Answer 21

Negative

Question 22

Why do non-rechargeable batteries run out?

Answer 22

• chemical reactants are fully used up.
• reactions that happen at the electrodes are irreversible
• over time the reacting particles (the ions in the electrolyte and the metal ions on the electrode) get used up and turned into products of reaction
• once any of the reactants has been used up - reaction can’t happen so no electricity is produced

Question 23

Are alkaline batteries rechargeable or non-rechargeable?

Answer 23

Non rechargeable

Question 24

What are rechargeable batteries?

Answer 24

• they are batteries and cells that can be recharged - chemical reactions are reversed when an external electrical current is supplied

Question 25

What is a fuel cell?

Answer 25

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

Question 26

How is a potential difference set up in a fuel cell?

Answer 26

- anode becomes electron-rich due to the fuel being oxidised, and the cathode becomes electron-deficient due to it reducing oxygen - difference in electron concentration between the electrodes sets up a potential difference

Question 27

How does a hydrogen-oxygen fuel cell work?

Answer 27

- Hydrogen goes into anode compartment and oxygen goes into cathode compartment - at anode - hydrogen becomes oxidised + loses electrons to produce H+ ions - electrons go through external circuit to cathode - hydrogen ions pass through electrolyte to cathode - allows oxygen to be reduced, gaining electrons supplied from anode through the external circuit and react with the H+ ions from the electrolyte to form water - electrons flowing through external circuit from anode to cathode is the electric current

Question 28

What is the half equation at the anode?

Answer 28

H2 ---> 2H+ + 2e-

Question 29

What is the half equation at the cathode?

Answer 29

02 + 4H+ + 4e- ---> 2H2O

Question 30

Overall reaction of a hydrogen-oxygen fuel cell?

Answer 30

2H2 + O2 --> 2H2O

Question 31

Advantages of hydrogen-oxygen fuel cells?

Answer 31

- they do not produce as many pollutants - no greenhouse gases. - fuel cells are less expensive than batteries - last longer than batteries - less pollutants to dispose of - fuel cells store more energy than than batteries - no power is lost in transmission

Question 32

Disadvantages of hydrogen-oxygen fuel cells?

Answer 32

- hydrogen is a gas, so it takes up more space to store than a rechargeable battery - hydrogen is explosive when mixed with air so its hard to store safely - hydrogen is often obtained by methods that involve the combustion of fossil fuels