energy changes (required practical)

Question 1

what has happened if the products of a reaction store more energy than the reactants?

Answer 1

energy has been taken in from the surroundings during the reaction.

Question 2

what has happened if the products of a reaction store less energy than the reactants?

Answer 2

energy has been released to the surroundings during the reaction.

Question 3

what is an exothermic reaction?

Answer 3

a reaction where heat is given out to the surroundings. such as combustion neutralisation and oxidation

Question 4

what is an endothermic reaction?

Answer 4

a reaction where heat is taken in form the surroundings. such as thermal decomposition

Question 5

how can you measure the amount of energy released by a chemical reaction? (required practical)

Answer 5

1. put 25cm³ of 0.25mol/dm³ hydrochloric acid and sodium hydroxide in separate beakers.
2. put both beakers in a water bath set to 25°c.
3. put a polystyrene cup in a beaker surrounded by cotton wool.
4. add the HCL and NaOH to the polystyrene cup and place a lid with a hole for a thermometer.
5. take the temperature of the mixture every 30 seconds.
6. repeat the experiment using 0.5mol/dm³ and 1mol/dm³ of hydrochloric acid.
   this test is for the effect of acid concentration on energy released however the experiment can be changed to investigate different variables

Question 6

what is a reaction profile?

Answer 6

a diagram that shows how the energy in a reaction changes over the course of the reaction.

Question 7

what causes changes in energy in a reaction?

Answer 7

bonds being broken. endothermic because energy is needed to break bonds
bonds being formed. exothermic because energy is released when bonds are formed

Question 8

what is an electrochemical cell?

Answer 8

a system of two electrodes touching an electrolyte.

Question 9

how does an electrochemical cell work?

Answer 9

1. 2 electrodes are in contact with an electrolyte that contains ions which react with the electrodes.
2. chemical reactions between the electrodes and electrolytes create a charge difference between electrodes.
3. if the electrodes are connected the charge is able to flow and electricity is produced.

Question 10

what can effect the voltage of a cell?

Answer 10

different electrode metals.
bigger difference in electrode metal reactivity. A and B are different
different electrolyte.

Question 11

why do non-rechargeable batteries get used up?

Answer 11

over time ions in the electrolyte run out. the products cannot be turned back to reactants

Question 12

what is a fuel cell?

Answer 12

an electrical cell supplied with a fuel and oxygen that uses energy from the reaction between them.

Question 13

how does a fuel cell work?

Answer 13

when fuel enters the cell it becomes oxidised by oxygen or air and creates a potential difference.

Question 14

what is a hydrogen fuel cell?

Answer 14

a fuel cell that combines hydrogen and oxygen to produce water and energy.

Question 15

how does a hydrogen fuel cell work?

Answer 15

1. hydrogen goes into the anode compartment and oxygen goes into the cathode compartment.
2. at the anode hydrogen loses electrons to produce H+ ions. oxidation
3. H+ ions in the electrolyte often an acid move to the cathode. electrolyte between electrodes
4. at the cathode oxygen gains electrons from the cathode and reacts with H+ ions to make water. reduction
5. the electrons from the electrodes flow through an external circuit from the anode to the cathode.
6. the overall reaction is 2H² + O² → 2H²O

Question 16

what are the advantages of hydrogen fuel cells?

Answer 16

don’t produce greenhouse gases. but batteries are polluting to dispose of
store more energy than batteries.

Question 17

what are the disadvantages of hydrogen fuel cells?

Answer 17

hydrogen is a gas and so takes up more space than a battery.
hydrogen is explosive and is hard to store safely.
hydrogen fuel is often made from hydrocarbons or electrolysis which uses electricity. polluting to generate