Ch7 Energy Changes

Question 1

Endothermic reacitons

Answer 1

temperature decreases

energy is transferred from surroundings to the reacting chemicals

Question 2

Exothermic reactions

Answer 2

temperature increases

energy is transferred to the surroundings from the reacting chemicals

Question 3

5 Examples of exothermic reactions

Answer 3

neutralisation
precipitation
displacement reaction
oxidation
combustion

Question 4

3 examples of endothermic reactions

Answer 4

thermal decomposition
dissolving salts
reaction of citric acid and sodium hydrogencarbonate

Question 5

Exothermic bond energy

Answer 5

energy released by forming bonds is greater than the energy used to break the existing bonds

Question 6

Endothermic bond energy

Answer 6

energy released by forming bonds is less than the energy used to break the existing bonds

Question 7

Overall energy change

Answer 7

total energy needed to break bonds - total energy released by forming new bonds

Question 8

breaking bonds

Answer 8

energy must be supplied

Question 9

forming products

Answer 9

energy is released

Question 10

activation energy

Answer 10

minimum amount of energy needed by particles when they collide for a reaction to occur

from reactants energy to peak

Question 11

overall energy change

Answer 11

energy of reactants - energy of products

Question 12

Uses of exothermic reactions

Answer 12

self-heating cans

hand warmers

Question 13

Uses of endothermic reactions

Answer 13

sports instant ice packs

Question 14

How chemical cells work

Answer 14

two metals are joined together by a wire and dipped into an electrolyte
electrons flow through the wire
the greater the difference in reactivity between the two metals used, the higher the voltage produced

Question 15

Batteries

Answer 15

two or more cells connected together in series

provide a greater voltage

Question 16

Rechargeable battery

Answer 16

an external electrical current is supplied

reverses the chemical reactions at each electrode when the cell is discharging

Question 17

Alkaline batteries

Answer 17

non-rechargeable

chemical reactions stop when one reactant has been used up

Question 18

Fuel cells

Answer 18

supplied by an external source of fuel

fuel is oxidised electrochemically within the cell to produce a potential difference

Question 19

hydrogen fuel cell process

Answer 19

Hydrogen is supplied to the negative electrode
It diffuses through the graphite electrode and reacts with hydroxide ions to form water
provides a source of electrons to an external circuit
2H2(g) + 4OH-(aq) –> 4H2O(l) + 4e-

oxygen gas is supplied to the positive electrode
it diffuses through the graphite and reacts with the electrolyte to from hydroxide ions
it accepts electrons from the external circuit
O2(g) + 2H2O(l) + 4e- –> 4OH-

The electrons and OH- ions cancel out. Left with overall change of the oxidation of hydrogen
2H2(g) + O2(g) –> 2H2O(l)

Question 20

Hydrogen fuel cell advantages

Answer 20

doesn’t produce pollutants
alternative to conventional fossil fuels
don’t need to be electrically recharged
can be a range of sizes for different uses

Question 21

Hydrogen fuel cell disadvantages

Answer 21

hydrogen is highly flammable
hydrogen is produced from non-renewable sources
hydrogen is difficult to store