5. energy changes

Question 1

exothermic rections

Answer 1

• negative sign

- bond formation

Question 2

endothermic reactions

Answer 2

• positive sign

- bond breaking (energy absorbed to break bonds)

Question 3

enthalpy change

Answer 3

energy absorbed to break bonds (+ve) + energy released during bond formation (-ve)

Question 4

bond energy

Answer 4

• measure of strength of bond

- stronger bond, more energy released during formation, more energy absorbed during breaking

Question 5

energy profile diagrams

Answer 5

• exothermic: goes up (Ea) then goes down lower than before. overall enthalpy change is negative
• endothermic: goes up (Ea) then goes down but above level before. overall enthalpy change is positive

Question 6

energy level diagrams

Answer 6

• no x axis
• exothermic: goes up to intermediate level then lower than before
• endothermic: goes up to immediate level then lowers but higher than before

Question 7

exothermic processes

Answer 7

• chemical energy to other energy
• combustion
• respiration
• simple cells, fuel cells
• freezing, condensing

Question 8

endothermic processes

Answer 8

• other energy to chemical energy
• photosynthesis
• electrolysis
• melting, boiling, evaporation

Question 9

combustion of fuels

Answer 9

• fuels are substances which burn in air to produce energy

Question 10

fossil fuels

Answer 10

• most common form of fuels

- carbon containing compounds

Question 11

coal

Answer 11

• mainly made of carbon

- burns in excess oxygen (complete combustion) to form carbon dioxide and energy

Question 12

natural gas

Answer 12

• mainly made of methane

- burns in excess oxygen (complete combustion) to form carbon dioxide, water and energy

Question 13

equations for complete combustion

Answer 13

1. balance number of C with CO2
2. balance number of H with H2O
3. balance O2 last cos its in excess

Question 14

incomplete combustion

Answer 14

• occurs when there is insufficient oxygen

- produces carbon particles (soot) and carbon monoxide

Question 15

combustion reactions

Answer 15

• are all exothermic

Question 16

selecting fuels

Answer 16

• not every chemical that burns is a good fuel

Question 17

good fuels:

Answer 17

• can produce large amounts of energy when burnt
• safe and convenient to store
• reasonable costs and easily accessible
• does not produce poisonous/ pollutant gases

Question 18

hydrogen

Answer 18

• possible fuel
• can be obtained from the catalytic cracking of long chain hydrocarbons from petroleum or crude oil
• or obtained from electrolysis of water

Question 19

advantages of hydrogen

Answer 19

• renewable source if obtained from water

- pollutant free because the only product formed is water

Question 20

disadvantages

Answer 20

• hydrogen needs to be liquified for transport and storage which is costly
• catalytic cracking requires alot fo energy and heat, releasing greenhouse gases from combustion of fossil fuels etc.

Question 21

another alternative to obtaining useful energy from fuels

Answer 21

• fuel cells