Energetics (3.1.4)

Question 1

what can reactions be?

Answer 1

endothermic or exothermic

Question 2

what is enthalpy change (△H)?

Answer 2

the change in heat energy measured under conditions of constant pressure

Question 3

what is standard pressure?

Answer 3

100 kPa

Question 4

what is standard temperature?

Answer 4

the stated temperature, but usually 298 K

Question 5

what is standard state?

Answer 5

the normal physical form of a substance under standard conditions eg. for carbon it is graphite

Question 6

what is standard enthalpy change (∆HƟ)?

Answer 6

the change in enthalpy measured under standard conditions with all substances being in their standard states

Question 7

what is the standard enthalpy of combustion (∆c H Ɵ)?

Answer 7

the enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions with all substances being in their standard state

Question 8

what is the balanced equation for the combustion of butanethiol (C4H9SH) where the enthalpy change is the standard enthalpy of combustion?

Answer 8

C4H9SH (l) + 7 1/2 O2 (g) -> 4 CO2 (g) + 5 H2O (g) + SO2 (g)

Question 9

what do you need to remember for balancing equations for standard enthalpy of combustion?

Answer 9

there must only be one mole of the substance reacting with oxygen, so other balancing numbers may involve fractions (balance normally and then divide)

Question 10

what is the standard enthalpy of formation (∆f H Ɵ )?

Answer 10

the enthalpy change when 1 mole of a substance is made from its elements under standard conditions with all substances being in their standard state

Question 11

what is the balanced equation for the formation of butanethiol (C4H9SH) where the enthalpy change is the standard enthalpy of formation?

Answer 11

4 C (s) + 5 H2 (g) + S (s) -> C4H9SH (l)

Question 12

what do you need to remember for balancing equations for standard enthalpy of formation?

Answer 12

the substance being formed should be the only product and there must only be one mole of it formed

Question 13

how do you calculate the enthalpy change in a reaction?

Answer 13

q = m x c x ∆T
q = heat energy change in that specific process (J)
m = mass of substance that is changing temperature (g)
c= specific heat capacity of substance changing temperature
∆T = change in temperature (degrees celsius or Kelvin)

Question 14

why isn’t q the same as ∆ H?

Answer 14

q is the enthalpy change for the amount of substance used, while ∆ H is the molar enthalpy change

Question 15

how do you convert from q to ∆ H?

Answer 15

∆ H = q/ moles (of substance providing energy) x 1000

Question 16

what do you need to remember when writing the final value for ∆ H?

Answer 16

the sign will be the opposite of whatever the sign for q was
ie. for an exothermic reaction ∆ T and therefore q will be positive, so ∆ H will be negative

Question 17

what does Hess’ Law state?

Answer 17

the energy change for a process is independent of the route taken - it only depends on the initial and final state

Question 18

how can you use Hess’ Law to find enthalpy changes for reactions?

Answer 18

1. write the chemical equation for which you are trying to find ∆ H
2. for each substance in that equation, identify what other information is provided (standard enthalpy of combustion or formation ie. is it being used or formed?) and add the appropriate arrows
3. fill in the second line (products of combustion or original elements)
4. label the arrow with the appropriate value from the data
5. check the balancing (do number of moles match data provided) and multiply up as appropriate
6. check whether the arrows are being followed in the same or opposite direction and show addition (same) or subtraction (opposite) as appropriate
7. do the sum

Question 19

how do you calculate ∆ H when given ∆ H f data?

Answer 19

𝛴 ∆ H f (products) - 𝛴 ∆ H f (reactants)

Question 20

how do you calculate ∆ H when given ∆ H c data?

Answer 20

𝛴 ∆ H c (reactants) - 𝛴 ∆ H c (products)