5.2.3 - calorimity

Question 1

what is calorimetry

Answer 1

an experimental method used to measure an enthalpy change

Question 2

in an exothermic reaction (calorimity)

Answer 2

1) the reaction releases heat energy
2) that heat energy is absorbed by the nearby water
3) the temperature of the water increases

Question 3

what formulae are used to convert measurements from calorimetry into ΔH

Answer 3

Q = mcΔT (heat energy = mass of water x specific heat capacity of water x temp change)

ΔH = -Q/moles (enthalpy change = -heat energy/moles)

Question 4

what is the value of c (specific heat capacity of water)

Answer 4

ALWAYS 4.18 J/g/degrees

(will be given it when needed to use

Question 5

combustion calorimetry:

Answer 5

involves combusting a fuel to heat up the water

for example, to measure ΔH for the combustion of the liquid fuel methanol (CH3OH) you would combust some methanol using a burner underneath a can of water:
method:
1) weigh out 50g of water and place in can above the burner
2) measure the initial mass of the burner and the initial temperature of the water
3) ignite the burner, stir water for 30s, then extinguish the burner
4) measure the final mass of the burner and the final (maximum) temperature of the water

measurements (fake):
- mass of water = 50g
- initial temp of water = 25 degrees
- final temp of water = 72 degrees
- initial mass of methanol burner = 37.699g
- final mass of methanol burner = 37.375g

use Q=mcΔT to calculate how much heat energy has been absorbed by the water

Q = 50 x 4.18 x (72-25) = 9823J

then use ΔH = -Q/moles
-> need to convert Q into kJ (9823/1000 = 9.823 kJ)
-> need to calculate moles of methanol (moles = mass/RFM = (37.699-37.375)/32)

ΔH = -9.823/0.01 = -970 kJ/mol

Question 6

what are two significant sources of error in combustion calorimetry

Answer 6

• heat loss - most of the heat dissipates into the air instead of the water
• incomplete combustion - this releases less heat energy than expected

Question 7

solution calorimetry:

Answer 7

sometimes the reaction you are measuring happens dissolved in some water

for example, to measure ΔH for the displacement reaction between Mg(s) and CuSO4(aq):

method:
1) pipette 25cm^3 of CuSO4(aq) into a polystyrene cup
1cm^3 solution has a mass of 1g
2) measure the initial temperature of the solution
3) add 1g of Mg(s) and stir continuously
4) measure the maximum temp of the solution

measurements (fake):
- volume of solution = 25cm^3
- initial temperature of solution = 25 degrees
- maximum temp of solution = 47 degrees
- mass of Mg = 1g

use Q = mcΔT to calculate how much heat energy has been absorbed by the water

Q = 25 x 4.18 x (47-25) = 2299J

then use ΔH = -Q/moles
-> need to convert Q into kJ
-> need to calculate moles of Mg

ΔH = -2.299/0.04 =-55kJ/mol

Question 8

what are potential sources of error in solution calorimetry

Answer 8

• heat loss to surroundings
• incomplete reaction
• the water being heated is no longer pure water - it has things dissolved in it
  -> this means the specific heat capacity is no longer exactly 4.18

Question 9

endothermic reactions in calorimetry experiments

Answer 9

you can do calorimetry experiments for endothermic reactions in solution
the reaction absorbs heat energy from the water, so the water cools down

this results in a negative temp change for the water
the minus sign gets passed through the equations so the final answer is a plus

endothermic reactions have positive enthalpy changes