RP2 - Measurement of enthalpy change

Question 1

What is the equation to find energy change?

Answer 1

q = mcΔT

Question 2

Describe the method to determine the enthalpy of combustion for a fuel like methanol

Answer 2

Measure 100cm^3 of water unto a calorimeter
Weight the mass of a spirit burner using a balance
Measure initial temp of water with thermometer
Ignite spirit burner to heat water
Stop heating when reasonable temp rise, stir and measure final temp
Reweigh spirit burner
Calculate temp change T2-T1 and energy change using q=mcΔT
Calculate mass of fuel used by subtraction and calculate moles used
Use energy change/moles to find enthalpy change

Question 3

Why may enthalpy of combustion values be less exothermic experimentally than in data books?

Answer 3

Heat loss to surroundings

Evaporation of water

Incomplete fuel combustuion

Question 4

Describe a method to find the enthalpy of neutralisation of HCl

Answer 4

Place polystyrene cup into glass beaker for support
Rinse measuring cylinder with HCl then add 25cm^3 of 1.0 moldm^3 HCl to polystyrene cup
Stir acid with thermometer and record initial temp
Rinse second cylinder with 1.0 mol dm^3 of NaOH and add 25cm^3 to the acid and stir
Record highest temperature reached
Calculate temp change using ΔT = ΔT2 - ΔT1 and then energy change with q = mcΔT
Calculate moles used and determine enthalpy of neutralisation

Question 5

How can accuracy of these experiments be improved?

Answer 5

Use draught shield to reduce heat loss

Use lid on calorimeter to reduce heat loss

Minimise distance between flame and calorimeter

Burn in supply of pure oxygen to prevent incomplete combustion

Question 6

What is an alternative method to find maximal temperature using a graph?

Answer 6

Extrapolation of a graph to estimate maximum temp