Module 3

Lesson 4 – Calorimetry and enthalpy change of combustion

Objectives

MustFollow a set of instructions to set up practical equipment safely and quicklyShouldUse the equipment provided to investigate the heat given out when a substance burnsCouldCalulate a value for ΔHc.

Equation for ΔHcBalance the following equations for burning methanol and ethanol in air and add the correct state symbols for the substances in their standard states:

__CH3OH + __O2 __CO2 + __H2O

__C2H5OH + __O2 __CO2 + __H2O

In an equation to represent ΔHc you must not put a balancing number in front of the substance being burnt. If you do, then more than 1 mol would have been combusted.

AfL – what are standard conditions?

The correct equations

CH3OH(l) + 1½O2(g) CO2(g) + 2H2O(l)

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)

Calorimetry

How does calorimetry work?

• A known volume of cold water is measured into the beaker/can

• The starting temperature of the water is recorded

• The water is heated using the flame from the burning fuel

• The final temperature of the water is recorded• The spirit burner containing the fuel is weighed

before and after the experiment.

Practical

• Follow the instructions on Activity sheet 26 to compare the enthalpy of combustion of methanol and ethanol.

Calculations

The amount of energy transferred from the burning fuel to the water (the surroundings) in the calorimeter can be calculated if you know:• the mass of water heated• the temperature rise

AfL – can you remember the equation required?

Calculations

ΔE = mcΔT

Energy transferred (joules, J) = mass of water heated × 4.2 × temperature rise

Use this equation to calculate the energy transferred for each of your experiments.

Energy per gram of fuel

• You can compare fuels by measuring the mass of fuel burned in the experiment. The best fuel is likely to release the most energy per gram of fuel. This is worked out using:

Energy released (J/g of fuel) = Energy transferred to water (J) ÷ Mass of fuel burned (g)

Errors

The figure you have calculated is lower than the figure found using bond energies (remember breaking and making bonds).

Why do you think this could be?

Errors

Why do experimental values for ΔHc deviate from their theoretical counterparts.

• Energy lost to the surroundings• Incomplete combustion• Conditions which are not standard (ie not

298K and 1atm pressure)

Examination questions (AS)

Mark scheme

Examination question

Mark scheme

Homework

• Complete the practical write up and examination paper questions.

• Short test next lesson on enthalpy changes – learn your definitions. YOU MUST DO THIS OR YOU WILL NOT UNDERSTAND THE NEXT BIT!