Required Practical 2 - Measurement of an enthalpy change

What equation do you use to calculate the energy change? Include units.

Q (j) = m(g)c delta T(k)

What is the calorimetric method?

A type of experiment in which substances are mixed in an insulated container and the temperature rise is measured when the substance reacts

Describe the general method of calorimetric method

Washes the equipment with the solutions to be used

Dry cup after walking

Put polysteryene cup in beaker for insulation and support

Measure out desired volumes with volumetric pipettes and transfer to a cup.

Clamp themometer into place

Measure initial temperature every minuite.

At minuite 3 add the second reagent to cup.

(If using a solid, use a before and after weighing method)

Stir solution- ensures all of the solution is at the same temperature

How do you obtain the exact remperature rise if the reaction is slow?

Take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.

Why do you also take the temperature of the reactants for a few minuites before they are added together?

To get a better average temperature.

What are the possible errors in this method?

Energy transfer from surroundings

Method assumes all solutions have the heat capacity of water

ignoring any heat absorbed by the apparatus

Density of solution is taken to be the same as water.

What would you do when finding out the energy change after doing q=mcdelta T

Divide by the moles of the reactant not in excess.

Describe the method for measuring the enthalpy change of solution of anhydrous copper (II) sulfate

Weigh out anhydrous copper(II) sulfate in a dry weighing bottle

Placse 25cc of deionised water into a polystyrene cup and record its temperature at the beginning. Start the timer and record the temperature every minute, stirring the liquid.

At the 4th minute, add the powdered anhydrous copper (II) sulfate to the water and continue to stir.

Reweigh the empty weighing bottle

At the 5th minuite (and for every minuite up to 15 mins) stir and record the temperature of the solution in the polystyrene cup.

Plot a graph of temperature against time. Draw two separate best-fit lines. Extrapolate to find the temperature change at the 4th minute. Use q=mcdeltaT

How would you measure enthalpied of combustion using flame calorimetry? What would you measure?

The fuel is burnt and the flame is used to heat up water in a metal cup. You need to measure the volume of water n the cup, the mass of the spirit burner before and after and the temperature change of water.

What are the errors in this method?

• energy transfer from surroundings (usually loss)

Incomplete combustion

Incomplete transfer of energy

Evaporation of fuel after weighing.

• approximation in specific heat capacity of solution. The method assumes all

solutions have the heat capacity of water.

• neglecting the specific heat capacity of the calorimeter- we ignore any

energy absorbed by the apparatus.

• reaction or dissolving may be incomplete or slow.

• Density of solution is taken to be the same as water.