Determine the enthalpy change of the decomposition of sodium hydrogen carbonate by thermochemical measurement and the application of Hess' Law.

using two identical pieces of apparatus, for each separate enthlapy change, will reduce the

chances of contamination.

Provided

Dry sodium hydrogen carbonate

Dry anhydrous sodium carbonate

2.0mol.dm-3 hydrochloric acid (HCl)

Method

* Collect all apparatus to be used and rinse using distilled water to avoid any

contamination. The apparatus may not have been cleaned satisfactorily from earlier

experiments and traces of chemicals may still be present.

* Accurately record the mass of sodium hydrogen carbonate to be used. This is achieved

by weighing the container and the sample together (mass one), and then weighing the

container after the sample has been removed (mass two). Mass two subtracted from

mass one will give the accurate mass of sodium hydrogen carbonate to be used. This

mass should be approximately 3.0g.

* Measure 30cm3 of HCl using a biurette. The acid needs to be in excess as it is

important that all of the sample of sodium hydrogen carbonate reacts. A biurette is the

most accurate way of measuring this volume and, in order to keep the experiment 'fair'

all variables need to remain constant.

* Place the acid into a plastic cup, which should be supported by a large beaker. The

large beaker also adds insulation.

* Take the temperature of the acid, using the thermometer, every thirty seconds for three

minutes. Record these readings.

* At 3.5 minutes add the sodium hydrogen carbonate and stir, using the thermometer, to

make sure that it all reacts.

* Record the first temperature of the mixture at four minutes.

* Continue recording the temperature of the mixture every thirty seconds for a further six

minutes.

* We are now ready to test the second sample of sodium carbonate.

* Measure accurately, approximately 2.0g of the sodium carbonate in the same way as

we measured the sodium hydrogen carbonate.

* Measure a further 30cm3 of HCl and place into a plastic cup supported by a large

beaker.

* Repeat the above steps of taking the temperature of the HCl every thirty seconds for

three minutes, adding the sodium carbonate at 3.5 minutes and recording the first

temperature of the mixture at four minutes before continuing to record the temperature

every thirty seconds for a further six minutes. Record all temperature readings.

Suggested Table of Results

When carrying out the practical the results should be recorded in a table similar to the one

below:

Plotting the Graph

To determine the enthalpy change of the reactions we need the maximum temperature change.

Unfortunaetly, it is impossible to get this maximum/minimum temperature using the thermometer

alone. The temperature recorded at four minutes is not 100% accurate as some of the reaction

has already been taking place since 3.5 minutes.

We therefore need to etrapolate the graph. To do this we plot our results as normal

(temperature on the 'y' axis, time on the 'x' axis). We then draw a line of best fit along for the

points from four minutes onwards. Where this line meets the line of mixing (a vertical line drawn

from 3.5 minutes) we find the maximum/minimum temperature.

This temperature is used in determining the maximum temperature.

Calculations

To find maximum temperature change:

i) for the exothermic reaction: maximum (final) temperature - initial temperature

ii) endothermic reaction: initial temperature - minimum (final) temperature

We then calculate the heat change using the following equation:

Heat Change = mass.c.temperature change

Where: mass = mass of solution (1ml = 1g)

c = heat capacity of solution (4.20J.g-1K-1)

heat change = heat change determined using the etrapolated line of the graph

To find out the heat change for one mole of sample we multipy the heat change by one over the

amount of moles present.

i.e. Energy Change x (1/n) where n = number of moles

To convert into Kj we multiply the energy change for one mole by 1/1000