As Expt 2.1(4) - Enthalpy of Formation of CaCO3

AS Experiment 2.1(4)

DETERMINING THE ENTHALPY OF FORMATION OF CaCO3 INTRODUCTION The molar enthalpy of formation, Hf, is defined as the enthalpy change when 1 mole of a compound is formed directly from its elements. Although some compounds (e.g., MgO) can be formed by direct synthesis from the elements, many cannot. Calcium carbonate is one of them. Below is the chemical equation representing the formation of 1 mole of CaCO3:Ca (s) + C (s) +

1½ O2 (g) 

CaCO3 (s)

The enthalpy change for this reaction can be found indirectly from reactions which can actually be carried out in the lab and then applying Hess’s Law. Both calcium metal and calcium carbonate react with dilute hydrochloric acid to form a solution of calcium chloride. In addition, calcium metal forms H2 gas and calcium carbonate forms CO2 and H2O. The enthalpy changes for these two reactions can be measured relatively easily in the lab. For the calculation of Hf for CaCO3, two more pieces of data are needed, i.e., the enthalpies of formation of water and carbon dioxide. METHOD 1) H1 for Ca + HCl  Weigh out accurately about 1g of calcium metal into a weighing bottle and record the mass.  Weigh a polystyrene calorimeter cup and record the mass.  Use a measuring cylinder to add 100cm3 of 1M HCl.  Reweigh the cup with the acid and record the mass.  Place the polystyrene cup inside a beaker for support, as shown in the diagram opposite.  Place the thermometer in the acid and start the stop-clock. Record the temperature every minute for 3 minutes.  On the 4th minute, quickly add all the calcium metal and stir to mix thoroughly, keeping the lid on top of the cup.  Record the temperature every minute from the 5th minute for a further 10 minutes. Stir the solution before each reading.  Empty and rinse the cup and thermometer and dry them with paper towel. 2) H2 for CaCO3 + HCl  Weigh out accurately between 2 and 3g of calcium carbonate powder into a clean weighing bottle and record the mass.  Using the same polystyrene cup as before, add a fresh lot of 100cm3 of 1M HCl using the measuring cylinder, weigh again and record the mass.  Repeat the steps in experiment 1, recording the temperature every minute and adding the solid on the 4th minute.  Empty and rinse the cup and thermometer and place them on the wet tray. Clear away all the apparatus and leave your bench clean and tidy.

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AS Experiment 2.1(4) RESULTS EXPERIMENT 1 Mass of calcium metal

EXPERIMENT 2 Mass of calcium carbonate

Mass of empty cup

Mass of empty cup

Mass of cup plus acid

Mass of cup plus acid

Mass of acid 1 (M1)

Mass of acid 2 (M2)

TIME (minutes)

TEMPERATURE (OC) EXPERIMENT 1 EXPERIMENT 2

0 1 2 3 4

ADD THE SOLID

5 6 7 8 9 10 11 12 13 14 15

FINDING THE TEMPERATURE CHANGES Plot two separate graphs of temperature against time, one for each experiment. Use a large scale to enable you to find the temperature change more accurately. Extrapolate the points find out the instantaneous temperature change at the 4th minute in each reaction. Call these T1 and T2.

T1 = …………………… OC

T2 = …………………… OC

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AS Experiment 2.1(4) CALCULATING THE ENERGY CHANGES  Take the Specific Heat Capacity (c) for the solutions to be 4.18 J.g-1.OC-1  Take the mass of the solutions to be the mass of acid used in each experiment  Ignore the sign of the temperature change (rise or drop) – just use the numerical value

1) For Ca metal Heat change

= (mass of solution 1)  c  T1 = ……………………………………………………………………. J = ……………………… kJ

Moles of Ca

= mass of Ca  Ar (Ar for Ca = 40.1) = ………………………

Energy per mole = Heat change  number of moles = ………………………………………………………….… kJ.mol-1 Now decide whether the reaction was exothermic (temperature rise) or endothermic (temperature drop). Use the correct sign convention to write your value for the enthalpy change for this reaction below:H1 = ……………………………………….. kJ.mol-1

2) For CaCO3 Heat change

= (mass of solution 2)  c  T2 = ……………………………………………………………………. J = ……………………… kJ

Moles of CaCO3 = mass of CaCO3  Mr (Mr = 100.1) = ……………………… Energy per mole = Heat change  number of moles = ………………………………………………………….… kJ.mol-1 Now decide whether the reaction was exothermic or endothermic. Use the correct sign convention to write your value for the enthalpy change for this reaction below:H2 = ……………………………………….. kJ.mol-1

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AS Experiment 2.1(4) 3) Draw a Hess Law cycle incorporating your values for H1 and H2 and the enthalpies of formation for water and carbon dioxide to calculate the enthalpy of formation for CaCO3 H2 (g) + ½O2 (g) 

H2O (l)

Hf = -285.8 kJ.mol-1 (call this H3)

C (s) + O2 (g) 

CO2 (g)

Hf = -393.5 kJ.mol-1 (call this H4)

Ca (s) + C (s) +

1½ O2 (g) 

CaCO3 (s)

Hf for CaCO3 = …………………………… kJ.mol-1 4) Compare your calculated value for the standard enthalpy of formation of CaCO3 with the accepted data book value of -1206.9 kJ.mol-1. Work out your experimental error and estimate the apparatus errors to see how accurate your experiment was. What are the main sources of error (other than apparatus errors) in your method? ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. …………………………………………………………………………………………….

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