There are many different kinds of chemical reaction. One type of reaction is called thermal decomposition. In such a reaction, heat is used to break a compound down into simpler products. A compound that can undergo thermal decomposition must be heated to a definite temperature before the reaction becomes feasible, that is, before the reaction becomes possible. This temperature can be calculated using thermodynamic data. The underlying equation is ΔG = ΔH − TΔS where ΔG is the standard free energy change for the reaction and ΔH and ΔS are the enthalpy and entropy changes respectively for the reaction. For the reaction to be just feasible, ΔG must be zero so that ΔH = TΔS and T = ΔH / ΔS.
The reaction that we will study is the breakdown, by heating, of sodium hydrogencarbonate to sodium carbonate, carbon dioxide, and water vapour. Since a gas is given off, the progress of the reaction can be monitored using a gas syringe.
- Open Yenka file Model 1. The model shows a test tube, containing 0.5 g of sodium hydrogencarbonate, above a Bunsen burner and connected to a gas syringe. The progress of the reaction is monitored by two graphs. One of these shows the volume of gas produced by the reaction as a function of time. The other shows the temperature of the sodium hydrogencarbonate as a function of time. When carried out in the laboratory, the heating is usually done using a bath of heated liquid paraffin. Suggest the reason for this.
AnswerThis allows the heating to be carried out in a more controlled manner. A Bunsen burner would heat the compound too rapidly.
- Now light the Bunsen burner using the control on its right. Allow the flame to touch the bottom of the test tube. Then start the reaction using the pause control in the main toolbar. When the reaction starts, as shown by the gas volume (upper graph) rising from zero, stop the reaction using the pause control. Use the lower graph to read the temperature at which the reaction commenced. Read the maximum temperature at the same time as the reaction started. At what temperature does the reaction appear to start?
AnswerIt is about 100 °C.
- Write a balanced equation for the reaction.
Answer2NaHCO3 → Na2CO3 + CO2 + H2O
- For this reaction, ΔH = +129 kJ mol-1.
Why is the '+' sign appropriate in this case?AnswerThis reaction required heat to be supplied before it could take place. It is an endothermic reaction, so the '+' sign is appropriate.
- The standard entropies for the chemicals involved are 102.1, 136.0, 188.7, and 213.6 JK-1mol-1 (in the order in which they appear in the equation in A3).
Calculate the entropy change for the reaction.AnswerThe entropy change is found by adding up the entropies of the products (538.3 J) and those of the reactants (204.2 J - remember, there are 2 moles of NaHCO3), then subtracting the reactant figure from the product figure, to give +334.1 J.
- Why is it appropriate that the above figure has a '+' sign?
AnswerSince the reaction involves the conversion of a solid to a mixture of solid and gases, the amount of disorder, that is, entropy, increases.
- Calculate the minimum temperature at which this reaction becomes thermodynamically feasible.
AnswerFor the reaction to be just feasible, ΔG must be zero so that ΔH = TΔS and T = ΔH / ΔS. Therefore, T = 129 / 0.3341 = 386 K. This is converted to °C by subtracting 273 so that the reaction becomes feasible at about 113 °C.
- Calculate the maximum volume of CO2 that could be produced from 0.5 g of NaHCO3. Take the molar volume of CO2 as 24 litres.
AnswerThe equation for the reaction is
2NaHCO3 → Na2CO3 + CO2 + H2O
24 / 168 litre = 0.143 litre
The temperature at which a reaction becomes feasible can be calculated using thermodynamic data. The underlying equation is ΔG = ΔH − TΔS where ΔG is the standard free energy change for the reaction and ΔH and ΔS are the enthalpy and entropy changes respectively for the reaction. For the reaction to be just feasible, ΔG must be zero so that ΔH = TΔS and T = ΔH / ΔS. Care has to be taken with units, because enthalpy values are usually given as kJ mol−1 but entropy values are given as J K−1 mol−1 so that the value of ΔS must be divided by 1000 before being used in the above equation.
- The prerequisite knowledge for this activity is knowledge of the relationship between free energy, enthalpy and entropy changes and some experience of carrying out related calculations. This activity models the PPA (prescribed practical activity) on the thermal decomposition of sodium hydrogencarbonate, and the agreement between theory and model is reasonable. The activity could be used either as an introduction to the PPA, or as a revision aid prior to examinations.