3.1.4 Energetics review Calorimetry calculation A student carried out a laboratory experiment to determine the enthalpy change when a sample of ethanol was burned. The heat produced was used to heat some water in a copper calorimeter. The student found that the temperature of 100 g of water increased by 8.75 °C when 0.150g of pure ethanol was burned in air. Use the student’s results to calculate a value, in kJ mol–1, for the enthalpy change when one mole of ethanol is burned. (The specific heat capacity of water is 4.18 J K–1 g–1)

3.1.4 Energetics review Mean bond enthalpies calculation Hydrogen reacts with nitrogen in the following equation: 1.5 H2 (g) + 0.5 N2 (g) --> NH3 (g) The enthalpy change for the reaction is -38 kJ mol-1 Calculate the mean bond enthalpy for the N-H bond.

3.1.4 Energetics review Hess’ law calculation

3.1.4 Energetics review LO- Review the energetics topic. There are lots of calculations as part of this topic but also some written answers and equations too. A) Questions requiring written answers 1) Definitions are potentially easy marks as they do not require the same higher level thinking that many questions will. Term Definition Enthalpy change Mean bond enthalpy Standard enthalpy of formation Standard enthalpy of combustion Hess’ law 2) A student carried out a calorimetry experiment but found the value from the Data Book showed the reaction to be more exothermic. Give two reasons for the difference. 3) A student calculated the enthalpy change for a reaction using mean bond enthalpies. The student found the value calculated from mean bond enthalpies was different to the value calculated experimentally. Explain why.

B) Equations Write equations for the complete combustion of the following substances. i) propanone- C3H6O ii) hexan-1-ol- C6H13OH For the following, write equations that represent the formation of one mole of different compounds from their constituent elements with all reactants and products in their standard states under standard conditions. i) gaseous C2H6 ii) solid AlCl3 iii) gaseous BrF3 iv) Why are the standard enthalpies of formation of Br2 (l) and F2 (g) zero? v) Why is the standard enthalpy of formation of Br2 (g) positive?

Energetics calculations Mean bond enthalpies calculations Calorimetry calculations Hess’ law calculations

A) Mean bond enthalpies calculations 1) Calculate the standard enthalpy change of combustion of ethanol, given by the equation: C2H5OH + 3O2 --> 2CO2 + 3H2O 2) Hydrazine has the structure below. When hydrazine reacts with hydrogen peroxide it forms nitrogen and water. The enthalpy change for the reaction is -789kJ mol-1. Use the mean bond enthalpy data to calculate the bond enthalpy value of the N≡N bond found in nitrogen. Bond N–H N–N N≡N H–O O–O Mean bond enthalpy/kJ mol–1 388 163 ? 463 146

B) Calorimetry calculations 1) A student carried out a simple laboratory experiment to measure the complete combustion of ethanol. The student showed that the temperature of 150 g of water increased by 9.0 °C when 0.51 g of pure ethanol was burned in air and the heat produced was used to warm the water. Use these results to calculate the value, in kJ mol–1, obtained by the student for this enthalpy change. (The specific heat capacity of water is 4.18 J K–1 g–1) Give one reason, other than heat loss, why the value obtained from the student’s results is less exothermic than a data book value. 2) In an experiment, an excess of solid calcium carbonate was added to 100 cm3 of 2.0 mol dm–3 hydrochloric acid. The equation is: CaCO3 + 2HCl --> CaCl2 + CO2 + H2O The initial temperature of the solution was 21 °C. After reaction, the temperature had risen to 53 °C. (The specific heat capacity of water is 4.2 J K–1 g–1) A) Use this information to calculate the enthalpy change for the reaction of one mole of calcium carbonate with hydrochloric acid. For your calculation you should assume that all the heat from the reaction is used to raise the temperature of 100 g of water.

2C(s) + 3H2(g) + 0.5O2 (g) → C2H5OH(l) Hess’ law calculations 1) Strontium is extracted from strontium oxide (SrO) by heating a mixture of powdered strontium oxide and powdered aluminium. Consider these standard enthalpies of formation. Use these data and the equation to calculate the standard enthalpy change for this extraction of strontium. 2) Calculate a value for the standard enthalpy of formation for liquid ethanol, C2H5OH. Use the equation given below and enthalpy of combustion data from the following table.   Substance C2H5OH(l) C(s) H2(g) ΔHc  / kJ mol–1 –1370 –394 –286 2C(s) + 3H2(g) + 0.5O2 (g) → C2H5OH(l)

1) -1279 kJ mol-1 2) 944 kJ mol-1 B) 1) 509 kJ mol-1 2) 133.8 kJ mol-1 C) 1) 101 kJ mol-1 2) -276 kJ mol-1