Calculating various enthalpy changes C2.1 Thermochemsitry 23 November 2018

State what is meant by the term standard conditions. Starter  is the symbol used to represent a reaction completed under standard conditions State what is meant by the term standard conditions.

Standard Enthalpy change of reaction Standard Enthalpy change of reaction (ΔrH ) is the enthalpy change when mole of a reactant is converted to products under standard conditions For these energy changes the reactions will always be stated. Worked example: Mg + 0.5O2  MgO ΔrH  = -602KJmol-1 Given the above equation and corresponding enthalpy change of reaction, determine ΔrH  values for the formation of 2, 10 and 20 moles of magnesium oxide.

Standard Enthalpy change of Combustion Standard Enthalpy change of combustion (ΔcH ) is the enthalpy change when one mole a substance reacts completely with oxygen under standard conditions Worked example: C4H10 + 6.5 O2  4CO2 + 5H2O ΔcH  = -2877 KJmol-1 Given the above equation, write equations for the combustion of propane and methylbutane, indicating the sign of the enthalpy change.

Knowledge check 1. Two enthalpy changes of reaction are shown below: N2(g) + 3H2(g)  2NH3 (g) ΔH = -92 kJmol-1 N2O4 (g)  2NO2 (g) ΔH = +58 kJmol-1 What is the enthalpy change of reaction for: ½N2(g) + 1½ H2(g)  NH3 (g) ½N2O4 (g)  NO2 (g) 2. Write equations for the change that occurs during the enthalpy change of combustion for each of the following compounds CH4(g) C3H8(g) CS2(l) CH3OH(l) C2H5OH(l)

Answers 1. What is the enthalpy change of reaction for: ½N2(g) + 1½ H2(g)  NH3 (g) ΔH = -46 kJmol-1 ½N2O4 (g)  NO2 (g) ΔH = +29 kJmol-1 2. Write equations for the change that occurs during the enthalpy change of combustion for each of the following compounds CH4(g) + 2O2(g) CO2(g) + 2H2O(l) C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) CS2(l) + 3O2(g)  CO2(g) + 2SO2(g) CH3OH(l) + 1½ O2(g) CO2(g) + 2H2O(l) C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)

Standard Enthalpy change of Formation Standard Enthalpy change of formation (ΔfH ) is the enthalpy change when one mole a substance is formed from its constituent elements in their standard states under standard conditions. For an element this value will always be 0! Worked examples: Write an equation for the reaction that occurs during the enthalpy change of formation of: Magnesium oxide Ethane

Knowledge check Write equations for the change that occurs during the enthalpy change of formation for each of the following compounds. C2H4(g) C3H8(g) C2H5OH(l) CaO(s) Al2O3(s)

Answers Write equations for the change that occurs during the enthalpy change of formation for each of the following compounds. 2C(s) + 2H2(g)  C2H4(g) 3C(s) + 4H2(g)  C3H8(g) 2C(s) + 3H2(g) + ½ O2(g)  C2H5OH(l) Ca(s) + ½ O2(g)  CaO(s) 2Al(s) + 1 ½ O2(g)  Al2O3(s)

Calculating enthalpy changes One way of calculating an enthalpy change is by using average bond enthalpies which we have seen previously. Enthalpy changes can also be measured directly using the following equation: q = m c T q is the symbol for heat energy and so gives the energy change. m is the mass of the liquid that changes temperature, usually water. c is the specific heat capacity which is the energy required to raise the temperature of a substance by 1 Kelvin T is the change in temperature during the reaction.

What is Kelvin? Kelvin is another scale used to measure temperature, similar to Celcius and Fahrenheit. The only difference with Kelvin is 0K means absolute zero energy. So the particle do not move or vibrate at all! 00C is 273K 250C is 298K

Worked Example Determine the enthalpy change of combustion of methanol. Mass of spirit burner and methanol before burning = 196.97g Mass of spirit burner and methanol after burning = 195.37 Mass of fuel burnt = 1.60g Volume of water = 150cm3 Initial temperature of water = 21.50C Final temperature of water = 62.50C For water, density = 1.00g cm-3, c = 4.18J g-1 K-1

Worked Example 2 An excess of zinc powder is added to 50cm3 of 1.00mol dm-3 copper (II) sulfate. The mixture is stirred until a maximum temperature is obtained. Find ΔrH for Zn + CuSO4  Cu + ZnSO4 Results: Initial temperature of solution = 22.5oC Final temperature of solution = 60.5oC For the solution, assume the density and specific heat capacity are the same as water (density = 1.00g cm-3, c = 4.18J g-1 K-1)

Knowledge Check Calculate the energy change, in KJ of the following: (density = 1.00g cm-3, c = 4.18J g-1 K-1) 50cm3 of water decreases in temperature from 620C to 190C 50cm3 of aqueous solution is mixed with 75cm3 of a second solution and the temperature increases from 23oC to 39oC. Combustion of 1.29g of C6H14 releases 34.2KJ of energy. Calculate ΔcH for one mole of C6H14. Combustion of 1.656g of ethanol, C2H5OH raised the temperature of 150g of water from 22.50C to 74.50C. Write an equation to represent the enthalpy change of combustion of ethanol. Calculate the enthalpy change of combustion of ethanol to three significant figures.