Enthalpy Calculations

Kaupapa / Learning Outcome At the end of todays lesson I will be able to use stoichiometry to calculate heats of reactions for known masses

Stoichiometry review Chemical equations give information about the chemicals involved and the amounts of those chemicals involved Example: 2SO2(g) + O2(g)  2SO3(g) This tells me that Sulfur Dioxide is reacting with Oxygen to form Sulfur Trioxide

2SO2(g) + O2(g)  2SO3(g) It also tell me the amount in moles that is reacting of each chemical 2 moles of SO2 reacts with 1 mole of O2 to form 2 moles of SO3

Question Fe2O3 + 3CO  2Fe + 3CO2 Q1. If 200g of Fe2O3 is reacted in the above equation what mass of Fe is produced? (Mr Fe2O3 = 160 gmol-1, Mr Fe = 55.85 gmol-1)

Fe Fe2O3 ratio 2 1 n 2.5 mol 1.25 mol m 140g 200g Mr 55.85g mol-1 160g mol-1

Thermochemical Equations These are balanced equations that include a ∆H value 2SO2(g) + O2(g)  2SO3(g) ∆rH = -188 kJ mol-1 This means that when 2 moles of SO2 reacts with 1 mole of O2 188 kJ of energy is released Because it is kJmol-1 the amount of energy is related to the number of moles in the reaction

Changing the amount of reactants 2SO2(g) + O2(g)  2SO3(g) ∆rH = -188 kJ.mol-1 If we halved the number of moles of SO2 then the energy released would halve as well SO2(g) + ½ O2(g)  SO3(g) ∆rH = -94 kJ.mol-1 If we doubled the amount of reactants the energy released would double etc…

Enthalpy change of a reaction Calculating ΔrH Energy per mol Number of moles ΔrH = ΔE / n Enthalpy change of a reaction Energy Change in kJ

Question Ethanol burns according to the equation: C2H5OH + 3O2  2CO2 + 3H2O ΔrH=-1370kJ mol-1 Calculate the amount of energy released when 9.2g of ethanol is burnt.

Step 1. Calculate the moles Mr(C2H5OH) =46g mol-1 n=m/Mr n=9 Step 1. Calculate the moles Mr(C2H5OH) =46g mol-1 n=m/Mr n=9.2g/46g mol-1 n= 0.200mol

ΔE= ΔrH x n ΔE= =-1370kJ * 0.200 mol ΔE= -247 kJ Step 2: Calculate the change in energy using the formula ΔrH = ΔE / n ΔE= ΔrH x n ΔE= =-1370kJ * 0.200 mol ΔE= -247 kJ

Question Calcium burns in oxygen to produce calcium oxide. The equation for this reaction is: 2Ca +O2  2CaO ΔrH=-1270 kJ mol-1 Calculate the mass of calcium that must react to release 116kJ of energy

n= ΔE / ΔrH n= -116kJ / -1270kJ mol-1 n= 0. 0913 mol m= n x Mr m= 0 n= ΔE / ΔrH n= -116kJ / -1270kJ mol-1 n= 0.0913 mol m= n x Mr m= 0.0913mol x 40.1g mol-1 m= 3.66g