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Enthalpy Calculations
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Kaupapa / Learning Outcome
At the end of todays lesson I will be able to use stoichiometry to calculate heats of reactions for known masses
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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
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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
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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 = gmol-1)
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Fe Fe2O3 ratio 2 1 n 2.5 mol 1.25 mol m 140g 200g Mr 55.85g mol-1 160g mol-1
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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
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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…
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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
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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.
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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
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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 * mol ΔE= -247 kJ
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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
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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= mol m= n x Mr m= mol x 40.1g mol-1 m= 3.66g
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