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Mass of H2SO4 Density x volume 184gcm3 x 3cm3 552 g |
Mass of H2O Density x volume 100gcm3 x 100cm3 100 g |
Moles of H2SO4 mass 552 g 00563 moles |
Molar mass of H2SO4 98g |
Enthalpy change H mass of acid water x specific heat capacity of water x T 100 552 g x 42 x 13oC 5761392 J 5761392 kJ |
1000 |
Hs of H2SO4 H 5761392 kJ 10233378kJmoles1 |
Moles of H2SO4 00563 moles |
eStandard enthalpyheat of formation Hf |
The molar enthalpy of formation Hf is defined as the energy change when one mole of a compound is formed from its elements at 298K25oC and 101325Paone atmospherepressure Hf is practically difficult to determine in a school laboratory |
It is determined normally determined by applying Hess law of constant heat summation |
Hess law of constant heat summation states that the total enthalpyheatenergy change of a reaction is the same regardless of the route taken from reactants to products at the same temperature and pressure |
Hess law of constant heat summation is as a result of a series of experiments done by the German Scientist Henri Hess18021850 |
He found that the total energy change from the reactants to products was the same irrespective of the intermediate products between ie |
As H1Cs As H2BsH3Cs |
Applying Hess law of constant heat summation then |
As H2 Bs |
H1 H3 |
Cs |
The above is called an energy cycle diagram It can be used to calculate any of the missing energy changes since |
i H1 H2 H3 |
ii H2 H1 H3 |
iii H3 H1 H2 |
Examples of applying Hess law of constant heat summation |
1Calculate the molar enthalpy of formation of methane CH4 given that Hc of carbongraphite is 3935kJmole1Hydrogen is 2857 kJmole1 and that of methane is 890 kJmole1 |
Working |
Carbongraphite hydrogen and oxygen can react to first form methane |
Methane will then burn in the oxygen present to form carbonIVoxide and water Carbongraphite can burn in the oxygen to form carbonIVoxide |
Hydrogen can burn in the oxygen to form water |
Cs 2H2 g2O2 g H1 CH4g 2O2g H2 CO2g2H2Ol |
Cs 2H2 g2O2 g H3 CO2g2H2Ol |
Energy cycle diagram |
Cs 2H2 g 2O2g H1Hc 8904kJ CH4g2O2g |
H3Hc 3935kJ H3Hc 2857kJ x 2 H2 Hf x |
CO2g 2H2Ol |
Substituting |
H3 H1 H2 |
3935 2857 x 2 8904kJ x |
x 745 kJ |
Heat of formation Hf CH4 745 kJmole1 |
2 Calculate the molar enthalpy of formation of ethyne C2H2 given Hc of carbongraphite 394kJmole1Hydrogen 286 kJmole1 C2H2 1300 kJmole1 |
Working |
Carbongraphite hydrogen and oxygen can react to first form ethyne |
Ethyne will then burn in the oxygen present to form carbonIVoxide and water Carbongraphite can burn in the oxygen to form carbonIVoxide |
Hydrogen can burn in the oxygen to form water |
2Cs H2 g2 O2 g H1 C2 H2 g 2 O2g H2 CO2gH2Ol |
2Cs H2 g 2 O2 g H3 2CO2gH2Ol |
Energy cycle diagram |
2Cs H2 g 2O2g H1Hf x C2 H22O2g |
H3Hc 394kJx 2 H3Hc 286kJ H2 Hc 1300kJ |
2CO2g H2Ol |
Substituting |
H3 H1 H2 |
394 x 2 286 1300kJ x |
x 244 kJ |
Heat of formation Hf CH4 244 kJmole1 |
3 Calculate the molar enthalpy of formation of carbonIIoxide CO given Hc of carbongraphite 3935kJmole1 Hc of carbonIIoxide CO 283 kJmole1 |
Working |
Carbongraphite reacts with oxygen first to form carbon IIoxide CO |
CarbonIIoxide CO then burn in the excess oxygen to form carbonIVoxide Carbongraphite can burn in excess oxygen to form carbon IV oxide |
Cs O2 g H1 CO g O2g H2 CO2g |
Cs O2 g H3 CO2g |
Energy cycle diagram |
Cs O2g H1Hf x COO2g |
H3Hc 3935kJ H2 Hc 283kJ |
CO2g |
Substituting |
H3 H1 H2 |
3935kJ 283kJ x |
x 110 kJ |
Heat of formation Hf CO 110 kJmole1 |
4Study the information below |
H2g O2g H2Ol H1286 kJmole1 |
Cs O2g CO2g H2393 kJmole1 |
2Cs H2g O2g C2H5OHl H3277 kJmole1 |
Use the information to calculate the molar enthalpy of combustion H4 of ethanol |
Energy cycle diagram |
2Cs 3H2 g 3O2g H3Hf 227kJ C2 H5OH 3O2g |
H2Hc 394kJx 2 H1Hc 286kJx 3 H4 Hc x |
2CO2g 3H2Ol |
Substituting |
H1 H2 H3 H4 |
394 x 2 286 x 3 277 x |
H4 1369 kJ |
Heat of combustion Hc C2H5OH 1369 kJmole1 |
5Given the following information below |
CuSO4s aq CuSO4aq H661 kJmole1 |
CuSO4s aq 5H2Ol CuSO4 5H2O aq H774 kJmole1 |
Calculate H for the reaction |
CuSO4aq 5H2O CuSO4 5H2O aq H774 kJmole1 |
Working |
CuSO4s aq 5H2Ol CuSO4aq 5H2Ol CuSO4 5H2O aq |
CuSO4s aq 5H2Ol CuSO4 5H2O aq |
Energy cycle diagram |
CuSO4s aq 5H2Ol H1661kJ CuSO4aq 5H2Ol |
H3 774kJ H2 x |
CuSO4 5H2O aq |
Substituting |
H3 H2 H1 |
774kJ x 661kJ |
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