HIGHER GRADE CHEMISTRY CALCULATIONS

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HIGHER GRADE CHEMISTRY CALCULATIONS

• Hesss Law

Hesss Law states that the enthalpy change for a
reaction depends only on the enthalpies of the
reactants and products and is independent of the
route taken for the reaction.
Worked example Use the enthalpies of combustion
in the data booklet to calculate the enthalpy
change for- C(s) 2 H2(g) ? CH4(g)
Use the enthalpies of combustion of carbon,
hydrogen and methane.

• C O2 -gt CO2 DH1 -394 kJ.
• H2 ½ O2 ? H2O DH2 -286 kJ.
• (3) CH4 2O2 ? CO2 2H2O DH3 -882 kJ.

Rearrange the equations to give the one we want.

• C O2 -gt CO2 DH1 -394 kJ.
• (2) x 2 2H2 O2 ? 2H2O DH4 -572 kJ.
• (3)rev CO2 2H2O ? CH4 2O2 DH5 882
  kJ.

Equations cancel out to give the one we want
DH (-394) (-572) (882) -84
kJ mol-1
2

• Calculations for you to try.
• Calculate the enthalpy of formation of ethane
  using the heats of combustion in the data booklet.

We want DH for 2C 3H2 ? C2H6
Use the enthalpies of combustion of carbon,
hydrogen and ethane.

• C O2 -gt CO2 DH1 -394 kJ.
• H2 ½ O2 ? H2O DH2 -286 kJ.
• (3) C2H6 3 ½ O2 ? 2CO2 3H2O DH3 -1560
  kJ.

Rearrange the equations to give the one we want.
(1) x 2 2C 2O2 -gt 2 CO2 DH4
-788 kJ. (2) x 3 3H2 1 ½ O2 ? 3H2O DH5
-858 kJ. (3)rev 2CO2 3H2O ? C2H6 3
½ O2 DH6 1560 kJ.
Adding these equations and cancelling out gives
the one we want DH (-788) (-858)
(1560) -86 kJ mol-1
Higher Grade Chemistry
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Calculations for you to try. 2. Calculate the
enthalpy of formation of methanol, CH3OH, using
the heats of combustion in the data booklet.
We want DH for C 2H2 ½ O2 ?
CH3OH
Use the enthalpies of combustion of carbon,
hydrogen and methanol.

• C O2 -gt CO2 DH1 -394 kJ.
• H2 ½ O2 ? H2O DH2 -286 kJ.
• (3) CH3OH 1 ½ O2 ? CO2 2H2O DH3 -727
  kJ.

Rearrange the equations to give the one we want.
(1) C O2 -gt CO2 DH1
-394 kJ. (2) x 2 2H2 O2 ?
2H2O DH4 -572 kJ. (3)rev CO2 2H2O ?
CH3OH 1 ½ O2 DH5 727 kJ.
Adding these equations and cancelling out gives
the one we want DH (-394) (-572)
(727) -239 kJ mol-1
Higher Grade Chemistry