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19/08/2013
Hesss Law
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Hesss Law
Hesss law is a version of the first law of thermodynamics, which is that energy is always conserved. Hesss law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place
chemrevise.org
19/08/2013
route 1 A+B E C+D
route 2
There are two routes to go from A+B to C+D. Hesss law states that the energy change for both routes will be the same.
H H+ (g) + Br - (g) a H (g) + Br (g) H+ (aq) + Br - (aq) d c HBr (g)
In this example one route is arrow a plus H The second route is shown by arrows c plus arrow d So a+ H = c + d And rearranged H = c + d - a
chemrevise.org
19/08/2013
Using Hesss law to determine enthalpy changes of reaction from enthalpy changes of formation. Enthalpies of formation can be used to calculate any enthalpy change for a reaction H reaction Reactants Hf reactants Products Hf products
Elements
In general H reaction = Hf products - Hf reactants
Standard enthalpy change of formation
The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kPa), all reactants and products being in their standard states Symbol Hf
Equations representing enthalpy change of formation Mg (s) + Cl2 (g) MgCl2 (s) 2Fe (s) + 1.5 O2 (g) Fe2O3 (s)
chemrevise.org
19/08/2013
Example What is the enthalpy change for this reaction ? Al2O3 + 3 Mg 3 MgO + 2 Al
Al2O3 (s)+ 3 Mg(s)
Hr
3 MgO (s) + 2 Al (s)
What goes in this box ?
Example What is the enthalpy change for this reaction ? Al2O3 + 3 Mg 3 MgO + 2 Al
Al2O3 (s)+ 3 Mg(s)
Hr
3 MgO (s) + 2 Al (s)
2 Al (s) + 3Mg (s) + 1.5 O2 (g) What do these arrows represent ?
chemrevise.org
19/08/2013
Example
What is the enthalpy change for this reaction ? Al2O3 + 3 Mg 3 MgO + 2 Al 1 Al2O3 (s)+ 3 Mg(s)
Hf (Al2O3)
Hr
3 MgO (s) + 2 Al (s)
3 x Hf (MgO)
2 Al (s) + 3Mg (s) + 1.5 O2 (g)
What are the two routes ?
Al2O3 (s)+ 3 Mg(s)
Hf (Al2O3)
Hr
3 MgO (s) + 2 Al (s)
3 x Hf (MgO)
2 Al (s) + 3Mg (s) + 1.5 O2 (g)
By applying Hesss law the enthalpy change for Route 1 = enthalpy change for route 2 3 x Hf (MgO) = Hf (Al2O3) + Hr rearrange to give Hr : = -129.4 kJ mol-1 Hr = 3 x Hf (MgO) - Hf (Al2O3) Hr = -129 kJ mol-1 to 3sf Hr = (3 x 601.7) - -1675.7
chemrevise.org
19/08/2013
Using Hesss law to determine enthalpy changes of reaction from enthalpy changes of combustion. Enthalpies of combustion can be used to calculate enthalpy changes for some reactions reaction H reaction Reactants HC reactants Combustion Products In general H reaction = Hcreactants - Hc products Products HC products
Standard enthalpy change of Combustion
The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely under standard conditions. (298K and 100kPa), all reactants and products being in their standard states Symbol Hc CH4 (g) + 2O2 (g) CO2 (g) + 2 H2O (l)
Incomplete combustion will lead to soot (carbon), carbon monoxide and water. It will be less exothermic than complete combustion.
chemrevise.org
19/08/2013
Example
Work out the enthalpy change for this reaction from heats of combustion? CO (g) + 2H2 (g) CH3OH (g)
CO (g) + 2H2 (g)
Hr
CH3OH (g)
What goes in this box ?
Example
Work out the enthalpy change for this reaction from heats of combustion? CO (g) + 2H2 (g) CH3OH (g)
CO (g) + 2H2 (g)
Hr
CH3OH (g)
CO2 (g) + 2H2O (l) What do these arrows represent
chemrevise.org
19/08/2013
Example
Work out the enthalpy change for this reaction from heats of combustion? CO (g) + 2H2 (g) CH3OH (g)
CO (g) + 2H2 (g) 1
HC(CO) + 2HC(H2 )
Hr
CH3OH (g)
HC(CH3OH)
CO2 (g) + 2H2O (l) What are the two routes?
CO (g) + 2H2 (g) 1
HC(CO) + 2HC(H2 )
Hr
CH3OH (g)
HC(CH3OH)
CO2 (g) + 2H2O (l)
By applying Hesss law the enthalpy change for Route 1 = enthalpy change for route 2
HC(CO) + 2 x HC(H2 ) = HC(CH3OH) +
Hr
rearrange to give Hr : = -102 kJ mol-1
Hr = HC(CO) + 2 x HC(H2 ) - HC(CH3OH) Hr = -102 kJ mol-1 to 3sf
Hr = -283+ 2x 245 - -671
chemrevise.org
19/08/2013
Using Hesss law to determine enthalpy changes of formation from enthalpy changes of combustion. Enthalpies of combustion can be used to calculate the enthalpy change of formation for a combustible compound H elements HC elements Combustion Products In general
formation
compound HC compound This is a variation of the other cycle using Hc
H formation = Hcelement - Hc compound
