Hess Law and

Advanced ChemistryMs. GrobskyEnthalpies of Reactions and Hess LawRecall that:Enthalpy is a measure of the heat that is released or absorbed in a chemical reaction at constant pressureYou cannot measure the actual enthalpy of a substanceYou can measure an enthalpy CHANGEWritten as the symbol DH , delta H THERMODYNAMICS - ENTHALPY CHANGESWhy is there a standard? Enthalpy values vary according to the conditionsA substance under these conditions is said to be in its standard state:Pressure1 atmosphereTemperature298K (25C)

Concentrationmol/L

As a guide, just think of how a substance would be under normal lab conditionsAssign the correct subscript [(g), (l) or (s) ] to indicate which state it is inTo tell if standard conditions are used we modify the symbol for DH:

Enthalpy Change Standard Enthalpy Change(at 298K)STANDARD ENTHALPY CHANGES

STANDARD HEATS (ENTHALPIES) OF FORMATIONDefinitionThe enthalpy change when ONE MOLE of a compound is formed in its standard state from its elements in their standard states

SymbolDHf

ValuesUsually, but not exclusively, exothermic

Example(s) C(graphite) + O2(g) > CO2(g)

H2(g) + O2(g) > H2O(l)

2C(graphite) + O2(g) + 3H2(g) > C2H5OH(l)

Note:Only ONE MOLE of product on the right-hand side of the equationElements In their standard states have zero enthalpy of formationSTANDARD HEATS (ENTHALPIES) OF FORMATIONDefinitionThe enthalpy change when ONE MOLE of a substance undergoes complete combustion under standard conditions. All reactants and products are in their standard states

SymbolDHcomb

ValuesAlways exothermic

Example(s)C(graphite) + O2(g) > CO2(g)

H2(g) + O2(g) > H2O(l)

C2H5OH(l) + 3O2(g) > 2CO2(g) + 3H2O(l)

NotesAlways only ONE MOLE of what you are burning on the LHS of the equationTo aid balancing the equation, remember:You get one carbon dioxide molecule for every carbon atom in the original and one water molecule for every two hydrogen atomsWhen you have done this, go back and balance the oxygenSTANDARD HEATS (ENTHALPIES) OF COMBUSTIONHeat of FusionThe heat absorbed by ONE MOLE of a substance in melting from a solid to a liquidDHfusIf ONE MOLE of a substance releases heat in freezing from a liquid to a solid, DHfus is negative!

Heat of NeutralizationWe will soon find out in lab!

Heat of VaporizationThe amount of heat necessary to vaporize ONE MOLE of a given liquid

Heat of CondensationThe amount of heat released when ONE MOLE of a vapor condenses

Heat of SolutionHeat change caused by dissolution of ONE MOLE of substanceOTHER HEATS (ENTHALPIES) OF REACTIONTheoryImagine that, during a reaction, all the bonds of reacting species are broken and the individual atoms join up again but in the form of products

Breaking a bond REQUIRES energyENDOTHERMIC

Forming a bond RELEASES energyExothermic

The overall energy change will depend on the difference between the energy required to break the bonds and that released as bonds are made

If energy released making bonds > energy used to break bondsEXOTHERMICProducts are lower in energy than the reactants If energy used to break bonds > energy released making bondsENDOTHERMICProducts are greater in energy than reactantsRelation of Enthalpy to BondsTurn to FRONT of page 165

Enthalpy change (DH) = Enthalpy of products - Enthalpy of reactants

Enthalpy of reactants > products Enthalpy of reactants < products DH = - DH = + EXOTHERMIC Heat given out ENDOTHERMIC Heat absorbed

REACTION CO-ORDINATEENTHALPY

REACTION CO-ORDINATEENTHALPYTHERMODYNAMICS - ENTHALPY CHANGESEnthalpy and Hess LawEnthalpy and Hess LawEnthalpy is a state functionAs such, DH for going from some initial state to some final state is pathway independent

Hess LawDH for a process involving the transformation of reactants into products is not dependent on pathway Therefore, we can pick any pathway to calculate DH for a reaction

Hesss Law12

StartFinishBoth lines accomplished the same result, they went from start to finishNet result = sameCampsite to Illustrate Altitude as a State Function

Hesss LawHesss law of heat summation states that for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation is the sum of the enthalpy changes for the individual steps. 143A B + CH = xB + C D H = y

A DH = ?H = x + y

The enthalpy change is independent of the path taken

HowThe enthalpy change going from A to B can be found by adding thevalues of the enthalpy changes forthe reactions A to X, X to Y and Y to B.

DHr = DH1 + DH2 + DH3

HESSS LAW

The enthalpy change is independent of the path taken

HowThe enthalpy change going from A to B can be found by adding thevalues of the enthalpy changes forthe reactions A to X, X to Y and Y to B.

DHr = DH1 + DH2 + DH3

If you go in the opposite direction of an arrow, you subtract the value ofthe enthalpy changeDH2 = - DH1 + DHr - DH3

The values of DH1 and DH3 have been subtracted because the route involves going in the opposite direction to their definitionHESSS LAW

The enthalpy change is independent of the path taken

UseApplying Hesss Law enables one to calculate enthalpy changes fromother data, such as:Changes which cannot be measured directlyLattice EnthalpyEnthalpy change of reaction from bond enthalpyEnthalpy change of reaction from DHcEnthalpy change of formation from DHfHESSS LAWEnthalpy diagram illustrating Hesss law

Hesss LawFor example, suppose you are given the following data:

Could you use these data to obtain the enthalpy change for the following reaction?

203Hesss LawIf we multiply the first equation by 2 and reverse the second equation, they will sum together to become the third.

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