Lesson 4 – Representing Enthalpy Changes

Method One

THERMOCHEMICAL EQUATIONS WITH ENERGY TERMSTHERMOCHEMICAL EQUATIONS WITH ENERGY TERMS

a balanced chemical equation that includes the heat transferred to a balanced chemical equation that includes the heat transferred to or from the surroundingsor from the surroundings

Ex. Ex. HH2 2 (g)(g) + ½O + ½O2 2 (g)(g) H H22OO(l)(l) + 285.8 kJ + 285.8 kJ

MgCOMgCO3 3 (s)(s) + 117.3 kJ + 117.3 kJ MgO MgO(s)(s) + CO + CO22(g)(g)

To write a thermochemical equation, you must knowTo write a thermochemical equation, you must know–the balanced chemical equationthe balanced chemical equation–the molar heat of reaction for one substancethe molar heat of reaction for one substance–if the reaction is endothermic or exothermicif the reaction is endothermic or exothermic

Sample Problem 1

From a calorimetry experiment, a student determines that the From a calorimetry experiment, a student determines that the specific heat of solution of KBrOspecific heat of solution of KBrO3 (s)3 (s) is an endothermic 0.25 kJ/g. is an endothermic 0.25 kJ/g.

Calculate the molar heat of solution and write the Calculate the molar heat of solution and write the thermochemical equation.thermochemical equation.

Sample Problem 2The combustion of magnesium releases 24.7 kJ per gram of The combustion of magnesium releases 24.7 kJ per gram of

magnesium. Calculate the molar heat of combustion of magnesium. Calculate the molar heat of combustion of magnesium and write the thermochemical equation.magnesium and write the thermochemical equation.

Sample Problem 3Use the following thermochemical equation to calculate the molar Use the following thermochemical equation to calculate the molar

heat of combustion of hexane.heat of combustion of hexane.

2C2C66HH14 14 (l)(l) + 19O + 19O2 2 (g)(g) 12CO 12CO2 2 (g)(g) + 14H + 14H22O O (g)(g) + 7086kJ + 7086kJ

Method Two

THERMOCHEMICAL EQUATIONS WITH THERMOCHEMICAL EQUATIONS WITH ΔΔH VALUESH VALUES

The enthalpy of reaction is indicated by a separate expression The enthalpy of reaction is indicated by a separate expression beside the chemical equationbeside the chemical equation

»Remember: exothermic reactions have -ΔH

endothermic reactions have +ΔHExamplesExamples::

HH2 2 (g)(g) + ½O + ½O2 2 (g)(g) H H22OO(l)(l) HHoorxnrxn = -285.8 kJ = -285.8 kJ

MgCOMgCO3 3 (s)(s) MgO MgO(s)(s) + CO + CO22(g)(g) HHoorxnrxn = 117.3 kJ = 117.3 kJ

Method ThreeMOLAR ENTHALPY OF REACTIONMOLAR ENTHALPY OF REACTION

the quantity of heat transferred in a reaction per mole of a the quantity of heat transferred in a reaction per mole of a specified substance specified substance

HHrr (kJ/mol) (kJ/mol)

to calculate the molar heat of reaction: to calculate the molar heat of reaction:

HHrr = specific heat of reaction x molar mass = specific heat of reaction x molar mass

standard molar enthalpy of reactionstandard molar enthalpy of reaction ( (HHrroo): molar enthalpy ): molar enthalpy

that is determined at SATPthat is determined at SATP

Sample Problem 4Ethane is the second largest component of natural gas. If its Ethane is the second largest component of natural gas. If its

specific heat of combustion is -51.85 kJ/g, what is the molar specific heat of combustion is -51.85 kJ/g, what is the molar heat of combustion of ethane?heat of combustion of ethane?

Method FourPOTENTIAL ENERGY DIAGRAMPOTENTIAL ENERGY DIAGRAM

a graphical representation of the energy transferred during a a graphical representation of the energy transferred during a physical or chemical changephysical or chemical change

exothermic reactionsexothermic reactions: products are lower in energy than : products are lower in energy than reactants (energy lost to surroundings)reactants (energy lost to surroundings)

endothermic reactionsendothermic reactions: products are higher in energy than : products are higher in energy than reactants (energy absorbed from surroundings)reactants (energy absorbed from surroundings)

Ep

Reaction Progress

Reactants

Products

Exothermic Reactions

H Ep

Reaction Progress

Reactants

Products

Endothermic Reactions

H