Chapter 17Chapter 17

ThermochemistryThermochemistry

BasicsBasics ThermochemistryThermochemistry

– Study of heat changes in a chemical reactionStudy of heat changes in a chemical reaction CalorimeterCalorimeter

– Instrument used to measure heat changes Instrument used to measure heat changes (actually measure temp changes because heat cannot be measured directly!)(actually measure temp changes because heat cannot be measured directly!)

– Styrofoam cup, “real” calorimeter (metal)Styrofoam cup, “real” calorimeter (metal) Temperature (T)Temperature (T)

– Measure of average KE of sampleMeasure of average KE of sample– Increased KE = Increased tempIncreased KE = Increased temp

Heat Heat – Measure of TOTAL KE of sampleMeasure of TOTAL KE of sample– Depends on the speed of the particles, the number of Depends on the speed of the particles, the number of

particles (the size or mass), and the type of particles in an particles (the size or mass), and the type of particles in an object. object.

Specific Heat (s or c)Specific Heat (s or c)– Amount of energy needed to raise 1g of sample 1Amount of energy needed to raise 1g of sample 1°C°C– A constant; a given value per substanceA constant; a given value per substance

CCH2OH2O = 4.186 J/g°C= 4.186 J/g°C

Calorimetry CalculationsCalorimetry Calculations Q = msQ = msΔΔTT

– Q = heatQ = heat– m = massm = mass– s = specific heats = specific heat– ΔΔT = change in tempT = change in temp

Basic Heat Example ProblemsBasic Heat Example Problems– Calculate the amount of heat needed to Calculate the amount of heat needed to

increase the temperature of 250g water increase the temperature of 250g water from 20°C to 56°C.from 20°C to 56°C.

– Calculate the specific heat of copper Calculate the specific heat of copper given that 204.75J of energy raises the given that 204.75J of energy raises the temperature of 15g of copper from 25°C temperature of 15g of copper from 25°C to 60°C.to 60°C.

Calorimetry CalculationsCalorimetry Calculations

When using a calorimeter to When using a calorimeter to calculate heat you use the energy calculate heat you use the energy transfer between item in transfer between item in calorimeter and the calorimeter calorimeter and the calorimeter itself:itself:

– Heat lost = heat gainedHeat lost = heat gained

– msmsΔΔTT(item)(item) = -(ms = -(ms ΔΔTT(calorimeter)(calorimeter)

+ ms + ms ΔΔTT(water)(water)))

Calorimetry CalculationsCalorimetry Calculations

1.1. A 25 g sample of a metal is heated in A 25 g sample of a metal is heated in a boiling water bath to 99.3a boiling water bath to 99.3°°C. A C. A calorimeter of ms value of 5.32 J/C is calorimeter of ms value of 5.32 J/C is used. 105mL of water (specific heat used. 105mL of water (specific heat = 4.184 J/g= 4.184 J/g°°C) at 22.7C) at 22.7°°C is placed in C is placed in the calorimeter. The metal is added the calorimeter. The metal is added to the calorimeter. The final to the calorimeter. The final temperature achieved is 44.8temperature achieved is 44.8°°C. C. What is the specific heat of the metal?What is the specific heat of the metal?

Heat of ReactionHeat of Reaction

Heat of Reaction (Q)Heat of Reaction (Q)– The amount of energy gained or lost during a The amount of energy gained or lost during a

chemical equationchemical equation

Thermochemical EquationThermochemical Equation– A reaction with heat information listedA reaction with heat information listed

Enthalpy (H)Enthalpy (H)– Measure of heat of reaction; usually per Measure of heat of reaction; usually per

mol of substance tested mol of substance tested

Heat of ReactionHeat of Reaction

ΔΔH = change in enthalpyH = change in enthalpy ΔΔH = H = ΔΔHHproductsproducts – – ΔΔHHreactantsreactants

If If ΔΔH is negative = exothermicH is negative = exothermic If If ΔΔH is positive = endothermicH is positive = endothermic Example:Example:

– 2H2H22O (g) O (g) 2H 2H22 (g) + O (g) + O22 (g) (g) ΔΔH =+483.6kJH =+483.6kJ Positive Positive ΔΔH means endothermic rxn – needs H means endothermic rxn – needs

energyenergy

Heat of ReactionHeat of Reaction

Stability of products vs. reactantsStability of products vs. reactants– If products have higher energy = If products have higher energy =

endothermicendothermic– If reactant have higher energy = If reactant have higher energy =

exothermicexothermicENDO – PUT IN ENERGY

EXO – ENERGY GIVEN OFF

Heat of ReactionHeat of Reaction

Heat of Formation (Heat of Formation (ΔΔHHff))– Energy change when forming 1mol of a Energy change when forming 1mol of a

compound from elements in most stable compound from elements in most stable formform

Heat of Combustion (Heat of Combustion (ΔΔHHcombuscombus))

– Energy change during burning in OEnergy change during burning in O22

Heat of Reaction (Heat of Reaction (ΔΔHHrxnrxn))– --ΔΔH are ‘favored’ (expect to happen)H are ‘favored’ (expect to happen)– Exothermic reactionsExothermic reactions

Calculating Calculating ΔΔH H

ΔΔHHrxnrxn = [ = [ΣΣ(coefficient)((coefficient)(ΔΔHHff products)] products)] – [– [ΣΣ(Coefficient)((Coefficient)(ΔΔHHff reactants)] reactants)]• ∑ ∑ = sum of= sum of

Practice problems (using Practice problems (using ΔΔHH charts)charts)

Hess’ LawHess’ Law

Hess’ LawHess’ Law– One big reaction that is the sum of One big reaction that is the sum of

several smaller reactionsseveral smaller reactions– Rules:Rules:

Flip a reaction = change sign of Flip a reaction = change sign of ΔΔHH Multiply by a coefficient = multiply Multiply by a coefficient = multiply ΔΔHH

Hess’ LawHess’ Law

Practice ProblemsPractice Problems

Driving Forces for ReactionsDriving Forces for Reactions

Enthalpy (Enthalpy (ΔΔH)H)– Exothermic (-) are favoredExothermic (-) are favored

Entropy (Entropy (ΔΔS)S)– DisorderDisorder– Gas – high entropyGas – high entropy– More disorder (+) are favoredMore disorder (+) are favored

EntropyEntropy

Practice Problems (Using Practice Problems (Using ΔΔS charts)S charts)

Free EnergyFree Energy

Gibb’s Free Energy (Gibb’s Free Energy (ΔΔG)G)– Available energyAvailable energy– Is the FINAL answer whether a reaction Is the FINAL answer whether a reaction

will occur or notwill occur or not– ΔΔG = G = ΔΔH – TH – TΔΔSS

Units must match!!Units must match!!– Temps in Kelvin!!Temps in Kelvin!!

If If ΔΔG is negative G is negative Reaction does occur Reaction does occur spontaneouslyspontaneously

If If ΔΔG is positive G is positive Reaction does not occur Reaction does not occur spontaneouslyspontaneously

Free EnergyFree Energy

Calculations (Using equation)Calculations (Using equation)

Free EnergyFree Energy

Calculations (Using Calculations (Using ΔΔG charts)G charts)

SummarySummary

--ΔΔHH are favoredare favored ++ΔΔS are favoredS are favored --ΔΔG are favoredG are favored