TThheerrmmooddyynnaammiiccss

The study of energy The study of energy transfers and chemical transfers and chemical

driving forcesdriving forces

EnthalpyEnthalpy Enthalpy (H)Enthalpy (H) the total E (KE + the total E (KE +

PE) of a system at constant PPE) of a system at constant P when a system reacts, when a system reacts, H = HH = Hfinalfinal - H - Hinitialinitial

for a chemical reaction:for a chemical reaction:HHrxnrxn = H = Hproductsproducts - H - Hreactantsreactants

HHH is a H is a state functionstate function - -

that is, what is the absolute that is, what is the absolute difference?difference?

the “history” of how it got the “history” of how it got there isn’t importantthere isn’t importantex: T, P, V, etc...ex: T, P, V, etc...

The only problem is...The only problem is...

The enthalpy of a system (H) The enthalpy of a system (H) cannot actually be measuredcannot actually be measured

KE = KE = 11//22mvmv22

the velocity of any object is always the velocity of any object is always relative to a relative to a frame of referenceframe of reference

the absolute velocity of the earth the absolute velocity of the earth cannot be determinedcannot be determined

But, we do know...But, we do know... For an For an endothermic endothermic reaction, reaction, H H

is (+)is (+) For an For an exothermicexothermic reaction, reaction, H H

is (-)is (-) so, so, H is all that is really H is all that is really

important, and it can be measured important, and it can be measured if we assume all the energy gained if we assume all the energy gained or lost is or lost is heatheat

Measuring Measuring HHBecause Because H = the H = the heatheat lost lost

or gained for each mole, or gained for each mole, if if we can measure the heat we can measure the heat lost or gained,lost or gained, we can we can know the value of know the value of H…H…

HH == q/nq/nAt constant At constant pressurepressure

What do all the What do all the H’s H’s mean?mean?

HHrxnrxn = the heat that is either = the heat that is either absorbed absorbed (+ (+ HHrxnrxn ) ) or or released by released by

(- (- HHrxnrxn ) ) the reactants during the the reactants during the course of a chemical reactioncourse of a chemical reaction

HHsolnsoln = the heat that is either = the heat that is either absorbed absorbed (+ (+ HHsolnsoln ) ) or or released by released by

(- (- HHsolnsoln ) ) a substance when it dissolves a substance when it dissolves

What do all the What do all the H’s H’s mean?mean?

HHfusfus = the heat that must be = the heat that must be added to change 1.0 mole of a added to change 1.0 mole of a substance from a substance from a solidsolid to to liquidliquid at at it’s melting pointit’s melting point

HHvapvap = the heat that must be = the heat that must be added to change 1.0 mole of a added to change 1.0 mole of a substance from a substance from a liquidliquid to to gasgas at at it’s boiling pointit’s boiling point

What do all the What do all the H’s H’s mean?mean?

Note:Note:all all HH’s are usually kJ/mol’s are usually kJ/moldivide the number of kJ by the divide the number of kJ by the

# of moles# of moles reverse process = same #, reverse process = same #,

opposite signopposite sign

calorimetrycalorimetry A calorimeter is a device A calorimeter is a device

used to measure the used to measure the T for T for a reacting systema reacting system

Often, filled with water to Often, filled with water to absorb or release absorb or release heatheat

calorimetrycalorimetry

Because the calorimeter (the water Because the calorimeter (the water inside it and the device itself) is the inside it and the device itself) is the surroundings, calculating the heat (q) surroundings, calculating the heat (q) that flows into or out of the water that flows into or out of the water allows us to infer the heat that allows us to infer the heat that flowed into or out of the system…flowed into or out of the system…

……which allows us to know the which allows us to know the H for H for the system itself (the system itself (H = q/n)H = q/n)

Because the Because the heatheat is absorbed by or is absorbed by or released mostly from the water, released mostly from the water, and a bit from the calorimeter, and a bit from the calorimeter, measuring measuring T of the water allows T of the water allows one to measure q for the reactionone to measure q for the reaction

qqrxnrxn = -( = -(qqH2OH2O + + qqcalcal))

qqrxnrxn = -( = -(msmsTT + + CCT)T)

Heat of reactionHeat of reaction The The entireentire energy change ( energy change (E) E)

for a reaction is often called the for a reaction is often called the heatheat of reaction (why? of reaction (why? We’ll seeWe’ll see))

The energy may be The energy may be absorbedabsorbed or or releasedreleased

the energy may be the energy may be heatheat, sound, , sound, light, electricity, etc.light, electricity, etc.

Thermochemical Thermochemical EquationsEquations

Since reactions involve the Since reactions involve the gain of gain of heatheat from from or or loss of loss of heatheat to to the surroundings, an the surroundings, an energy term may be energy term may be included on the reactant or included on the reactant or product side of a chemical product side of a chemical equationequation

endothermic reaction endothermic reaction heatheat is a is a reactantreactantH is (+)H is (+)

products

reactants

H = (+) number kJ

Energy

exothermic reactions exothermic reactions heatheat is a is a productproductH is (-)H is (-)

reactants

H = (-) number kJ

products

Energy

Example...Example...

When four moles of ammonia When four moles of ammonia burns in air, 1170kJ of burns in air, 1170kJ of heatheat are are producedproduced..

4 NH4 NH3(g)3(g) + 5 O + 5 O2(g)2(g) 4 NO 4 NO(g)(g) + 6 H + 6 H22OO(l) (l) + + 1170 kJ1170 kJ

HHrxnrxn = -1170 kJ = -1170 kJ

How many kJ are released How many kJ are released into the surroundings for into the surroundings for each gram of ammonia that each gram of ammonia that reacts?reacts?

1.00gNH1.00gNH33 x 1 mol/17.04gx 1 mol/17.04gx 1170kJ/4 mol NHx 1170kJ/4 mol NH33

= 17.2 kJ= 17.2 kJ

How many grams of nitric oxide How many grams of nitric oxide have been produced according have been produced according to the above reaction if 5000 kJ to the above reaction if 5000 kJ of heat has been generated?of heat has been generated?

5000 kJ5000 kJx 4 mol NO/1170 kJx 4 mol NO/1170 kJx 30.01g/1 molx 30.01g/1 mol= 513g NO= 513g NO