ThermochemistryThermochemistryChapter 6Chapter 6

The Nature of EnergyThe Nature of Energy

Energy is the capacity to do work Energy is the capacity to do work or produce heat.or produce heat.

Total energy of the universe is constant.

Energy lost = Energy gained by something else.

Potential Energy = energy due to position= mgh

Kinetic Energy = energy of motion = ½ mv2

Heat= transfer of energy due to temperature

differences

Work = force used to move an object a

distance.

Heat is a transfer of Heat is a transfer of energyenergy

System vs SurroundingsSystem vs Surroundings

State Function – a property that depends State Function – a property that depends only on the present state of the system…not only on the present state of the system…not on the changes it has or will experience.on the changes it has or will experience.

Internal energyInternal energy PressurePressure VolumeVolume EnergyEnergy

System

Surroundings

Heat and work are NOT state functions!

Thermodynamic Thermodynamic QuantitiesQuantities

Consist of two partsConsist of two parts

1) the number – indicates how much1) the number – indicates how much

2) the sign- indicates direction of flow2) the sign- indicates direction of flow

HEATq

WORKw

ENTHALPYH

Internal Energy

E

Negative Values = flow out of system

Positive Values = Flow into system

Heat Lost & Heat Heat Lost & Heat GainedGained

Draw…

…a graph of energy vs. reaction timefor a reaction that gives energy to

the surroundings AND

a second one for a reaction that absorbs energy from the surroundings

Exothermic and Exothermic and EndothermicEndothermic

Exothermic Exothermic ReactionReaction

Feels hotFeels hot Heat transferred Heat transferred

to surroundings to surroundings (lost by system)(lost by system)

Negative Negative enthalpy and enthalpy and heat valuesheat values

Endothermic Endothermic ReactionReaction

Feels coldFeels cold Heat transferred Heat transferred

to system to system (gained by (gained by system)system)

Positive enthalpy Positive enthalpy and heat valuesand heat values

PV Work PV Work

Work (w) = Force*displacementWork (w) = Force*displacement

W = F * d = F * W = F * d = F * hh

W = P * A * W = P * A * hh

P = Force/Area

VOLUME!

When pressure of system does not change

W = -PV

Try Me ProblemTry Me Problem

A balloon is inflated to its maximum capacity by A balloon is inflated to its maximum capacity by heating. If the volume changes from 4.0 x 10heating. If the volume changes from 4.0 x 1066L to 4.5 L to 4.5 x 10x 1066L by addition of 1.3 x 10L by addition of 1.3 x 1088J energy as heat. J energy as heat. Assuming that the balloon expands against constant Assuming that the balloon expands against constant 1.0 atm pressure. Calculate Internal Energy.1.0 atm pressure. Calculate Internal Energy.

(1 L*atm = 101.3 J)(1 L*atm = 101.3 J)

How is enthalpy How is enthalpy different?different? Enthalpy, H, is the amount of Enthalpy, H, is the amount of

energy capable of doing work in a energy capable of doing work in a system.system.

The amount of energy contained within The amount of energy contained within the bonds of chemicals involved in the the bonds of chemicals involved in the system.system.

H = E + PVH = E + PVAnswer

Now

Compare the equation for total internal energy with the equation

for enthalpy listed above.How can you alternately define

Enthalpy?

Enthalpies of FormationEnthalpies of Formation

The enthalpy of formation (HThe enthalpy of formation (Hffoo) for an ) for an

element in its standard state is ZERO.element in its standard state is ZERO.

Standar

d Sta

te

is a

t

1 atm

and 2

5o C

The more negative the value of Hf

o, the more stable the compound.

Hrxn = [np(Hfoprod)]-[nr(Hf

orct)]

Sample ProblemsSample Problems

Try Me 1Try Me 1Find the enthalpy for the Find the enthalpy for the

reaction:reaction:4NH4NH3(g) 3(g) + HCl+ HCl(l)(l) 4NH 4NH44ClCl(s)(s)

Try Me 2Try Me 2Find the enthalpy for the Find the enthalpy for the

reaction:reaction:2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(s)

Experimental Determination Experimental Determination of Heat & Enthalpyof Heat & Enthalpy

Specific/Molar Heat of CombustionSpecific/Molar Heat of Combustionq = mcq = mcTT q = ncq = ncTT

-The heat needed to raise the -The heat needed to raise the temperature of 1 g (or 1 mol) of temperature of 1 g (or 1 mol) of substance 1 degree K.substance 1 degree K.

Another way to find Another way to find HHrxnrxn

Experimentally! Experimentally! rxnrxn = = HHproductsproducts – – HHreactantsreactants

How do you measure this stuff?How do you measure this stuff? CALORIMETRY!CALORIMETRY!

Hess’s LawHess’s Law

Enthalpy is a state functionEnthalpy is a state function The value will be the same regardless of how many The value will be the same regardless of how many

steps are needed to complete the reaction.steps are needed to complete the reaction.

Hess’s Law States:

The enthalpies of individual stepsin a reaction mechanism can beadded together to calculate theenthalpy of the overall reaction.

Fundamentals for Applying Hess’s Fundamentals for Applying Hess’s LawLaw

Reverse the reaction, reverse the Reverse the reaction, reverse the sign on enthalpy.sign on enthalpy.

Multiply the reaction by a Multiply the reaction by a coefficient, multiply the enthalpy coefficient, multiply the enthalpy by the same coefficient.by the same coefficient.

Add the reactions together, add Add the reactions together, add the enthalpies together.the enthalpies together.

Try Me!Try Me!

Overall: NOverall: N22OO4(g)4(g)=>N=>N2(g)2(g) + 2O + 2O2(g)2(g)

Reaction Mechanism:Reaction Mechanism:

NONO2(g) 2(g) ½ N ½ N2(g)2(g) + O + O2(g)2(g) H=-84.75 kJ H=-84.75 kJ

2NO2NO2(g) 2(g) N N22OO4(g)4(g) H=-145.5 kJ H=-145.5 kJ

Thisis fun! Let’s do

some more!