Chpt 6 - Thermochemistry• Energy and framework (definitions)

• Internal energy, heat & work

• Enthalpy & Calorimetry

• Hess’s Law

• Standard Enthalpies

• Applications?• HW: Chpt 6 - pg. 275-283, #s 14, 16, 24, 27, 28, 32, 34, 38,

41, 42, 44, 49, 54, 62, 72, 73, 80, 84, 112 Due Wed Oct. 7

Thermodynamic Framework

• Energy - capacity to do work or produce heat

• Law of conservation of energy

• PE (generally chemical potential energy) and KE

• Heat involves a transfer of energy - Very different from temperature (a measure of motions of particles) Heat is NOT a substance

• Work is force acting over a distance

State Functions

Energy = work + heat– Ball rolling down hill

example - heat and work are different depending on pathway

State Functions - con’t

• State function or state property only depends on current state not pathway– Energy is state function – Heat and work are not state functions– Another example, Elevation vs. Distance

Chemical Energy

• System and surroundings

• Exothermic and endothermic– Chemical PE <--> thermal energy

• Recall energy reaction diagrams– Only concerned with Energy of Reactants

and Products (not pathway) Internal Energy

E = q + w q=heat, w=work

• 1st Law of thermodynamics– Energy of Universe is constant

Universe Energy is Constant

Exothermic or Endothermic?

Chemical Energy - con’t• Quantities have sign and magnitude

– System’s perspective for sign, thus endo is flow into system so q is positive (gaining heat)

E <0 exothermic, E>0 endothermic– Work done on system is positive, w>0– Work done by system is negative, w<0

PV Work

• Common types of work are expansion by a gas and compression on a gas

• PV work,

• w = - PV if volume is expanding

• w = PV if volume is compressingV = Vfinal - Vinitial,

PV Work - derivation

Example that shows Pressure=force/area = F/A so F = P x AWork is force x dist = F x h so W = P x A x h volume of cylinder = A x h

Thus W = PV

The sign is (-) for expanding gas, since work is done by system on surroundings

Enthalpy, H H = E + PV, since E, P and V are state

functions H is also a state function

At constant pressure H = qp (qp is heat at const p) In general, for open laboratory chemical reactions pressure is constant, so the change in enthalpy is used interchangeably with the heat of a reaction.

For a chemical reactionH = Hproducts - Hreactants Exothermic means enthalpy, H < 0Endothermic means enthalpy, H > 0

Calorimetry

The science of measuring heat. Substances absorb heat differently; heat capacity, C, measures this

C = heat absorbed / increase in temp

Specific heat capacity is per gram substance

A calorimeter measures heat change.

q = C x m x T our text uses ‘s’ for C

The AP test will use C, actually Cp

Calorimeter

Simple styrofoam cup calorimeter is easily used for lab measurements and constant pressure measurements.

Bomb Calorimeter

A “Bomb” calorimeter schematic - it only looks like a bomb. It is used when constant volume measurements are needed.

Hess’s Law• Since enthalpy is a state function, heat of

reactions can be calculated from a known set of simple chemical rxns combined together to get the final rxn.

One step:

N2(g) + 2O2(g) --> 2NO2(g) H = 68kJ

Two distinct steps

N2(g) + O2(g) --> 2NO(g) H = 180kJ

2NO(g) + O2(g) --> 2NO2(g) H = -112kJ

Total these reactions

N2(g) + 2O2(g) --> 2NO2(g) H = 68kJ

Hess’s Law schematic

The overall reaction enthalpy is independent of pathway

Hess’s Law rules

Characteristics of H for a rxn

• If a reaction is reversed, the sign of H is also reversed.

• The magnitude of H is directly proportional to the quantities of reactants and products in the rxn– i.e. if the coefficients are multiplied by an

integer, H is multiplied by same integer.

Example 6.8 pg. 254We want to calculate the H for the synthesis of

diborane, B2H6, from its elements.2B(s) + 3H2(g) --> B2H6 (g) H = ?

Use the following data:

2B(s) + 3/2O2(g) --> B2O3(s) H = -1273 kJ

B2H6(g) + 3O2(g) --> B2O3(s) +3H2O(g) H = -2035 kJ

H2(g) + 1/2O2(g) --> H2O(l) H = -286 kJ

H2O(l) --> H2O(g) H = 44 kJHints - 1) work backward from the required/desired reaction, 2) reverse any

reaction as needed to align reactants and products, 3)multiply any reaction as necessary to get correct coefficients.

Recall H is a state function (independent of pathway)

Standard Enthalpies of Formation

Hfo of a substance is the change in enthalpy that

accompanies the formation of 1 mole of the substance from its elements in their standard states.

Standard state: gas is 1 atm, liquid or solid is pure substance, solutions are 1M. Elements are state at 1atm and 25oC.

CalculatingH from Hfo

Since enthalpies are state functions independent of pathway, in chemical reactions, the reactants can be taken apart into their elements and the products can be constructed from their elements.

For a chemical reactionH = npHf

o (products) - nrHfo (reactants)

Data found in Appendix 4 - pg A19 - A22

Example H from Hfo

4 NH3(g) + 7 02(g) --> 4 NO2(g) + 6 H2O(l)

NH3(g) -46 kJ/mol NO2(g) 34kJ/mol

H2O(l) -286kJ/mol O2(g) 0 kJ/mol

H = Products - Reactants…

4x34kJ/mol + 6x(-286kJ/mol) - 0 - 4x(-46kJ/mol)

H = -1396kJ/mol

Greenhouse Effect