First Law of Thermodynamics

Enthalpy -H, Hf

Second Law

Third Law

Entropy S, Free Energy G

Spontaneity

Chap 18. Thermodynamics and Equilibrium

Thermochemistry

Heat changes during chemical reactions Thermochemical equation. eg.

H2 (g) + O2 (g) ---> 2H2O(l) H =- 256 kJ; is called the enthalpy of reaction.

if H is + reaction is called endothermic

if H is - reaction is called exothermic

Why is it necessary to divide Universe into System and Surrounding Universe = System + Surrounding

Why is it necessary to divide Universe into System and Surrounding

Universe = System + Surrounding

What is the internal energy What is the internal energy change (change (U) of a system?U) of a system?

U is associated with changes in atoms, molecules and subatomic particles

Etotal = Eke + E pe + U

U = heat (q) + w (work) U = q + w U = q -P V; w =- P V

What forms of energy are found in the Universe? mechanical thermal electrical nuclear mass: E = mc2

others yet to discover

What is 1st Law of Thermodynamics Eenergy is conserved in the universe

All forms of energy are inter-convertible and conserved

Energy is neither created nor destroyed.

What exactly is H?

Heat measured at constant pressure qp

Chemical reactions exposed to atmosphere and are held at a constant pressure.

Volume of materials or gases produced can change. ie: work = -PV

U = qp + w; U = qp -PV

qp = U + PV; w = -PV

H = U + PV; qp = H(enthalpy )

How do you measure U?

Heat measured at constant volume qv

Chemical reactions take place inside a bomb. Volume of materials or gases produced can not change. ie: work = -PV= 0 U = qv + wqv = U + o; w = 0U = qv = U(internal energy )

What is Hess's Law of Summation of Heat?

To heat of reaction for new reactions. Two methods? 1st method: new H is calculated by

adding Hs of other reactions. 2nd method: Where Hf ( H of

formation) of reactants and products are used to calculate H of a reaction.

Method 1: Calculate H for the reaction:

SO2(g) + 1/2 O2(g) + H2O(g) -----> H2SO4(l) H = ?

Other reactions: SO2(g) ------> S(s) + O2(g) H = 297kJ H2SO4(l)------> H2(g) + S(s) + 2O2(g)

H = 814 kJH2(g) +1/2O2(g) -----> H2O(g) H = -242 kJ

SO2(g) ------> S(s) + O2(g);H1 = 297 kJ - 1

H2(g) + S(s) + 2O2(g) ------> H2SO4(l) H2 = -814 kJ - 2

H2O(g) ----->H2(g) + 1/2 O2(g) H3 = +242 kJ - 3

______________________________________ SO2(g) + 1/2 O2(g) + H2O(g) -----> H2SO4(l) ªH = H1 + H2 + H3

ªH = +297 - 814 + 242 ªH = -275 kJ

Calculate Heat (enthalpy) of Combustion: 2nd method

C7H16(l) + 11 O2(g) -----> 7 CO2(g) + 8 H2O(g) ; H = ?

Hf (C7H16) = -198.8 kJ/mol

Hf (CO2) = -393.5 kJ/mol

Hf (H2O) = -285.9 kJ/mol

Hf O2(g) = 0 (zero)

What method? 2nd method

ªH = [3n ( ªHof) Products] -

[3n ( ªHof) reactants]

ªH = [ 7(-393.5 + 8 (-285.9)] - [-198.8 + 11 (0)]

= [-2754.5 - 2287.2] - [-198.8] = -5041.7 + 198.8 = 4842.9 kJ = 4843 kJ

Why is ªHof of elements is zero?

ªHof, Heat formations are for compounds

Note: ªHof of elements is zero

What is 2nd Law of Thermodynamics

Entropy (S) of the Universe increases during spontaneous process.

What is entropy Entropy (S) : A measure of randomness or

disorder of a system. Spontaneous Process: A process that occurs

without outside intervention.

Definitions The Universe: The sum of all parts under

consideration. System: Part of the Universe we are

interested in and a change is taking place. Surrounding: Part of the Universe we are

not interested in or can not observe.

Entropy S

Suniv = entropy of the Universe

Ssys = entropy of the System

Ssurr = entropy of the Surrounding

Suniv = Ssys+ Ssurr

Suniv= Ssys + Ssurr

Suniv Ssys Ssurr

+ + +

+ +(Ssys>Ssurr) -

+ - + (Ssurr>Ssys)

Entropy Change

Entropy (S) normally increase (+) for the following changes:

i) Solid ---> liquid (melting) ii) Liquid ---> gas iii) Solid ----> gas iv) Increase in temperature v) Increasing in pressure vi) Increase in volume ( at constant temperature

and pressure)

What is G ?

Free Energy

G = - TSuni.

G = H - TS.

How do you get:G = H - TS. suniv= ssys +ssurr

ssurr = -Hsys/T)

Suniv = Ssys -Hsys/T

Suniv x T = TSsys -Hsys : x T

-Suniv x T = -TSsys +Hsys : x -1

-Suniv x T = Hsys-TSsys

-Suniv x T = G;

G = Hsys-TSsys or G = H - TS.

What G means

If G is - for a change it will take place spontaneously If G is + for a change it will not take

place If G is 0 for a change it will be in

equilibrium

Qualitative prediction of S of Chemical Reactions Look for (l) or (s) --> (g) If all are gases: calculate n = 3n(gaseous prod.) -3 n(gaseous reac.) N2 (g) + 3 H2 (g) --------> 2 NH3 (g)

n = 2 - 4 = -2 If n is - S is negative (decrease in S) If n is + S is positive (increase in S)

Predict S!

2 C2H6(g) + 7 O2(g)--> 4 CO2(g) +

6H20(g).

2 CO(g) + O2(g)-->2 CO2(g).

HCl(g) + NH3(g)-->NH4Cl(s).

H2(g) + Br2(l) --> 2 HBr(g).

Calculating S of reactions

Based on Hess’s Law second method:

So=3So(prod.) - 3S o(react.)

How do you calculate G

There are two ways to calculateG for chemical reactions. i) G = H - TS. ii) Go = 3 Go

f (products)

- 3 G of (reactants)

How do you calculate G at different T and P G = Go + RT ln Q Q = reaction quotient at equilibrium G = = Go + RT ln K Go = - RT ln K If you know Go you could calculate K