Post on 26-Dec-2015
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