First Law of Thermodynamics Enthalpy - H, H f Second Law Third Law Entropy S, Free Energy G Spontaneity. Thermochemistry

Transcription

1 Chap 18. Thermodynamics and Equilibrium First Law of Thermodynamics Enthalpy - H, H f Second Law Third Law Entropy S, Free Energy G Spontaneity Thermochemistry Heat changes during chemical reactions Thermochemical equation. eg. H 2 (g) + O 2 (g) ---> 2H 2 O(l) H =- 256 kj; Η 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 1

2 Why is it necessary to divide Universe into System and Surrounding Universe = System + Surrounding What is the internal energy change ( U)( of a system? U is associated with changes in atoms, molecules and subatomic particles E total = E ke + 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 = mc 2 others yet to discover 2

3 What is 1 st 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 q p Chemical reactions exposed to atmosphere and are held at a constant pressure. Volume of materials or gases produced can change. ie: work = -P V U = q p + w; U = q p -P V q p = U + P V; w = -P V H = U + P V; q p = H(enthalpy ) How do you measure U? Heat measured at constant volume q v Chemical reactions take place inside a bomb. Volume of materials or gases produced can not change. ie: work = -P V= 0 U = q v + w q v = U + o; w = 0 U = q v = U(internal energy ) 3

4 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 H f ( H of formation) of reactants and products are used to calculate H of a reaction. Method 1: Calculate H for the reaction: SO 2 (g) + 1/2 O 2 (g) + H 2 O(g) -----> H 2 SO 4 (l) H =? Other reactions: SO 2 (g) > S(s) + O 2 (g) H = 297kJ H 2 SO 4 (l)------> H 2 (g) + S(s) + 2O 2 (g) H = 814 kj H 2 (g) +1/2O 2 (g) -----> H 2 O(g) H = -242 kj SO 2 (g) > S(s) + O 2 (g); H 1 = 297 kj - 1 H 2 (g) + S(s) + 2O 2 (g) > H 2 SO 4 (l) H 2 = -814 kj - 2 H 2 O(g) ----->H 2 (g) + 1/2 O 2 (g) H 3 = +242 kj - 3 SO 2 (g) + 1/2 O 2 (g) + H 2 O(g) -----> H 2 SO 4 (l) ªH = H 1 + H 2 + H 3 ªH = ªH = -275 kj 4

5 Calculate Heat (enthalpy) of Combustion: 2 nd method C 7 H 16 (l) + 11 O 2 (g) -----> 7 CO 2 (g) + 8 H 2 O(g) ; H =? H f (C 7 H 16 ) = kj/mol H f (CO 2 ) = kj/mol H f (H 2 O) = kj/mol H f O 2 (g) = 0 (zero) What method? 2 nd method ªH = [3n ( ªH o f ) Products] - [3n ( ªH o f ) reactants] ªH = [ 7( (-285.9)] - [ (0)] = [ ] - [-198.8] = = kj = 4843 kj Why is ªH o f of elements is zero? ªH o f, Heat formations are for compounds Note: ªH o f of elements is zero 5

6 What is 2 nd 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 S univ = entropy of the Universe S sys = entropy of the System S surr = entropy of the Surrounding S univ = S sys + S surr 6

7 S univ = S sys + S surr S univ S sys S surr ( S sys > S surr) ( S surr > S sys) 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 = - T S uni. G = H - T S. 7

8 How do you get: G = H - T S. s univ = s sys + s surr s surr = - H sys /T) S univ = S sys - H sys /T S univ x T = T S sys - H sys : x T - S univ x T = -T S sys + H sys : x -1 - S univ x T = H sys -T S sys - S univ x T = G; G = H sys -T S sys or G = H - T S. 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.) N 2 (g) n = 2-4 = H 2 (g) > 2 NH 3 (g) If n is - S is negative (decrease in S) If n is + S is positive (increase in S) 8

9 Predict S! 2 C 2 H 6 (g) + 7 O 2 (g)--> 4 CO 2 (g) + 6H 2 0(g). 2 CO(g) + O 2 (g)-->2 CO 2 (g). HCl(g) + NH 3 (g)-->nh 4 Cl(s). H 2 (g) + Br 2 (l) --> 2 HBr(g). Calculating S of reactions Based on Hess s Law second method: S o =3 S o (prod.) - 3 S o (react.) How do you calculate G There are two ways to calculate G for chemical reactions. i) G = H - T S. ii) G o = 3 G o f (products) - 3 G o f (reactants) 9

10 How do you calculate G at different T and P G = G o + RT ln Q Q = reaction quotient at equilibrium G = 0 0 = G o + RT ln K G o = - RT ln K If you know G o you could calculate K 10