chemical reactions two basic questions: 1) What is the driving force behind a chemical reaction? 2) How fast does a chemical reaction proceed?

Transcription

1 chemical reactions two basic questions: + + 1) What is the driving force behind a chemical reaction? 2) How fast does a chemical reaction proceed?

2 Chemical thermodynamics - What drives a chemical reaction? The key parameter is the change of the Free Enthalpy G

3 E Epot + Ekin Etherm one way

4 kinetic energy kinetic energy + thermal energy one way disorder!

5 general driving force for all spontaneous processes: the decrease of order, the decrease of free enthalpy G Ordnung one way G Ordnung < 0 Unordnung Unordnung decrease of order decrease of free enthalpy

6 the decrease of free enthalpy connected to chemical reactions - internal and external contributions: educts heat products + internal contribution to G H, leads to external contribution to G

7 Chemical reactions primarily driven by the emission of heat: 2Cu + O 2 2Na + Cl 2 2CuO 2NaCl Chemical reactions primarily driven by the generation of internal disorder: H 2 CO 3 H 2 O + CO 2 NH 4 Cl HCl + NH 3

8 Summary The key criterium is the drcrease of the free enthalpy (or G < 0 ). Two contributions to a negative enthalpy change are important: - the emission of heat - the generation of internal disorder

9 Chemical kinetics - how fast are chemical reactions? v = velocity of a chemical reaction The velocity of a given chemical reaction generally depends on three parameters: - the concentration of the educts - the temperature - the presence or the absence of a catalyst

10 time frame of important chemical reactions slow s fast typical reactions in organic chemistry: substitutions, additions... enzymatically catalyzed reactions typical reactions in inorganic chemistry: dissociations, reductions, oxidations diffusion controlled elementary reactions molecular rotation molecular vibration electronic excitation

11 Chemical kinetics - dependence of v on concentrations v generally depends on the concentration of the reaction educts A, B, C,... (with concentrations c A c B c C... ) according to the equation: a b c v = k c A c B c C... with k being the rate constant of the reaction. The exponents a, b, c,... characterize the individual dependencies of v on the concentrations of A, B, C,... The sum s = a+b+c+... is called the order of the reaction.

12 case A: zero order kinetics v = k v is independent on the concentrations model assumption: t =

13 educt 1 zero order kinetics: v = k 2 rule of the game : Start with an ensemble of dice and remove two of them at each step of the game. These dice are called the product

14 example: decomposition of ammonia on a platinum surface NH3 H2 N Pt

15 case B: first order kinetics v = kc A v depends linearly on a single concentration v educt t

16 educt 1 first order kinetics: v = kc A rule of the game : 2 All dice are rolled, each dice showing 6 points is removed and called product 3 4 5

17 example: replication of bacteria t = example: radioactive decay

18 case C: second order kinetics v = kc A c B v linearly depends on two different concentrations or on the square of one concentration v model assumption: 90% B 50% / 50% 90% A

19 1 second order kinetics: v = kc A c B rule of the game : 2 All dice are rolled, each pair of dice with mixed colors showing points is removed and called product 3 4 5

20 interpretation of the rate constant k: k k = A * frequency factor: The fraction of molecules that fulfill sterical conditions - collision frequency - sterical factors (conformation, molecular orientation) n e - E a /RT Boltzmann-factor: The fraction of molecules With an energy higher than E a n E>E a a E = e - E a /RT a E

21 slow s classical time frame of important chemical reactions: observation techniques flow methods temperature relaxation pressure relaxation ultrasonics spectroscopy fast typical reactions in organic chemistry: substitutions, additions... enzymatically catalyzed reactions organic chemistry: dissociations, reductions, oxidations diffusion controlled elementary reactions molecular rotation molecular vibration electronic excitation

22 Summary The velocity v of a given chemical reaction generally depends on: - the concentration of the educts: zero order reactions: v = k first order reactions: v = kc A second order reactions: v = kc A c B or v = kc A 2 - the temperature: according to k = A exp(-e a /RT) - the presence or the absence of a catalyst: a catalyst lowers the activation energy E a