Lecture Notes J: Chemical Equilibrium I

Transcription

1 Lecture Notes J: Chemical Equilibrium I 1) Motivation: Binding/Unbinding of biological molecules How do we think about reactions where two molecules stick together/ unstick together: Protein + Drug Protein:Drug The larger the binding energy, the more Protein:Drug complex we expect to find Duplex-DNA single strand DNA The DNA helix better not be too strongly bound or it will never unravel to expose its sequence for transcription 2) Law of Mass Action For a reaction: aa + bb cc + dd Consider starting with 1 atm of H 2 and 1 atm of I 2. These will react according to the following reaction: H 2 (g) + I 2 (g) 2HI (g) At equilibrium, you find 0.213atm each of H 2 and I 2 and atm of I 2 : H 2 (g) + I 2 (g) 2 HI (g) Initially 1atm 1atm 0 At equilibrium 0.213atm 0.213atm 1.573atm If we then add 1atm of I 2 (raising it to atm), the system will adjust and at equilibrium, the pressures will be as follows, H 2 (g) + I 2 (g) 2 HI (g) Initially 0.213atm 1.213atm 1.573atm At equilibrium atm 1.061atm 1.878atm Lecture Notes J Distributed on: Monday, February 21, 2005 Page 1 of 11

2 Consider the reaction: A 2 + B 2 2AB The reactants are mixed together and the following is a plot of the reactant and product concentrations with time: What is the value of the equilibrium constant? (A) 1/2 (B) 1 (C) 2 (D) 4 3) Equilibrium is dynamic Lecture Notes J Distributed on: Monday, February 21, 2005 Page 2 of 11

3 4) Reaction Quotient When the mass action expression is not equal to K, its value if denoted Q. aa + bb cc + dd Test Consider the reaction: N 2 +3H 2 2NH 3 The equilibrium constant at 400 o C is K=0.5. Suppose we make a mixture with the following concentrations: [NH 3 ] = 1.0M, [N 2 ] = 1.0M, [H 2 ] = 1.0M In which direction will the reaction go? a) N 2 +3H 2 2NH 3 b) N 2 +3H 2 2NH 3 5) Heterogenous Equilibrium (a reaction involving more than one phase) C(s) + CO 2 (g) 2CO(g) Lecture Notes J Distributed on: Monday, February 21, 2005 Page 3 of 11

4 Test Consider the equilibrium expression for the following reaction: CaCO 3 (s) CaO(s) + CO 2 (g) Which of the following is true: a) The ratio of CaO(s) to CaCO 3 (s) will be the same in all samples of this solid. b) The vapor pressure of CO 2 (g) above a mixture of CaCO 3 (s)/cao(s) is independent of the relative amount of these two solids. c) You need both CaO(s) to CaCO 3 (s) to produce CO 2 (g). 6) Relation between the equilibrium constant and G o. G reac prod G reac prod Lecture Notes J Distributed on: Monday, February 21, 2005 Page 4 of 11

5 The binding constant for a drug-protein interaction is the equilibrium constant for the reaction: Protein + Drug Protein:Drug K binding G o binding How big must the binding energy ( G o binding) be for K binding to be equal to 1000 at body temperature? What range of G o binding gives K between 1/1000 and 1000? Lecture Notes J Distributed on: Monday, February 21, 2005 Page 5 of 11

6 If G o binding is doubled, what happens to K binding? a) K stays the same b) new K=10,000 c) new K= 100,000 d) new K= 1000*e 2 If G o binding is positive, no drug binds to the protein. true false 7) Properties of Equilibrium Constants: Consider the following reactions A + 2B C Multiply reaction coefficients by n 2A + 4B 2C Reverse reaction C A + 2B Lecture Notes J Distributed on: Monday, February 21, 2005 Page 6 of 11

7 Add two reactions 1) A+B C G 1 2) B+C D G 2 3) A+2B D G 3 From a chemical handbook you find the equilibrium constants that give the concentrations of Iodine vapor above water and oil: I 2 (water) I 2 (g) K water I 2 (oil) I 2 (g) K oil But you want the equilibrium constant for the partitioning of I 2 between water and oil I 2 (water) I 2 (oil) K You can obtain it as: a) K= K water +K oil b) K= K water K oil c) K= K water /K oil d) K= K oil /K water Hemoglobin can bind up to 4 oxygen molecules. If the equilibrium constant for binding a single oxygen is K 1, what is the equilibrium constant for binding 4 oxygen molecules? a) 4K 1 b) K 1 4 c) 4 2 K 1 d) K 1 Lecture Notes J Distributed on: Monday, February 21, 2005 Page 7 of 11