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CHEM 1A03 (173)


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McMaster University
David Brock

Chapter 15: Dynamic Chemical Equilibrium - N2O4 (g)equilib arrows  2NO2 (g) o At low T, we have mostly N2O4 o At high T, we have mostly NO2 o I general we have a mixture of the two gases Equilibrium Constant (K) - K is a thermodynamic parameter characterizing the tendency for a chemical reaction or process to occur - Chemical processes are reversible – the extent of reaction (amt of product formed - yield) is determined by K - K is a ratio of product to reactant activities, a (effective concentration of pressure ) at equilibrium – the form of K depends on the balanced chemical equation - General Reaction in Aqueous Solution: - Activity = [Solute] (assuming gamma & [S] areoboth 1) - Activity of a gas: a = gamma P G P ,O,e use a = P G Activity and Equilibrium Concentrations - Example: dissolution of CaCO3(s) 2- 2- - Pure splid  CaCO (s3  Ksp  CO 3 (aq) + Ca (aq) - Ksp = [CO 3[Ca ]  concentrations at eqbm - K appears as unitless b/c all pressure and concentrations are relative to reference values - Pure solids and liquids have an activity of 1 and don’t include them in equilib calculations K vs Kc and Kp - We reserve the substrict on K to denote the type of reaction – eg. Ksp for dissolution reactions - K expression may include concetrations (aqueous species ) and/or pressures (gases) - Kc when activities are concentrations - Kp when activities are partial pressures - Defines to what extent a reaction is taking place – equilibrium expression Reaction Quotient, Q - K = ratio of products to reactants at equilibrium (a constant at a given temp, T) - Q = ratio of products to reactants at any instant ( not a constant, can hold any value) - Relative magnitudes of Q to K determines which direction a reaction will proceed – specifically.. - Reaction proceeds in the forward direction if QK - Q is the exact same as K equation, calculate it the same way Reaction Quotient and Direction of Equilibrium Q = products / reactants k = products / reactants - Q = 0 when products/reactants = 0/x (purely reactants) - Qc < Kc ,Q must get bigger to reach equilibrium which means more products have to be produced therefore reaction goes towards the right to reach equilib so its left of equilibrium - If Q > K, Q must decrease to reach equil so it must go left aka reverse reaction aka right of equil. - Qc = infinity, when products / reactiants = x / #approaching 0 aka pure products K ow & log K ow - Kow is the octanol-water partition coefficient and gives a measure of lipophilicity of solute (S) molecule - Large Kow = high lipophilic - Kow = [S organic[Saqueous - Lipophilicity – assign to molecules to get a general idea… solubility in fat, high lipophilicity goes straight to fat and stays there and accumulates, low lip
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