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Chapter 15: Chemical Equilibrium Thursday, February 2, 2017 3:27 PM The Concept of Equilibrium Equilibrium: a state in which there are no observablechanges as time goes by Chemical equilibrium is achieved when: 1. The rates of forward and reverse reactions are equal and 2. The concentrationsof the reactants and the products remain constant When a reaction has reached equilibrium it does not mean there are equal concentrations of reactants and products. Kfwill be larger than K r Forward reaction:rate = k [A]f Reverse reaction:rate = k [B]r The only way k [Af=k [Br at equilibrium is if the concentrationof B is higher than A. After equilibrium is established for a chemical reaction, the forward reaction rate is equal to the reverse reaction rate. Equilibriumwill be established regardless of the starting concentrations of reactants or products. The Equilibrium Constant Kcis a ratio of product concentrationsto reactant concentrations Kcis sometimesK or jeqt K p r K = [Products] / [Reactants] K is independent of concentration changes Dependent on the temperature K= k fk r Equilibrium constant expression can be written directly from the balanced equation and is independent of mechanism Kpis a ratio of gaseous product partial pressures after an equilibrium is reached K p K (cT)^Δn Δn applies to RT only Δn = moles of gaseous products - moles of gaseous reactants = (c+d) - (a+b) When Δn = 0, K = Kc p When: Equilibrium Will K >> 1 Shift Right Favor products K<<1 Shift