Reactivity 2.3

  • Reversible reactions - chemical reactions that can proceed both forward and backward

    • represented by 2 arrows pointing in opposite directions on top of each other

      • also means equilibrium 

  • extent of reaction - how far a reaction proceeds in the forward/reverse direction


  • physical equilibrium - physical change ocurring at equal rates in both forward + reverse directions

    • changes of state/dissolution of solute

    • REQUIRES CLOSED SYSTEM

      • no matter can escape

  • Chemical equilibrium - chemical change occurring at equal rates in both forward + reverse directions

    • involves chemical change

    • rate of forward = rate of reverse

    • concentrations of reactants/products remain constant

      • rates are equal

      • concentrations are constant

        • not equal usually

    • can be achieved from either direction

  • reaction vessel - sealed container where a chem reaction occurs

    • closed system

  • Equilibrium Constant Kc - ratio providing info about composition of a reaction mixture at equilibrium 

    • quantifies extent of reaction 

    • NO UNITS

    • K - equilibrium constant

    • c - terms of concentration

    • provides info about

      • extent of reaction

      • position of equilibrium

        • Lies to right (products) → greater concentration of products at equilibrium

          • Kc > 1

            • If exponent of 10^(4) is more than 4 → almost exclusively products

        • Lies to left (reactants) → greater concentration of reactants at equilibrium 

          • Kc < 1

            • If exponent of 10^(-4) is less than -4 → almost exclusively reactants

  • raised to the power of the molar ratio

  • products on top + reactants on bottom

  • does not include state symbols in equation


  • Kc of reverse reaction = reciprocal of Kc of forward reaction


  • Reaction quotient Q - measure of relative amounts of reactants/products for a reaction not at equilibrium 

    • determines which direction reaction proceeds to reach equilibrium 

    • compare to Kc to see if system reached equilibrium 

    • Equation for Q is same as Kc

      • Kc used AT equilibrium

      • Q used NOT AT equilibrium


  • Relationship btwn Q and Kc

    • Q < K

      • greater concentration of reactants

      • forward reaction favored

      • reaction proceeds right

    • Q = K

      • equilibrium

      • rate forward = rate reverse

    • Q > K

      • greater concentration of products

      • reverse reaction favored

      • reaction proceeds left


  • Le Chatelier’s Principle - a system at equilibrium is subjected to a change → system responds to minimize effect of change

    • system tries to counteract change made

    • changes that can be made

      • concentration

        • increase concentration → shift right (away)

          • extra needs to be used

        • decrease concentration → shift left (towards)

          • need more substance

        • Kc NO CHANGE

      • pressure

        • increase pressure → shifts to side with fewer moles of gas species

          • high pressure → more collisions → faster rate

        • decrease pressure → shifts to side with more moles of gas species

          • low pressure → fewer collisions → slower rate

        • Kc NO CHANGE

      • Volume

      • temperature

        • increase temp → away from heat/energy (favors endothermic) 

          • increase heat (reactant) → right

          • increase heat (product) → left

        • decrease temp → towards heat/energy (favors exothermic) 

          • decrease heat (reactant) → left

          • decrease heat (product) → right

        • Kc DOES CHANGE

          • shifting right → more products + bigger Kc

          • shifting left → more reactants + smaller Kc

      • catalysts

        • no shift

          • the rates are increased by the same amount for both forward and reverse reactions

        • only provides alternative pathway to lower Ea

        • Kc NO CHANGE



  • ICE Table - calculates Kc when not at equilibrium

    • I - initial

    • C - change

      • based on coefficients from balanced chemical equations

      • reactants subtract, products add

      • If Kc is very small (less than 10^(-4)) 

        • reaction barely proceeds 

        • concentration of products very small

        • assume reactant initial = reactant equilibrium

          • x is negligible

    • E - equilibrium


  • Gibbs free energy (J or kJ) is negative (G < 0) → reactions are spontaneous 

  • R has to be in kJ (8.31/1000)

  • Standard Gibbs free energy units: J mol^(-1)


  • If at equilibrium 

    • ΔG = 0

    • Q = K

    • ΔG⦵ = -RTlnK

      • cannot leave K as e^x 


  • Relationship btwn Gibbs and Kc

    • chemical reaction will always move to achieve the most stable state possible under a given set of conditions

      • thermodynamically at equilibrium

        • entropy = max value

          • system in its most disordered state

            • more disorder = more favorable

        • Gibbs free energy = min value

          • system in lowest possible energy state

            • lowest energy = most stable

    • G < 0, Kc > 1

      • high concentration of products

    • G > 0, Kc < 1

      • high concentration of reactants

    • G = 0, Kc = 1

      • neither favored

  • Spontaneous reactions

    • equilibrium favors product side

      • products > reactants

    • min value of Gibbs lies more to product side


  • Nonspontaneous reactions

    • equilibrium favors reactants side

      • products < reactants

    • min value of Gibbs lies more to reactants side