Calculating Reaction Quotient, Effect of Temperature on Equilibrium

This is part of the HSC Chemistry course under the topic Calculating Equilibrium Constant. 

HSC Chemistry Syllabus

  • Deduce the equilibrium expression (in terms of Keq) for homogeneous reactions occurring in solution.

    • Perform calculations to find the value of Keq and concentrations of substances within an equilibrium system, and use these values to make predictions on the direction in which a reaction may proceed. 

      • Qualitatively analyse the effect of temperature on the value of Keq

      • Conduct an investigation to determine Keq of a chemical equilibrium system, for example:

      – Keq of the iron (III) thiocyanate equilibrium

       

        How to Calculate Equilibrium Quotient (Q)

        In this video, we will be exploring how to calculating the reaction quotient and determine what changes are necessary for a non-equilibrium to go to equilibrium. 

        What is Reaction Quotient (Q)?

        For chemical reactions that are not yet at equilibrium, the concentration/partial pressure of reactants and products can be expressed in the form of an equilibrium quotient (Q).

        $$Q=\frac{{[C]}^c{[D]}^d}{{[A]}^a{[B]}^b}$$

         

        • If Q < Keq, the forward reaction rate of the chemical system will be greater than the reverse reaction rate. This is so that the concentration of product(s) increases while that of reactant(s) decreases. Eventually, Q will increase in value and become Keq.

          • If Q > Keq, the reverse reaction rate of the chemical system will be greater than the forward reaction rate. This is so that the concentration of reactant(s) increases while that of product(s) decreases. Eventually, Q will decrease in value and become Keq.

            Therefore, a chemical system will proceed in such a way that quotient Q will eventually equal to the equilibrium constant Keq.

             

            Effect of Temperature on Equilibrium Constant

            • Temperature is the only factor (change in condition) that affects the equilibrium constant of a reaction.

             

            Endothermic reactions

            • An increase in temperature increases the equilibrium constant of an endothermic reaction. Vice versa. 

            Consider a generic reversible reaction at equilibrium:

             

             $$A_{(g)}+B_{(g)}\rightleftharpoons AB_{(g)} \;\;\; \Delta{H} >0$$

             

            The equilibrium expression of this reaction is: 

             $$K_{eq}=\frac{[AB]}{[A][B]}$$

             

            An increase in temperature favours the forward reaction because it has a higher activation energy compared to the reverse reaction. So, reactants A and B will be consumed to form AB.

             

             
            `[A_{(g)}]`
            `[B_{(g)}]`
            `[AB_{(g)}]`

            Initial
            `[A]`
            `[B]`

            `[AB]`

            Change

            		 `-x` `-x` `+x`

            Equilibrium

            		 `[A]-x` `[B]-x` `[AB]+x`

             

             

            After heating, the new equilibrium constant becomes:

             $$K_{eq}=\frac{[AB]+x}{([A]-x)([B]-x)}$$


            As the numerator increases and denominator decreases, the equilibrium constant increases in value.

             

            Exothermic reactions

            • An increase in temperature decreases the equilibrium constant of an exothermic reaction. Vice versa.

            Consider a generic reversible reaction at equilibrium:

             $$A_{(g)}+B_{(g)}\rightleftharpoons AB_{(g)} \;\;\; \Delta{H} <0$$

             

            The equilibrium expression of this reaction is:

             

            $$K_{eq}=\frac{[AB]}{[A][B]}$$

             

            An increase in temperature favours the reverse reaction because it has a higher activation energy compared to the forward reaction. So, product AB will be consumed to form A and B.

             

            		 `[A_{(g)}]` `[B_{(g)}]` `[AB_{(g)}]`

            Initial

            		 `[A]` `[B]`

            `[AB]`

            Change

            		 `+x` `+x` `-x`

            Equilibrium

            		 `[A]+x` `[B]+x` `[AB-x]`

             

            After heating. the new equilibrium constant becomes:

             

             $$K_{eq}=\frac{[AB]-x}{([A]+x)([B]+x)}$$

             

            As the numerator decreases and denominator increases, the equilibrium constant decreases in value.

             


            Previous section: Calculating Equilibrium Constant (ICE Table) 

            Next section: Using Colourimetry to Calculate Equilibrium Constant

             

            BACK TO MODULE 5: EQUILIBRIUM AND ACID REACTIONS