CHM1011 Lecture Notes - Lecture 9: Magic 2Ch, Dynamic Equilibrium, Reaction Quotient

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31 Jul 2018

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Thermodynamic equilibria: do all chemical reactions go to completion, compare these two reactions: (i) mg(s) + 2 hcl(aq) mgcl2(aq) + h2(g) This reaction goes to completion, which is indicated by the single headed arrow only going in one direction (ii) ch3cooh(aq) + ch3ch2oh(aq) ch3cooch2ch3(aq) + h2o(l) The double headed arrow indicates the reaction occurs in both directions, i. e. it is an equilibrium. Starting with just n2 and h2 in the reaction vessel, the following occurs during the reaction: Meaning of kckc indicates the extent of a reaction (i. e. how far the forward reaction proceeded before equilibrium was established) If kc is small (< 10-3), then the reactants are favoured. If kc is large (> 103), then the products are favoured. If 10-3

Related Questions

Very few chemical reactions go to completion. Instead, most reach a state called chemical equilibrium in which both products and reactants are present. The concentrations of reactants and products can be predicted using the equilibrium constant, Kc, which is a mathematical expression based on the chemical equation. For example, in the reaction

aA(aq)+bB(aq)âcC(aq)+dD(aq)

where a, b, c, and d are the stoichiometric coefficients, the equilibrium constant is

Kc=[C]c[D]d[A]a[B]b

where [A], [B], [C], and [D] are the equilibrium concentrations. If the reaction is not at equilibrium, the quantity can still be calculated, but it is called the reaction quotient, Qc, instead of the equilibrium constant, Kc.

Part A

A mixture initially contains A, B, and C in the following concentrations: [A]=0.600mol Lâ1, [B]=1.25mol Lâ1, and [C]=0.400mol Lâ1. The following reaction occurs and equilibrium is established:

A(aq)+2B(aq)âC(aq)

At equilibrium, [A]=0.450mol Lâ1 and [C]=0.550mol Lâ1. Calculate the value of the equilibrium constant, Kc .

Learning Goal:

To determine equilibrium concentrations from initial conditions.

The reversible reaction

XY(aq)âX(aq)+Y(aq)

has a reaction quotient Qc defined as

Qc=[X][Y][XY]

Because the reaction is reversible, both the forward and reverse reactions will occur simultaneously. The reaction will eventually reach equilibrium, at which point the concentrations do not change, and Qc is equal to a constant known as Kc.

Part C

Based on a Kc value of 0.180 and the data table given, what are the equilibrium concentrations of XY, X, and Y, respectively?

Express the molar concentrations numerically.

CHM1011 Lecture Notes - Lecture 9: Magic 2Ch, Dynamic Equilibrium, Reaction Quotient

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