Writing Equilibrium Expressions for Chemical Reactions

To write an equilibrium expression for a chemical reaction, examine the chemical equation and follow the preceding definition. For example, suppose we want to write an equilibrium expression for the reaction  

The equilibrium constant is [NO2] raised to the fourth power multiplied by [O2] raised to the first power divided by [N2O5] raised to the second power. 

Notice that the confidents in the chemical equation become the exponents in the equilibrium expression. 

EXAMPLE 15.1 Writing Equilibrium Expressions for Chemical Reactions 

The equilibrium expression is the concentration of the products raised to their stoichiometric coefficients divided by the concentrahon of the reactants raised to their stoichiometric coefficients. Notice that the expression is a ratio of products over reactants. Notice also that the coefficients in the chemical equation are the exponents in the equilibrium expression. 

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In accordance with the general discussion of chemical equilibrium given in Chapter 19, we may write fhe follo%ving equilibrium expression for this reaction ... 

Describe how we write the equilibrium expression for a reaction. Give three examples of balanced chemical equations and the corresponding expressions for their equilibrium constants. 

Write chemical equations and equilibrium expressions for the reactions of each of the following weak acids with water (a) acetic acid, CH3COOH, (b) propanoic acid, C2H5COOH, (c) hydrofluoric acid, HE, (d) hypochlorous acid, HCIO, (e) carbonic acid, H2CO3... 

The problem asks for a qualitative analysis of a chemical equilibrium. We must visualize what takes place at the molecular level, describe the system in words, draw pictures that summarize the reactions, and then use the ideas developed for the NO2 /N2 O4 reaction to write an expression for the equilibrium constant. 

C16-0055. Write each of the chemical reactions of Problem in the opposite direction and determine the equilibrium constant expressions for these reactions. 

This criterion of equilibrium provides a general method for determination of equilibrium states. One writes an expression for G as a function of the numbers of moles (mole numbers) of the species in the several phases, and then finds the set of values for the mole numbers that minimizes G subject to the constraints of mass conservation. This procedure can be applied to problems of phase, chemical-reaction, or combined phase and chemical-reaction equilibrium it is most useful for complex equilibrium problems, and is illustrated for chemical-reaction equilibrium in Sec. 15.9. 

Write a chemical equation that represents the ionization of a weak acid, HA. Write the equilibrium constant expression for this reaction. What is the special symbol used for this equilibrium constant ... 

When steam is passed over iron filings, solid iron(III) oxide and gaseous hydrogen are produced in a reversible reaction. Write the balanced chemical equation and the equilibrium constant expression for the reaction, which yields iron(III) oxide and hydrogen gas. 

Explain how water is an amphoteric substance. Write the chemical equation for the autoionization of water. Write the expression for the equilibrium constant, Kyf, for this reaction. What values does have at 25 °C What are [H ] and [OH ] in pure water at 25 °C How does [H ] compare to [OH ] in an acidic solution How does [H ] compare to [OH ] in a basic solution ... 

Compare homogeneous and heterogeneous equilibria. Give a balanced chemical equation and write the corresponding equilibrium constant expression as an example of each of these cases. How does the fact that an equilibrium is heterogeneous influence the expression we write for the equilibrium constant for the reaction ... 

What is the equilibrium concentration of the weak acid HONO in a 0.80 M solution of HONO Write the chemical reaction of HONO with water. Write the expression for What is the numerical value of the hydronium ion concentration ... 

Equation 15.17 is the reciprocal of the expression in Equation 15.16. The equilibrium-constant expression for a reaction written in one direction is the reciprocal of the expression for the reaction written in the reverse direction. Consequently, the numerical value of the equilibrium constant for the reaction written in one direction is the reciprocal of that for the reverse reaction. Both expressions are equally valid, but it is meaningless to say that the equilibrium constant for the equilibrium between NO2 and N2O4 is 0.212 or 4.72 unless we indicate how the equilibrium reaction is written and specify the temperature. Therefore, whenever you are using an equilibrium constant, you should always write the associated balanced chemical equation. 

Analyze We are asked to write the equilibrium-constant expression for a reaction and to determine the value of given the chemical equation and equilibrium constant for the reverse reaction. 

Being able to write an expression relating equilibrium concentrations in chemical reactions provides a powerful tool for understanding chemical equilibrium. We will soon see how to evaluate these expressions for several classes of reactions. But first, let s look at what we can learn from the numerical values of equilibrium constants. For example, we could ask the question, Which reaction is more likely to produce hydrogen gas at relatively low temperature—the BMA process or the Fluohmic process How can we use the equilibrium constants for these reactions to find the answer ... 

When we write chemical equations for reactions that go to completion, the choice of reactants and products seems logically obvious. But for an equilibrium reaction, the fact that both forward and backward reactions are important makes the choice seem much more arbitrary. We can write equations for equilibria in either direction. Neither choice is wrong, provided we also realize that the equilibrium constant and expression we use must match the equation we choose. Consider the precipitation of silver cyanide, which we ve seen in the previous example problems. We can write the equilibrium expressions for both forms of the chemical equation ... 

Given the following equilibrium expressions for a pair of gas phase reactions, find the equilibrium expression for the sum of the two reactions, and write its chemical equation. 

C -P D, is unknown. Is it still possible to apply the law of mass action to the reaction Explain, (b) Write the chemical reaction involved in the Haber process. Why is this reaction important to humanity (c) Write the equilibrium-constant expression for the reaction in part (b). 

Analyze and Plan We are given the molarity of a weak acid and are asked for the pH. From Table 16.2, for HCN is 4.9 X 10 . We proceed as in the example just worked in the text, writing the chemical equation and constmcting a table of initial and equilibrium concentrations in which the equiUbiium concentration of H is our unknown. Solve Writing both the chemical equation for the ionization reaction that forms H cuj) and the equilibiium[Pg.631]

To write the expression for K p, first write the chemical reaction showing the solid compound in equilibrium with its dissolved aqueous ions. Then write the equilibrium expression based on this equation. 

Now you substitute into the equilibrium-constant expression for the reaction. From the chemical equation, you write... 

Write the chemical equilibrium expression for the following reaction ... 

Initially, there are 3.0 mol NO2 and no N2O4 present. Write two expressions for the extent of the reaction, and one expression that must be satisfied in order for chemical equilibrium to exist. 

We can now describe how to write the equilibrium constant expression for the reaction of H2 and I2 that forms HI. First, we need to write the balanced chemical equation with a double arrow between the reactants and the products. 

Explain how dissolving a slightly soluble salt to form a saturated solution is an equilibrium process. Give three balanced chemical equations for solubility processes and write the expressions for K -p corresponding to the reactions you have chosen. When writing expressions for K p, why is the concentration... 

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