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IC S2 K2 Interpret the equilibrium constant expression from the chemical equation of equilibrium reactions. P3 Process and present information from secondary sources to calculate K from equilibrium conditions

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The equilibrium constant (Kc) is a number that indicates the relationship between the concentrations of reactants and products at equilibrium

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The equilibrium constant K c for a particular reaction remains the same at constant temperature.

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In this example, a moles of substance A react with b moles of substance B.

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The products are c moles of substance C and d moles of substance D.

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To work out the expression for K c, we look at the products first.

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The square brackets show that we are using the concentration of substance C in mol dm -3.

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We need to raise this to the power c.

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We do the same thing for substance D. Notice that the two values will be multiplied together.

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We need to raise this to the power a.

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We do the same thing for substance B to complete the expression.

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Let’s apply this to a chemical equation.

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This is the equation for one of the reactions involved in the manufacture of sulphuric acid.

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We use the product first.

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Its concentration must be raised to the power of 2 – it must be squared.

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Next, we look at the reactants. We use the concentration of SO 2 in mol dm -3.

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Its concentration must be raised to the power of 2 – it must be squared.

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Notice that the concentration of O 2 does not need to be raised to any power, because there is no number in front of it in the equation.

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This is the chemical equation for one of the stages in the manufacture of sulphuric acid.

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This is the chemical equation for one of the stages in the manufacture of ammonia by the Haber process.

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Calculating the equilibrium constant

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K c =[COCl 2 ] [CO][Cl 2 ]

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K c =[COCl 2 ] [CO][Cl 2 ]

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K c =[COCl 2 ] [CO][Cl 2 ]

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K c =[COCl 2 ] [CO][Cl 2 ] K c =3.2=200.40

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This is the chemical equation for the hydrolysis of ethyl ethanoate.

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This is the corresponding expression for the equilibrium constant, K c.

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The table shows experimental data for the equilibrium concentrations of each substance at 293 K.

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Substitute the correct values for each equilibrium concentration.

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Remember to raise numbers to the appropriate power, if necessary.

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In this example, it is not necessary to raise any of the numbers to a power.

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This is the value of K c for this chemical reaction, given the data in the table.

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In this example, all the units of concentration cancel out, so K c has no units. If K c does have units, you must work them out.

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