CHEMICAL EQUILIBRIA Chemical equilibrium reversible reactions A Chemical equilibrium occurs for...

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Transcript of CHEMICAL EQUILIBRIA Chemical equilibrium reversible reactions A Chemical equilibrium occurs for...

  • Katumba J. 2020 elbow grease is the best polish 1

    CHEMICAL EQUILIBRIA

    Chemical equilibrium: Is a state in which the rates of the forward and reverse

    reactions are equal and the concentrations of the reactants and products remain

    constant.

    A Chemical equilibrium occurs for reversible reactions. Many reactions do not go

    to completion since the products of the reaction themselves react to form

    original reactants.

    A reversible reaction is a reaction that takes place in both forward and backward

    directions, hence does not go to completion. For example;

    In a closed container/vessel, calcium carbonate decomposes as follows;

    πΆπ‘ŽπΆπ‘‚3(𝑠) β‡Œ πΆπ‘Žπ‘‚(𝑠) + 𝐢𝑂2(𝑔) {Reversible reaction}

    Explanation

    When calcium carbonate is heated at a fixed temperature in a closed container;

    at first, πΆπ‘ŽπΆπ‘‚3 decomposes faster than the products recombine. After a while,

    the amounts of πΆπ‘Žπ‘‚ and 𝐢𝑂2 build up to a level at which the rate of combination

    of πΆπ‘Žπ‘‚ and 𝐢𝑂2 is equal to the rate at which πΆπ‘ŽπΆπ‘‚3 dissociates, and hence, the

    system has reached a state of dynamic equilibrium.

    Dynamic equilibrium is a state of equilibrium in which the conversions of

    reactants to products and products to reactants are still going on, although there

    is no net change in the number of reactant and product molecules.

    However, if a reaction proceeds in only one direction and goes to completion, it is

    referred to as irreversible reaction.

    For example, the combustion of methane to form carbon dioxide and water

    𝐢𝐻4(𝑔) + 2𝑂2(𝑔) β†’ 𝐢𝑂2(𝑔) + 2𝐻2𝑂(𝑙)

    Characteristics of chemical equilibria

    οƒ˜ Chemical equilibria occur in reversible reactions

    οƒ˜ At equilibrium state, the rates of forward and backward reactions are

    equal

    οƒ˜ A state of dynamic chemical equilibrium occurs in a closed system.

    οƒ˜ At equilibrium state, the concentrations of both reactants and products

    remain unchanged

    οƒ˜ Chemical equilibria occur at constant temperature.

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    οƒ˜ The observable properties such as pressure, concentration, colour, density,

    viscosity etc of the system remain unchanged with time

    HOMOGENOUS AND HETEROGENEOUS EQUILIBRIA.

    A homogenous equilibrium is an equilibrium in which all the reactants and products

    are in the same phase or physical state. This may exist in liquid or gaseous phase

    or aqueous state. For example;

    οƒ˜ The reaction between sulphur dioxide gas and oxygen gas to form sulphur

    trioxide.

    𝑆𝑂2(𝑔) + 𝑂2(𝑔) β‡Œ 𝑆𝑂3(𝑔)

    οƒ˜ The reaction between ethanol and ethanoic acid to form ethyl ethanoate

    /diethyl ether (esterification) 𝐢𝐻3𝐢𝑂𝑂𝐻(𝑙) + 𝐢𝐻3𝐢𝐻2𝑂𝐻(𝑙) β‡Œ 𝐢𝐻3𝐢𝑂𝑂𝐢2𝐻5(𝑙) + 𝐻2𝑂(𝑙)

    οƒ˜ The reaction between hydrogen gas and gaseous iodine to form hydrogen

    iodide 𝐻2(𝑔) + 𝐼2(𝑔) β‡Œ 2𝐻𝐼(𝑔)

    οƒ˜ Decomposition of Dinitrogen tetroxide to form nitrogen dioxide. 𝑁2𝑂4(𝑔) β‡Œ 2𝑁𝑂2(𝑔)

    οƒ˜ Reaction between iodine solution and potassium iodide to form potassium

    triiodide complex 𝐼2(π‘Žπ‘ž) + 𝐼

    βˆ’(π‘Žπ‘ž) β‡Œ 𝐼3 βˆ’(π‘Žπ‘ž)

    A heterogeneous equilibrium is an equilibrium in which two or more phases are

    involved. For example;

    οƒ˜ Decomposition of calcium carbonate to form calcium oxide and carbon

    dioxide.

    πΆπ‘ŽπΆπ‘‚3(𝑠) β‡Œ πΆπ‘Žπ‘‚(𝑠) + 𝐢𝑂2(𝑔)

    οƒ˜ Reaction between iron and steam to form triiron tetraoxide and hydrogen 3𝐹𝑒(𝑠) + 4𝐻2𝑂(𝑔) β‡Œ 𝐹𝑒3𝑂4(𝑠) + 4𝐻2(𝑔)

    οƒ˜ Reaction between bismuth chloride and water to form bismuth

    oxychloride and hydrochloric acid 𝐡𝑖𝐢𝑙3(π‘Žπ‘ž) + 𝐻2𝑂(𝑙) β‡Œ 𝐡𝑖𝑂𝐢𝑙(𝑠) + 2𝐻𝐢𝑙(π‘Žπ‘ž)

    Note: If only ions are involved in an equilibrium, an ionic equilibrium is established.

  • Katumba J. 2020 elbow grease is the best polish 3

    THE EQUILIBRIUM LAW (LAW OF MASS ACTION)

    The law states that, β€œfor a reaction at equilibrium, the ratio of product of the

    concentration of the products to the product of the concentrations of the

    reactants each raised to the power of their stoichiometric coefficients is

    constant at constant temperature”

    Derivation of equilibrium constant expressions (Application of the law)

    Consider a hypothetical reversible reaction below;

    𝒂𝑨(𝒂𝒒) + 𝒃𝑩(𝒂𝒒) β‡Œ 𝒄π‘ͺ(𝒂𝒒) + 𝒅𝑫(𝒂𝒒)

    The rate of the forward reaction, 𝑹𝒇 = 𝑲𝒇[𝑨] 𝒂[𝑩]𝒃

    [𝑨] = concentration of 𝑨, [𝑩] = concentration of 𝑩

    The rate of the backward reaction, 𝑹𝒃 = 𝑲𝒃[π‘ͺ] 𝒄[𝑫]𝒅

    [π‘ͺ]= concentration of π‘ͺ, [𝑫]= concentration of 𝑫

    At dynamic chemical equilibrium;

    Rate of forward reaction, 𝐾𝑓 = Rate of backward reaction, 𝐾𝑏 and substituting

    for, 𝑅𝑓 and 𝑅𝑏

    𝑲𝒇[𝑨] 𝒂[𝑩]𝒃 = 𝑲𝒃[π‘ͺ]

    𝒄[𝑫]𝒅

    𝑲𝒇

    𝑲𝒃 =

    [π‘ͺ]𝒄 [𝑫]𝒅

    [𝑨]𝒂 [𝑩]𝒃 where

    𝑲𝒇

    𝑲𝒃 = 𝑲𝒄

    Therefore, the concentration equilibrium constant, 𝑲𝒄 = [π‘ͺ]

    c [𝑫]

    d

    [𝑨] a

    [𝑩] b

    The equilibrium constant, 𝑲𝒄, is the ratio of product of the equilibrium

    concentrations of products to the product of the equilibrium concentrations of

    reactants each raised to the power of their stoichiometric coefficient at a

    constant temperature.

    For a similar reaction taking place in a gaseous phase/state;

    𝒂𝑨 (π’ˆ) + 𝒃𝑩(π’ˆ) β‡Œ 𝒄π‘ͺ(π’ˆ) + 𝒅𝑫 (π’ˆ)

    The pressure equilibrium constant, 𝑲𝒑 = 𝑷π‘ͺ

    𝒄 𝒙 𝑷𝑫 𝒅

    𝑷𝑨 𝒂 𝒙 𝑷𝑩

    𝒃

    Where 𝑷𝒄, 𝑷𝑫, 𝑷𝑨 and 𝑷𝑩 are partial pressures of π‘ͺ, 𝑫, 𝑨 and π‘ͺ respectively.

    Units of equilibrium constants

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    The units depend on the number of moles involved.

    Case I: If the number of moles of the reactants and products are the same, then

    both 𝑲𝒄 and 𝑲𝒑 have no units. For example;

    π‘΅πŸ(π’ˆ) + π‘ΆπŸ(π’ˆ) β‡Œ πŸπ‘΅π‘ΆπŸ(π’ˆ)

    𝑲𝒄 = [π‘΅π‘ΆπŸ]

    2

    [π‘΅πŸ] [π‘ΆπŸ] =

    (π’Žπ’π’π’βˆ’πŸ) 2

    (π’Žπ’π’π’βˆ’πŸ) (π’Žπ’π’π’βˆ’πŸ) = 𝑡𝒐 π’–π’π’Šπ’•π’”

    𝑲𝑷 = π‘·π‘΅π‘ΆπŸ

    𝟐

    π‘·π‘΅πŸ .π‘·π‘ΆπŸ =

    (π‘΅π’Žβˆ’πŸ)

    (π‘΅π’Žβˆ’πŸ) (π‘΅π’Žβˆ’πŸ) = 𝑡𝒐 π’–π’π’Šπ’•π’”

    Case II: If the number of moles is not the same, the units will depend on the

    difference in the number of moles. For example;

    𝑷π‘ͺπ’πŸ“(π’ˆ) β‡Œ 𝑷π‘ͺπ’πŸ‘(π’ˆ) + π‘ͺπ’πŸ(π’ˆ)

    𝑲π‘ͺ = [𝑷π‘ͺπ’πŸ‘] [π‘ͺπ’πŸ]

    [𝑷π‘ͺπ’πŸ“] =

    (π’Žπ’π’π’βˆ’πŸ) (π’Žπ’π’π’βˆ’πŸ)

    (π’Žπ’π’π’βˆ’πŸ) = π’Žπ’π’π’βˆ’πŸ

    𝑲𝒑 = 𝑷𝑷π‘ͺπ’πŸ‘ .𝑷π‘ͺπ’πŸ

    𝑷𝑷π‘ͺπ’πŸ“ =

    (π‘΅π’Žβˆ’πŸ)(π‘΅π’Žβˆ’πŸ)

    (π‘΅π’Žβˆ’πŸ) = π‘΅π’Žβˆ’πŸ

    NB: When writing expressions for equilibrium constants, the following should

    be noted;

    οƒ˜ Check whether only the concentrations /moles in a given volume are given

    or total pressure at equilibrium

    οƒ˜ If only concentration/ moles are given, then an expression for 𝑲𝒄 should

    be written.

    οƒ˜ If total pressure at equilibrium is given, then expression for 𝑲𝒑 should be

    written.

    οƒ˜ Solids do not appear in the equilibrium constant expression since their

    concentration is assumed to be constant.

    οƒ˜ For a 𝑲𝒑 expression, only gaseous reactants and products appear.

    οƒ˜ If water is one of the reactants, and its concentration is not given, or

    remains unchanged, it is assumed to be present in a large excess hence

    does not appear in the equilibrium constant expression. If water is in

    gaseous state, then include it in the expression.

    οƒ˜ When only the expression is required, do not write the equation also as

    part of the answer.

    οƒ˜ In the case of 𝑲𝒄 , strictly square brackets must be used.

  • Katumba J. 2020 elbow grease is the best polish 5

    Question: In each of the following, write the equation for the equilibrium, the

    equilibrium constant expression in terms of either 𝑲𝒄 and 𝑲𝒑 or both , depending

    on what is indicated in brackets and state the units.

    a) Reaction between ethanol and ethanoic acid in presence of an acid catalyst

    (𝑲𝒄)

    b) The reaction between hydrogen gas and gaseous iodine. (both 𝑲𝒄 and 𝑲𝒑)

    c) De