Unit 5
• State Le chatellier's principle.
• Use Le chatelier's principle to explain the effects of concentration change, pressure, temperature on the equilibrium position and the equilibrium constant of a reversible reaction.
5.2. Reversible Reactions
Reversible reactions are reactions that do not proceed to completion and the conversion of the products to reactants is also possible under a set of reaction conditions. They are represented by between the reactants and the products. Reactions in which the reactants go virtually to give products and there is no tendency for products to revert to the reactants are irreversible reactions . They are represented by
between the reactants and the products.
The reaction of magnesium with dilute hydrochloric acid, the precipitation of silver chloride when aqueous solutions of silver nitrate and sodium chloride are mixed and the reaction of aqueous solutions of acid and alkali are examples of irreversible reactions.
Mc
+ 2HCkm) —I- Mg C1240) +H2(8) AgNo300 + Nackm) Aga,) +
NaNO30,0 NaOH00) + HC1(.0 NaCk.0) + H2 000 5.2.1 Common examples of reversible reactions(a) Heating of hydrated copper (II) tetraoxosulphate (vi) crystals.
CuSO 4 .5H2 0 CuS0 4(,) + 5H2 0 (1)
When the hydrated salt is heated, water vapour is given off The blue colour of the hydrated salt changes to white and the crystals turn to powder. When a few drops of water are added to the cold white powder, the blue colour returns as the hydrated salt is formed.
(b) Heating ammonium chloride crystals
NH Sl w NH3(g) + HCL
When ammonium chloride crystals are heated the salt dissociates into ammonia gas and hydrogen chloride gas. As the gases cool, they recombine to form the solid. One test for ammonia gas is to hold the stopper of a concentrated hydrochloric acid near the source of ammonia. A dense white fume of ammonium chloride is formed.
(c) Formation of calcium hydrogen trioxocarbonate(iv)
When excess carbon (iv) oxide is bubbled through lime water (Ca(OH) 2(aq) an initial white precipitate turns clear as calcium hydrogen trioxocarbonate (iv) is formed. When this solution is heated, the precipitate returns as the insoluble calcium trioxocarbonate(iv) forms.
Ca CO3(,) + 112 00) + CO2 Ca(HCO 3)2(n) (d) Reaction of iron with steam.
When steam is passed over heated iron a slow reaction occurs producing hydrogen gas and an oxide of iron.
The reverse reaction also occurs when hydrogen is passed over the heated oxide.
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3Fe (S) + 4H20(g) Fe ,04(s) + 4H2(g) 5.2.2 Reversible reactions and the equilibrium state
When a chemical reaction occurs spontaneously, it continues until a state of dynamic equilibrium is reached in which both the forward and reverse reactions are taking place at the same rate. The concentrations of the reactants and products no longer change with time. The reaction is said to be at equilibrium or that a states
of equilibrium has been reached.
This equilibrium can only be established in a sealed system, where no ' chemicals can enter, or leave the system. Such a system is called a closed system, The equilibrium state remains unless it is disturbed. The equilibrium state can be approached from the reactant or product direction.
The equilibrium state is a dynamic state in which there is no net change in properties such as the density or concentration of the various parts of the system.
At the equilibrium state the free energy change is equal to zero (AG = 0) -Recall AG = AH — TAS
IfAG = 0
AH = Teq AS Teq equilibrium temperature AS = AH
T eq
The entropy change that accompanies the reversible reaction can be calculated if the enthalpy change is known.
5.2.3 The reversible reaction and the equilibrium constant
There is a fixed relationship, at a given temperature, between the molar concentration of the products and the reactants in the equilibrium mixture. The above is the equilibrium law.
For a reaction of the form
+B(g) g) — concentration equilibrium constant kc = [C] [D]
[A] [B] ] molar concentration
From the above you see that the equilibrium constant (Ic c) is an index of how far the reaction goes in the direction of C (g) and D(g). Very high value of k implies that the reaction goes virtually to completion.
Very small values of kc implies that the reaction does not go far in favour of C f:) and D(0 The equilibrium constant for the reverse reaction.
C + D(5) A
(5.1 (5)+B
(q (
kc = [A(v] [B(51 ] [C(g [Dig ,]
The equilibrium constant of the reverse reaction is the reciprocal of the requilibrium constant of the forward reaction.
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Now let us look at real reactions.
(a) H2(g) +12(g) "r" )
['2(g) [Hag) + 3H22NI-13(g) (g)
kc = [NH3(g) [Ng) EF12(o 3
Note that the molar concentrations are raised to the power of the reactant coefficient in the equation.
A correctly balanced equation is required to enable you write the correct equation for Ice Now write the equilibrium constant equation for the reaction.
2S0 +0 2(g) 2(g) 2S0 2(8)
For gas reactions we can also write the equilibrium constant expression using the partial pressures of the gases
For example
H2tg) + 1 2(g) 2H1
k = [PH'(01 P— partial pressure
[P112(8)1 [Pi 2(g)]
k and ke are not equal but are related. Recall that for a gas, pressure is directly proportional to the concentration.
Tile units of k and k depend on the particular reaction that is considered.
For example
H2(g) + I2(g) 2H1
(g)' k and k have no unit because the number of molecules is the same on reactant and product side.
For the decomposition reaction
N2 04Ig) 2NO2(g)
ke and k will have units of mol dm-3 and Nm-2 or mm Hg (i.e. pressure unit) K = [NO2( }2
[N2 04(p) ] kp = [Pg102(8) ?
— The equilibrium constant is related to the free energy change of the reaction AG = AG° + RT In K AG° — free energy standard At equilibrium AG = 0 and
AG° = — RT In k (b)
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