value of gas constant in the unit of bar dm3 k -1 mole -1
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Answer:
kf p(H2)p(I2) = kb p(HI)p(HI), so that we can now define the equilibrium constant K as the ratio of the rate constants kf /kb:
kf /kb = p(HI)2/p(H2)p(I2) = K
In general, this can be written as:
K = p(products)POWERS/p(reactants)POWERS
. The molar concentration of AgCl in pure solid AgCl is therefore 5600/143.32 = 39.07 mol/litre.
Gas Phase Reactions
When a reaction occurs entirely in the gas phase, the activities of the gaseous reactants and products are replaced by their partial pressures. These partial pressures are the partial pressures which actually prevail at equilibrium, not the partial pressures of the gases which may have been mixed initially.
Example. The equilibrium constant for the reaction H2(g) + Cl2(g) larrow.GIF (55 bytes)rarrow.gif (63 bytes) 2HCl(g) is written as:
K = a(HCl)a(HCl)/a(H2)a(Cl2) = a2(HCl)/a(H2)a(Cl2)
K = p2(HCl)/p(H2)p(Cl2)
Explanation of Le Chatelier's Principle: Added Reactants
The effect of adding reactants to a system at chemical equilibrium is to increase the concentration or partial pressures of the products; the equilibrium concentrations of the reactants will be less than the sum of the original concentrations and those added due to the equilibrium reaction.
Example. The equilibrium for the formation of hydrogen iodide, H2(g) + I2(g) larrow.GIF (55 bytes)rarrow.gif (63 bytes) 2HI(g), is governed by the equilibrium constant K = p2(HI)/p(H2)p(I2). If the equilibrium partial pressures of hydrogen and iodine are taken to be, or made to be, exactly 1.0 on some pressure scale (atmospheres, bars, torr, pascals, or kilopascals) then the equilibrium partial pressure m of hydrogen iodide is given by K = x2/(1.0)2. If the value of the equilibrium constant is known, the value of x can be calculated.
If an additional partial pressure z of the reactant hydrogen is added, then the partial pressure of hydrogen would become 1.0 + z. The partial pressure of hydrogen having increased, there is a stress on the equilibrium which is relieved by removal of some partial pressure of hydrogen, y, through the chemical reaction. The stoichiometry of the reaction and the equilibrium constant would then give
K = (x + 2y)2/(1.0 + z - y)(1.0 - y)
because x + 2y is the new equilibrium partial pressure of hydrogen iodide, 1.0 + z - y is the new equilibrium partial pressure of hydrogen, and 1.0 - y is the new equilibrium partial pressure of iodine. The new equilibrium partial pressure of hydrogen is m + 2y because two moles of HI are produced for every mole of H2 which reacts. If the value of x is calculated as above, and the value of the added partial pressure z is known, then the only remaining unknown is y. One equation in one unknown can be solved for y and then the values of the actual partial pressures at the new position of chemical equilibrium can be calculated. We will carry out these calculations in another section.
The effect of adding a reactant to a reaction at equilibrium is to decrease the concentrations of all of the reactants (as well as the concentration of the reactant added, though only to the extent of part of the original increase) and to increase the concentrations of all products. A chemical equilibrium shifts to the right when a component is added on the left, or reactant, side.
Explanation of Le Chatelier's Principle: Added Products
The effect of adding products to a system at equilibrium is the reverse of adding reactants. The equilibrium will shift to the left, increasing the pressures or concentrations of the reactants.
Example. For the example already used above, let z now represent the added partial pressure of the product HI. Then K = p2(HI)/p(H2)p(I2) with the equilibrium state before the addition being, as before, K = x2/(1.0)2. The partial pressure of hydrogen iodide after addition and re-equilibration is then x + z - 2y, where y is again the change in partial pressure of hydrogen or iodine due to reaction. These partial pressures are both originally taken as 1.0 (atmospheres, bars, torr, pascals, or kilopascals), so
K = (x + z - 2y)2/(1.0 + y)2
The effect of addition of product is to shift the equilibrium to the left, back toward the reactants. The final concentrations of hydrogen iodide, iodine, and hydrogen will all be higher after addition and re-equilibration than before the addition of hydrogen iodide.
Explanation of Le Chatelier's Principle: Total Pressure
All chemical equilibria are affected to some degree by pressure, but in most cases the equilibrium constant varies only slightly with pressure and this is the only effect of pressure on the equilibrium. When gases are involved in the equilibrium, however, the effect of pressure can be much more significant. This greater effect can be understood in terms of the principle of Le Chatelie.
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