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If a state of equilibrium is reached as a result of the balancing of two opposing reactions, it is the same no matter which of the two groups of substances separated by the sign <=> we bring together in the first instance. The same state of equilibrium is reached on heating hydrogen iodide at 444° for a sufficient time as on heating a mixture of hydrogen and iodine vapour, in equivalent proportions, at the same temperature: H2 + I2 <=> 2HI.
This is shown in.
 | Fig: Curves illustrating attainment of equilibrium state |
A CD represents the amounts of hydrogen iodide left after various times when that gas is heated; BCD represents the amounts of hydrogen iodide formed from hydrogen and iodine Both curves gradually coalesce to a horizontal line, CD, where equilibrium is reached. No further change then occurs: H2 + I2 <=> 2HI. Equilibrium is a state which is independent of time. This example shows that both reactions can go on under the same conditions; in tne equilibrium state both are still proceeding, but the amount of hydrogen iodide formed in any instant is exactly equal to the amount decomposed. The two reactions are balanced, and a state of kinetic equilibrium is attained.
The conception of equilibrium as the balance of two opposing reactions follows from the kinetic theory. A liquid comes into equilibrium with its vapour when as many molecules leave the liquid as return to it in a given interval. A salt is in equilibrium with its saturated solution when as many molecules break away from the solid per second as are caught up again, possibly in a different part of the crystal. Barium peroxide heated in a closed vessel at a constant temperature, breaks up into baryta and oxygen: 2BaO2 => 2BaO + O2. The oxygen molecules, by collision with the baryta, reproduce molecules of barium peroxide. The higher the oxygen pressure, the more frequent are collisions of oxygen molecules on the baryta and the greater is the rate of recombination. The rate at which the peroxide molecules break up is constant at a given temperature, hence at a certain pressure of oxygen the rate at which peroxide is reproduced ' becomes equal to the rate at which it is decomposed. A state of equilibrium is therefore set up at a definite pressure of oxygen, called the dissociation pressure: 2BaO2 &lr;=> 2BaO + O2. If the oxygen pressure is raised, the collisions become more frequent, additional combination takes place, and if the pressure is maintained above the dissociation pressure, all the oxygen is reabsorbed by the baryta. If the pressure of the oxygen is decreased more peroxide decomposes, since less oxygen returns to it by collisions, and if gas is continuously pumped off all the peroxide is ultimately decomposed.
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