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In many cases a reaction appears to go to completion instead of to a state of equilibrium. Berthollet remarked that this often results from some disturbance of the equilibrium state, by one or more of the products of the reaction being removed from the sphere of action by their "volatility, or insolubility. As soon as they leave the system, passing into the gaseous state or depositing as solids, they cease to exert any influence and the reaction by which they are produced, being no longer opposed, cannot become balanced and proceeds until the change becomes nearly if not quite complete.
When sulphuric acid is poured over common salt, a state of equilibrium is set up: NaCl + H2SO4 <=> NaHSO4 + HCl↑. The hydrochloric acid, however, escapes from the liquid as a gas (shown by the upward-pointing arrow), the state of equilibrium is disturbed, and the reaction proceeds. When the hydrochloric acid gas is expelled by heating, further reaction occurs.
When sulphuric acid is added to barium chloride solution, double decomposition ensues: BaCl2 + H2SO4 <=> 2HCl + BaSO4↓. The barium sulphate, being very sparingly soluble, is precipitated (shown by the downward-pointing arrow); in this way it is removed from the sphere of action and the reaction proceeds. The sulphate, however, is really very slightly soluble, so that when the amount dissolved is in equilibrium with the solid: BaSO4 <=> BaSO4 (dissd.), a state of equilibrium is set up. The four substances are then in solution: BaCl2 + H2SO4 <=> 2 HCl + BaSO4 (dissd.) <=> BaSO4 (ppd.). Precipitated barium sulphate is appreciably soluble in concentrated hydrochloric acid.
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