S.4 Chem. Rate Of Reaction And Equilibrium Caltec S.4

1 RATE OF REACTION AND EQUILIBRIUM Rate of reaction Some reactions proceed very fast. For example, when aqueous ammonia is added to a solution of lead(II) salts, a white precipitate forms immediately. Some reactions proceed at moderate speed. For example, it takes some time for the reaction between calcium carbonate and dilute hydrochloric acid to come to completion. Other reactions are slow. For example, it takes iron a few days to rust in moist air. The above mentioned reactions proceed at differ ent rates. The rate of a chemical reaction is the progress of the reaction in unit time. In other words, the rate of a chemical reaction is the rate at which products are formed or the rate at which reactants are used up in the reaction. Rate of reaction = concentration in moles per litre time in seconds Units for the rate of reaction are moles/litre/second i.e mol/l/s. Determination of rate of reaction Let us consider the reaction between magnesium and dilute hydrochloric acid. Mg(s) + 2HCl (aq) → MgCl 2 (aq) + H 2(g) The determination of the rate of this reaction can be done by either measuring the volume of hydrogen evolved with time or by measuring the time a given length of magnesium ribbon takes to dissolve in varying concentrations of the acid. Determination of rates of reaction by measuring the volume of the gas evolved with time A known mass of magnesium and a known volume of dilute hydrochloric acid in a test -tube tied with a thread, are placed in a conical flask and the experiment is set u p as shown in figure 11.1. The stopper is opened for a moment so that thread is free. The test -tube drops pouring hydrochloric acid into the conical flask. At the same time, the …