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For any given chemical reaction, the rate of the reaction can be measured by either the disappearance of any of the reactants or by the formation of any of the products. If measuring the disappearance of the reactants, the change in concentration is negative so the number is negated to make a positive rate. If measuring the appearance of the products, the change in concentration is positive so the rate calculation will automatically be positive. If the different species have different coefficients in the balanced equation, then the rate will be different depending on the coefficient of the species measured. To normalize this to a general rate (called the unique rate of reaction), we divide the rate of the reactant or product by its coefficient. So in this case we are measuring the formation of Cl- as 3.6 M/min. Since the coefficient of Cl- in the balanced equation is 2, we divide 3.6 by 2 to get 1.8 M/min as the unique rate.
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