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The reaction rate is represented as change in concentration over change in time. Since the concentration of reactants decreases as time goes on, the change in concentration is a negative value. To make the reaction rate a positive value, we always place a negative sign in front of the change in concentration of reactants.
The concentration of the reactants will decrease over the reaction, so without the negative sign, the reaction rate for the reactants would be a negative number, as it is calculated from the change in concentration over the change in time. However, we want to work with positive reaction rates, so by adding a negative sign in front of the change in concentration of the reactants, we make the reaction rate positive.
Since we are calculating the rate of the forward reaction, we are assuming that the reactants are being used up and converted into the products. Therefore, the concentration of reactants is decreasing and the change in concentration will be negative.
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