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Rate of Reaction

All chemical reaction proceeds with separate speed. Some reactions happen very quickly such as potassium reacting with water, while some reactions are very slow such as the rusting of iron or the setting of concrete. During any chemical reaction, the concentration of reactant decreases, at the same time concentration of product increases.


For a given reaction;A + B ABAs in the given reaction, the reactants A and B are consumed and the concentration of product AB increases. The reaction rate can be determined by measuring how fast the concentration of reactant decreases, or by how fast the concentration of product increases.Reaction Rate Definition: The reaction rate represents the change in concentration of reactant and product with respect to time. 


The rate curve shows hypothetical reaction profile in which red line show the consumption of the reactants and blue line show the formation of product.Rate of Reaction Equation can be represents by given expression;aA + bB  cC + dDHence rate of reaction can be represents in terms of concentration of reactant as well as product. 


The unit of rate is given by mole/litre/sec.At some instant in time, the rate of reaction is called as instantaneous rate of reaction.Instantaneous rate = -d[reactant]/dt or d[product]/dt.An instantaneous rate at time t can be calculated by the negative of the slope of the curve of concentration of a reactant or product versus time at time t as in the given curve.As the reactant consume in a reaction, reaction rate decreases with time due to decrease in the concentration of reactant. 



The instantaneous rate at the start of the reaction i.e. at t = 0 is called as initial rate of reaction. It is equal to the negative of the slope of the curve of reactant concentration versus time at t = 0.The rate of a reaction can relates to the concentrations of the reactants by using expression of rate law of reaction which based on law of active mass given by Guldberg and Waage. According to law of active mass, the reaction rate is directly proportional to the active mass of reactant. For the given reaction;aA + bB  cC + dDThe rate law will be Rate = K [A]a [B]bWhere k = reaction rate constant, and [A], [B] are active mass or molar concentration of reactants. 


By using the rate law equation, we can determine the order of reaction which is sum of stoichiometry coefficient of reactant molecules involve in rate law of reaction.For example;A BRate = K [A]Since rate law involves only one reactant molecule with stoichiometry coefficient one, hence order will be one and reaction is called as first order reaction and rate law is first order reaction rate law. Some other important terms for first order reaction are as follows;Important terms First-OrderRate Law Rate="k" [A] Integrated Rate LawUnits of Rate Constant (k):Sec-1 Linear Plot to Determine (k):ln[A] versus time Half-life:ln 2/k