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In chemistry, there are plenty of reactions that occur at different rates. Generally, they can be categorized into three categories, which are instantaneous reactions, moderate rate reactions, and very slow reactions. Simply, the rate of a chemical reaction can be known as how fast the reaction occurs.
Chemical reaction is a process where reactants are converted into products. In this process, reactants are consumed to form products. Therefore, the concentration of the products is decreasing with time, while the concentration of the products is increasing.
The rate of a chemical reaction can be defined in terms of the concentration of the reactants or products. The rate of the reaction is the amount of consumed reactants in a unit time or the amount of formed product within a unit time.
The amount of consumed reactants can be given by the change in concentration of the reactants. Since the concentration of the reactants is decreasing with time, the change in concentration of the reactants is a negative value. Therefore, that value should be multiplied by -1 in order to get a positive value.
The amount of products is increasing with time, and the in-product concentration is a positive value. Therefore, the reaction rate can be expressed as follows,
Let’s consider a reaction where reactant A converts into product B. If the stoichiometric ratio between A and B is 1:1, the reaction can be written as follows,
In the above reaction, when one mole of A is consumed, one mole of B is formed. Therefore, the rate of consumption of A is equal to the rate of formation of B.
Now let's consider a reaction where reactant X converts into product Y and the stoichiometric ratio between X and Y is 2:1. This means 2 mol of X are consumed in order to form 1 mol of Y. This means the rate of consumption of X is two times high as the rate of formation of B. To make the consumption rate and the formation rate equal, the rate of consumption of X should be divided by 2.
That means the change in concentration must be divided by the stoichiometric ratio of each component to get the overall reaction rate. In the following example, A and B are the reactants while C and D are the products. Also, a, b, c, and d are the stoichiometric ratios of A, B, C, and D, respectively.
When a reaction proceeds, the reactant rate decreases and the product concentration increases (Figure 01). But the change in concentration of both reactants and the products is decreasing. In other words, the amount of reactants consumed or he amount of products formed in a unit time decreases.
That means the reaction rate decreases with time. Therefore, it can be observed that different rates at the different stages of the reaction.
The average rate is the rate of the reaction over a given period of time. The average rate can be calculated from the slope of the line that joins any two points of the concentration vs time graph.
Instantaneous rate is the rate at a certain time of the reaction (not within a period of time). This gives the rate of the reaction at a particular instant. The instant rate is given by the slope of the tangent line at a certain point of the concentration vs time graph.
The initial rate is the rate of the reaction when the reactants are just mixed or when the reaction is just started. At this point, the formed amount of product is negligible. Therefore, the rate is expressed in terms of the concentration of the reactants. The initial rate is the instantaneous rate at t = 0.
It is given by the slope of the tangent line to the curve at t = 0 in a concentration vs time graph. Since the slope of the tangent line is a negative value, it is multiplied by -1 in order to get a positive value.
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