When studying many chemical processes it is essential to know the mechanisms by which different concentrations affect the rate of a reaction. The term "order of reaction" refers to how the concentration of one or more reactants (chemicals) affects the speed with which the reaction develops. The overall reaction order is the sum of the orders of all reactants present; While looking at a balanced chemical equation will not help you determine this value, you can still get all the information you need by studying the kinetic equation or plotting the reaction itself.
Steps
Method 1 of 3: Analyzing the Kinetic Equation
Step 1. Distinguish the kinetic equation from that of the reaction
You can only determine the order of reaction from this formula, which shows the increase or decrease of a certain substance over time. The other reaction-related equations are not particularly useful for this purpose.
Step 2. Recognize the order of each reagent
Each compound listed in the reaction has an exponent which can be 0, 1 or 2 (those above 2 are very rare). These exponents define the order of the reagent they accompany. In detail:
- An exponent of 0 indicates that the concentration of that reagent has no effect on the kinetics of the reaction.
- A value of 1 corresponds to a compound whose concentration increases the reaction rate in a linear manner (doubling the reagent doubles the rate).
- An exponent equal to 2 indicates a reaction rate that progresses quadratically with respect to the change in concentration (doubling the reagent the rate quadruples);
- Null-order reactants are often not listed in the kinetic reaction, since any number raised to 0 equals 1.
Step 3. Add up all the reagent orders
The overall order of the reaction corresponds to the sum of all these values, it is therefore sufficient to proceed with a simple addition of all the exponents. Typically, the final value is 2 or less.
For example, if one reactant is first order (exponent 1) and the next is also first order (exponent 1), the reaction is second order (1 + 1 = 2)
Method 2 of 3: Draw the Graph
Step 1. Find the variables needed to draw a linear graph of the reaction
When the graph is linear, it means that there is a constant variation; in other words, the dependent variable changes in a manner directly proportional to the independent one. A line graph produces a line.
Step 2. Draw the graph of concentrations versus time
By doing so, you determine the amount of reactant that remains in the various stages of the reaction. If the graph is linear, it means that the concentration of this substance does not affect the speed of the process; consequently it is possible to affirm that the compound is of null order.
Step 3. Plot the natural logarithm of the concentration of a reactant versus time
If the path is a straight line, you can say that the substance is of the first order. This means that the concentration of this compound plays a role in the speed of the reaction; if you don't get a straight line, you need to verify that the reagent is second order.
Step 4. Draw a graph showing the variation of the reciprocal of the concentration of a reagent with respect to time
This means that the rate of the reaction increases by the square of each increase in concentration. If the graph obtained is not linear, you must try to plot that of reactions of zero or equal to 1 degree.
Step 5. Find the sum of the orders of all reagents
Once you have identified the linear graph of each substance, you know its order; then you just need to add these values and find the total order of the reaction.
Method 3 of 3: Solving Practical Problems
Step 1. Determine the order of a reaction when, doubling the concentrations of all reactants, the rate doubles
You must know that when the concentration of the compound influences the kinetics in a linear way, you are faced with a first order reactant. This means that both reactants are of first order and consequently the sum of the exponents is equal to 2; the reaction is second order.
Step 2. Find the order of reaction in case doubling both reactants does not trigger any change in kinetics
If changing the concentrations of the substances does not produce changes in the speed of the reaction, it means that these substances are of null order; in this case, they have exponent equal to 0 and the reaction itself has a null order.
Step 3. Identify the order of reaction in case doubling the concentration of a reagent quadruples the rate
When a substance produces this effect, it means that it is of the second order; the other reagent does not generate any effect and for this reason it is of null order. The sum between the exponents of the compounds therefore corresponds to 2 and the reaction is second order.
