Method of Initial Rates
Definition and meaning of Method of Initial Rates in chemistry.
The method of initial rates is a highly practical laboratory technique. Chemists use it to find the exact rate law for a chemical reaction. It also helps them determine the specific reaction orders for each reactant involved.
In more detail
Chemists measure the reaction speed right after mixing the chemicals together. This measured starting speed is officially called the initial rate. They run the exact same reaction several times in separate trials.
In each trial, they change the starting amount of just one reactant. They keep the amounts of all other reactants exactly the same. This careful control lets them see how one specific chemical affects the speed.
By comparing these starting speeds across trials, chemists figure out the reaction order. A reaction order is a number that shows the strength of that effect. If changing a reactant does not change the speed, its order is zero.
If doubling the reactant doubles the speed, its order is one. If doubling the reactant makes the speed four times faster, its order is two. A common mistake is thinking you can guess the rate law from the balanced chemical equation.
You must find the rate law through actual experiments like this method. Gathering these experimental orders gives the complete mathematical rate law equation.
Key facts
| Main Purpose | Finding experimental rate laws |
|---|---|
| Key Measurement | Reaction speed at time zero |
| Independent Variable | Starting concentration of one reactant |
| Dependent Variable | Initial reaction rate |
| Final Outcome | Reaction orders for every reactant |
A student studies a reaction between chemical A and chemical B. In the first trial, the student measures the starting reaction speed. In the second trial, the student doubles the amount of chemical A. The starting amount of chemical B remains completely unchanged during this trial. The student observes that the new initial speed is four times faster. This observation proves that the reaction is second order for chemical A. The student then repeats this testing process by changing chemical B. Once both orders are known, the student writes the final rate law.
Frequently asked questions
Why do chemists measure the speed at the very beginning of the reaction?
The exact amounts of starting chemicals are completely known at the very start. Also, products have not formed yet to run the reaction backward.
What exactly is a reaction order?
A reaction order is a specific number that shows how strongly a chemical changes the reaction speed. It is found only through experiments.
Can I find the rate law just by looking at the chemical equation?
No, you cannot guess the rate law from a regular balanced equation. You must use an experimental method like this one to find it.