Reaction Rates — Definition & Overview

Reaction Rates

We know that a chemical reaction converts reactants to products. The atoms or molecules collide. If they have the proper energy and orientation, they react. Bonds are broken and reformed. And if the reaction is spontaneous, the products are more stable than the reactants. But, just because a reaction occurs does not mean it's a quick process. When I was in college, I performed a reaction that took three days! We started it Friday afternoon and let it react over the weekend!

We're beginning to study chemical kinetics. Kinetics examines how quickly reactions occur. The reaction rate is the speed at which a reaction converts compounds. We measure the reaction rate by the change in concentration per unit of time. We can use the change in concentration of either the reactants or products.

Rate of reaction:

$Rate=\frac{Concentration\, change}{Time\, change}=\frac{\Delta [C]}{\Delta t}$

The [ ] indicates a concentration. Typically rates are measured in $M/s$ or molarity per second.

Reaction rates vary greatly between different reactions. For example, the rusting of iron is an oxidation-reduction reaction. But, the chemical change can take years. In contrast, the reaction between sodium and water Is a violent reaction! It occurs immediately.

There are various ways to measure the change in concentration. For example, consider a reaction involving gases. If the moles of gas per volume changed, we could monitor the pressure. We could use the ideal gas law to convert between pressure and concentration. In reactions with color changes, the absorbance can be used to determine concentration.

Different Rates in a Reaction

Rates can measure the increase in products or the decrease in reactants over time. The rates between products and reactants are not the same values but are related to each other.

For example, consider the generic balanced chemical equation.

$w A+ x B \rightarrow y C+z D$

In this equation $A$ and $B$ are reactants and $C$ and $D$ are products. The coefficients are represented by $w, x, y,$ and $z$. If we compare the rate of the formation of $D$ to the other reaction species we get:

$\frac{\Delta [D]}{\Delta t}\ =\ \frac{1}{y} \bigg( \frac{\Delta [C]}{\Delta t} \bigg)\ =\ - \frac{1}{x} \bigg( \frac{\Delta [B]}{\Delta t} \bigg)\ =\ - \frac{1}{w} \bigg( \frac{\Delta [A]}{\Delta t} \bigg)$

Notice that there is a negative sign in front of the reactants. The rate of reaction for reactants is a decomposition. The reactant is being consumed. So, the final concentration minus the initial concentration will be a negative number. But, the reaction rate is a positive value. So we add a negative sign to decomposition rates. Including a negative sign offsets the negative value from the concentration change. The rate of formation is always a positive value.

The rate can be evaluated experimentally between different time points. This is important because the rate changes as a reaction proceeds. Usually, as the reaction continues, the rates decrease as the reactants are consumed. If we were to graph the rate, what would we see? The graph would curve. An instantaneous rate is the rate at a particular time. This is found by taking the slope of a line tangent to a time, $t$, on a graph. The initial rate is the rate at $t=0$.

Do you want to learn more about chemical kinetics? In future lessons, we'll study...

• Writing rate law equations.

• Determining the reaction orders.

• Factors that affect the reaction rate.

• How to combine stoichiometry and kinetics. It turns out, there is a relationship between rate and stoichiometry.

(Video) GCSE Chemistry - Rates of Reaction #38

by Cognito

This video will give you a good overview of what a reaction rate is and how we measure it. Remember, reactions can be measured by how quickly reactants are used, or by how quickly products are formed.