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The rate, or speed, at which chemical reaction occur can vary widely. A **reaction rate** is the change in concentration of a reactant or product with time.

For example, say that substance \begin{align*}A\end{align*} underwent a reaction with its concentration decreasing. The rate of reaction can be expressed as:

\begin{align*}\text{rate}=-\frac{\Delta [A]}{\Delta t}=-\frac{[A]_{\text{final}}-[A]_{\text{initial}}}{\Delta t}\end{align*}

**Note:** delta (∆) means "the change in," the brackets around the \begin{align*}A\end{align*} refers to the concentration in molarity (M) of \begin{align*}A\end{align*}, and a negative sign is used in this case because we know that the concentration of \begin{align*}A\end{align*} is decreasing.

If you started with 1.10 M of \begin{align*}A\end{align*} and you only had 0.88 M after 15.0 seconds, what would the reaction rate be? (To check your answer, highlight the following: 0.015 M/s)

If your initial concentration of hydrochloric acid (HCl) is 2.14 and the rate of reaction after 11.0 seconds is 0.012 M/s, what is your final concentration of HCl? (To check your answer, highlight the following: 2.01 M)

You can click here to find more information on chemical reaction rates and check your answers.