Reaction Rate Definition
The reaction rate, sometimes called the rate of reaction, is the speed at which chemical substances react with one another. The reaction rate can be calculated as the instantaneous reaction rate or the average reaction rate, though it is more common to determine the average rate because it is easier to measure.
As a matter of designation, reaction rate values are always displayed as positive. This is true whether it is a reactant or product being measured. If the reactant is being measured, make sure you understand that in the forward reaction the reactant will be decreasing by the reaction rate, where the product will be increasing by the reaction rate.
Reaction Rate Equation
The equation for the average reaction rate is simple, but you must have an understanding of basic chemistry in order to use it. The reaction rate equation can be seen below:
Reaction rate = Moles of a substance used or produced / Number of seconds taken to complete
In this simple equation, it can be seen that the reaction rate is the number of moles of a substance used as a reactant or created as a product divided by how long it took to make or use that amount of substance. This is considered the average reaction rate.
In order to calculate the reaction rate of any given substance within a chemical reaction, you simply need to know how many moles of a substance have been used or created within a given timeframe. However, this is not always straightforward or easy to calculate. Some reactions happen in a fraction of a second while others take centuries to complete. Further, in order to accurately measure the reaction rate you must accurately measure and weigh the products or reactants of a reaction which can be complicated by losses during the experimental procedure.
What Determines the Reaction Rate?
Many things determine the reaction rate, such as temperature, acidity within the solution, or other environmental modifiers. These modifications of the environment prevent or enhance reactions between individual chemical species, which in turn affects how fast the overall reaction takes place. Further, many reaction rate measurements will change as the reaction takes place due to the effects of changing concentrations of products and reactants.
For example, the breakdown of sucrose, or table sugar, is a natural process. Sucrose breaks down into glucose and fructose, two smaller sugar molecules. Typically, however, the reaction rate for this process is abysmally low. In fact, it would take thousands of years for this reaction to take place naturally. Luckily for living organisms, there is an enzyme known as sucrase. This enzyme has the ability to lower the energy needed to break sucrose, which changes the reaction rate from thousands of years to a few seconds.
In non-biological reactions, other chemicals within the solution can change the reaction rate of an individual species by changing the way that those molecules react. For instance, metal ions are used in a number of reactions to help facilitate the reaction rate and increase the conversion of reactants to products.
Reaction Rate Examples
Methane is a flammable gas with the chemical formula CH4. When methane burns it is oxidized, releasing many of the hydrogen atoms. The full equation for the reaction is as follows:
CH4 + 2O2 → CO2 + H2O
Therefore, as you can see by the balanced equation above, it takes two moles of oxygen for every mole of methane burned. So, to calculate the reaction rate for any of the above species, you would simply need to measure how many moles of each species are used during a specified time period.
For example, let us pretend that you started with 4 moles of methane. After 1 minute, all 4 moles of methane are gone. To determine the reaction rate of methane, simply divide 4 moles by 60 seconds.
4 moles of methane / 60 seconds = 0.067 moles/second
This reaction rate will be the same for all of the other species found in the reaction, besides oxygen. The other species (carbon dioxide and water) are produced in equivalent amounts to the amount of methane introduced. However, in order to combust the methane, two moles of oxygen are needed for every one mole of methane. Thus, 8 moles of oxygen were used in the 1 minute the reaction took place. Therefore, the reaction rate of oxygen would be:
8 moles of oxygen / 60 seconds = 0.134 moles/second
The rate is twice that of methane, because two molecules of oxygen are being used for every one mole of methane.
Measuring the Instantaneous Rate
Though the average reaction rate is commonly used to describe how quickly common reactions take place, the instantaneous reaction rate is used to determine how quickly the reaction is proceeding at various concentrations.
The instantaneous reaction rate is often calculated graphically. By plotting the amount of product and reactant over time, scientists can produce a graph which shows exactly how much change there is between every measurement. The tangent line, or a line that runs parallel to the graph at any point, will show the instantaneous change at that point.
The slope of this line is the instantaneous reaction rate. The instantaneous reaction rate can change depending on the concentration of products and reactants, and generally slows as the reaction nears completion. On the other hand, the instantaneous rate is often higher than the average at the beginning of the reaction because of the high concentration of reactants.