A chemical reaction rate quantifies how quickly reactants are converted into products. This rate is defined as the change in the concentration of a substance, either a reactant or a product, over a specific period of time. The field of chemical kinetics studies these reaction speeds, providing insights into the molecular mechanisms and steps involved in a chemical transformation. Understanding the rate allows chemists to control reactions, optimize industrial processes, and predict chemical behavior under varying conditions. The reaction speed is not constant and typically slows down as the concentration of reactants decreases.
Tracking Observable Changes
Measuring a reaction rate requires continuous or periodic monitoring of a measurable property that changes as the reaction progresses. Scientists typically track either the decrease in the concentration of a starting material (reactant) or the increase in the concentration of an end product. The resulting data plots the concentration against the elapsed time, forming the basis for all subsequent rate calculations.
For reactions that are relatively slow, samples can be periodically withdrawn from the reaction vessel for analysis. This process often requires “quenching” the sample, which means rapidly stopping the reaction, usually by cooling it quickly or adding a substance that halts the chemical process. Faster reactions, however, necessitate non-invasive techniques that allow continuous monitoring directly within the reaction vessel without disturbing the chemical environment. These continuous methods are generally preferred because they provide a complete, real-time picture of the concentration change as the reaction unfolds.
Measuring Rate Through Physical Properties
One direct way to monitor a reaction rate is by tracking changes in physical properties that are easy to observe and quantify.
Gas Volume Measurement
If a reaction produces a gaseous product, the rate is determined by measuring the volume of gas evolved over time. This is commonly done using a gas syringe connected to the reaction flask, allowing for the recording of gas volume at regular time intervals. Alternatively, the gas can be collected over water, displacing the liquid into a measuring cylinder for accurate volume determination.
Pressure Monitoring
For reactions conducted in a sealed, rigid container where the number of gaseous molecules changes, monitoring the total pressure provides a reliable measure of the reaction’s progress. As the concentration of gaseous reactants decreases or the concentration of gaseous products increases, the overall pressure within the fixed volume changes proportionally. A pressure sensor records this change as a function of time, directly correlating to the changing molecular count.
Mass Loss
Another technique involves monitoring the total mass of the reaction system if a volatile product is allowed to escape into the atmosphere. For example, when a carbonate reacts with an acid, carbon dioxide gas is produced and leaves the solution, resulting in a measurable decrease in the total mass of the flask and its contents over time. Tracking the rate of mass loss using a sensitive balance provides an indirect measure of the product formation rate.
Measuring Rate Through Chemical Properties
More sophisticated methods rely on monitoring changes in specific chemical properties that are sensitive to the concentration of dissolved species.
Spectrophotometry
Spectrophotometry is a widely used instrumental technique, particularly when one of the reactants or products absorbs light in the visible or ultraviolet regions of the spectrum. The intensity of light absorbed by the solution is directly proportional to the concentration of the light-absorbing substance present. By setting a spectrophotometer to a specific wavelength, scientists continuously measure the light absorption, or absorbance, as the reaction proceeds. Changes in absorbance provide a real-time record of the concentration change over time.
Electrical Conductivity
Another effective approach involves measuring the electrical conductivity of the solution, which is governed by the presence and mobility of dissolved ions. If a reaction consumes highly conductive ions or produces ions with different mobilities, the total electrical conductivity of the solution will change. A conductivity probe records this change, which can then be directly related to the changing concentration of the ionic species involved in the chemical process.
pH Monitoring
If a reaction involves the consumption or production of hydronium (\(H^+\)) or hydroxide (\(OH^-\)) ions, a pH meter can be used to monitor the rate. Since pH is a direct measure of the acidity or basicity of the solution, its change over time provides a precise way to track the concentration of these specific ions.
Interpreting the Data
Once the raw data—such as absorbance, volume, or pressure—is collected against time, the next step is to convert this information into a meaningful reaction rate.
The simplest calculation is the average rate, determined by calculating the total change in concentration over a large, defined time interval. This provides a general idea of the reaction speed but obscures the moment-to-moment variations that occur as the reaction progresses.
A more scientifically useful measure is the instantaneous rate, which represents the reaction speed at one specific point in time. This value is determined by plotting the concentration of the monitored species against time and then calculating the slope of the tangent line drawn to the curve at the desired time point. Because the reaction rate generally decreases as reactants are consumed, the instantaneous rate changes continuously throughout the experiment.
A particularly important instantaneous rate is the initial rate, measured at the very beginning of the reaction when time equals zero. This initial rate is often used because the concentrations of the reactants are known precisely at this moment, simplifying the mathematical analysis required to determine the overall rate law for the chemical process.