You can measure the rate of a chemical reaction by examining the ratio between the amount of substance or products formed and the time it took to produce them. Products can either be measured by mass per unit time or by volume per unit time. Measuring reaction rates is crucial in both analytic and synthetic aspects of chemistry. For instance, plastics manufacturers can use reaction rates to determine how much propane and ethane are required to produce the right amounts of propylene and ethylene.Knowing the rate of chemical reactions under certain conditions, for example, temperature and pressure, also enables you to optimise production and minimise waste. This is very important in industrial production, such as chemical manufacturing, where a miscalculation could potentially result in millions of pounds worth of losses.As we discover later on, there are various ways to measure the rate of a reaction. For example, laboratory-scale reactions can be measured in smaller units like grams per second, whereas large-scale production might be measured in terms of metric tons per day.Comprehensive techniques for measuring reaction ratesWhen examining reaction rates, you can either measure products by mass per unit time or by volume per unit time. This is especially true for reactants that produce gases as byproducts. These include gaseous fuels such as hydrogen, propane, butane, and ethylene. It’s important to take accurate and precise measurements because they can have a huge impact on an operation’s cost-effectiveness, production efficiency, profitability and, crucially, safety.1. Measuring the volume of gasTwo of the main challenges when measuring gas by mass are buoyancy and diffusion. Some gases, like helium, are too buoyant in air for their weight to be measured, while others diffuse easily and need to be kept in special containers.One of the ways to measure the volume of gas is through the displacement method under one atmosphere pressure and at 25°C. For example, the hydrogen gas that’s produced through the electrolysis of water can be collected using an inverted test tube with water in it. When the test tube is completely filled with gas, the volume of the gas is equivalent to the volume of water it displaces. 2. Measuring the volume of gas given off by a reaction over timeMeasuring the volume of gas given off over time is a good indicator of the proportions of reactants and the degree or intensity of other factors. For example, in a biochemical oxygen demand (BOD) experiment, the amount of organic materials that are decomposed by aerobic bacteria can be estimated based on the total volume of gases that are generated by the aerobic respiration of the bacteria.Common units for measuring reaction ratesThe units for the rate of a chemical reaction include both mass per unit time and volume per unit time. In a laboratory setting, rates of reaction are typically measured in smaller units like grams and cubic centimetres. On the other hand, industrial-scale production is typically measured in larger units like metric tons per day or week. Here are some examples of the most commonly used units of measurement for rates of reaction. 1. Grams per second (g/s) The grams per second unit is particularly useful when measuring solid products in a fast reaction. For example, in a precipitation reaction, solid precipitates can be separated from the mixture and then weighed. Solid products can also be calculated based on the relationship between the reactants in a balanced chemical equation.2. Grams per minute (g/min)The grams per minute reaction rate can either be directly measured from the amount of products produced per minute or it can be extrapolated from the smaller or larger measurements.3. Cubic centimetres per second (cm3/s)The cubic centimetre per second, which can also be converted into millilitres per second (mL/s), is a good measurement for fast reactions that produce gaseous byproducts. This can either be the result of a decomposition reaction or a combination reaction. For example, in a combustion reaction of methane, the rate of carbon dioxide production can be measured in terms of cubic centimetre per second.4. Cubic centimetres per minute (cm3/min)Again, the cubic centimetre per minute is useful for measuring the rate of reaction of reactants that produce gaseous products. It’s ideal for relatively slow reactions and biological processes, like the production of oxygen by plant leaves. The unit of reaction can either be measured directly or extrapolated from other units of measurement.How to calculate the rate of reaction using a formulaIt’s important to note that the rate of a chemical reaction follows a nonlinear relationship. So if you were to chart the product produced against the duration of the reaction, the graph will be curved. This is because the proportional amounts of reactants and products are not always present at a 1:1 ratio.There’s also an optimal amount of reactants that can be produced given certain conditions. For example, not all carbon dioxide molecules dissolved in water completely react with the water to produce carbonic acid (only about 1% of the total dissolved carbon dioxide is actually converted into carbonic acid).In a simple chemical reaction that converts A → B, the general equation for the rate of reaction can be written as: This means that as the amount of reactant decreases, the amount of product increases proportionally. The change in amount of the product is also directly proportional to the change in time. Application of the rate of reaction formulaOne of the most important applications of the rate of reaction formula is in the synthesis of new substances from reactants. Using this formula, chemists can make the synthesis of new substances standardised and more efficient. Optimal conditions, such as concentrations and temperature, can be determined. Chemists can calculate the time required to complete a particular reaction given the relative quantities (concentrations) of the reactants.Examples and practice problemsExample 1: First order decomposition of (N2O5). Determine the constant k at 25∘C assuming that the half-life of dinitrogen pentoxide at that temperature is 4.03×104 seconds. Calculate the percentage of the molecules that will not have decomposed in one day. Try this problem before checking the answer here.Example 2: Find the rate of reaction for the decomposition of water into hydrogen and oxygen if k is equal to k = 1.14 x 10-2 and [H2O] = 2.04M. Try solving this problem on your own before checking here for the answer.Factors influencing the rate of reactionThere are several physical factors that affect the rate of reaction. These include the state of matter of the reactants, the temperature, concentration of the reactants, pressure, and the presence of a catalyst (see more on catalysts below). These factors are either directly proportional to the rate of reaction or inversely proportional. For example, increasing the temperature also tends to increase the rate of reaction. However, all of these factors have certain limits.The effect of temperature on reaction rateThe rate of reaction typically increases as the temperature increases, within a certain range. However, when the optimal temperature is reached, the reaction rate goes down as the higher temperature breaks the chemical bonds in the products. There are also other factors to consider, such as the solubility of substances. The solubility of gaseous substances like carbon dioxide tend to decrease in water as the temperature increases within a certain range. Lower solubility results in slower chemical reactions given a certain medium or solvent. The impact of concentration on reaction rateTypically, the rate of reaction increases as the concentration of the reactants increases up to a certain point. However, there’s always an optimal level of concentration that limits the rate of reactions. A higher concentration means there are more molecules and, consequently, there’s a greater chance of them colliding with each other.Nonetheless, the stoichiometry of, or relationship between, reactants limits their optimal level of reactions. Not all reactants will be converted to products. In some cases, a reaction may be reversible when it reaches a certain level. For example, when the reaction between lead (II) nitrate and sodium iodide reaches a stoichiometrically balanced state to produce lead (II) iodide and sodium nitrate, the reaction reverses and achieves equilibrium.The role of surface area in reaction ratePowdered chemicals and chemicals in mist form tend to react faster due to their greater surface area. When a substance is converted into smaller particles, it’s more exposed to other chemicals, which means there’s a greater chance of contact between the reactants. For example, sawdust burns faster than wood because of the larger combined surface areas of the individual particles that are exposed to air.How does a catalyst increase the rate of a reaction?The presence of catalysts may either decrease (inhibit) or increase the rate of reaction depending on the type of catalyst and reactants. For example, zeolites are commonly used as a catalyst to synthesise ammonia from nitrogen and hydrogen using the Haber process. Catalysts affect the rate of reaction by either increasing or decreasing the activation levels of the substances involved. They also provide greater surface areas for reactions to take place.The effect of pressure on reaction ratePressure generally has a directly proportional relationship with the rate of a reaction. When substances are pressurised, their molecules tend to be squeezed together. This is especially true for gaseous reactants. That means the probability of the molecules bumping into each other is also significantly increased.ConclusionYou can measure the rate of a chemical reaction by examining how much of a substance is converted into products within a certain unit of time. The units for the rate of a chemical reaction include both mass per unit time and volume per unit time. Key variables such as temperature, concentration, and surface area can all affect the rate of a chemical reaction.
