Factors That Affect the Chemical Reaction Rate

Factors That Affect the Chemical Reaction Rate Its useful to be able to predict whether an action will affect the rate at which a chemical reaction proceeds. Several factors can influence the chemical reaction rate. In general, a factor that increases the number of collisions between particles will increase the reaction rate and a factor that decreases the number of collisions between particles will decrease the chemical reaction rate. Concentration of Reactants A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increased reaction rate (except for zero-order reactions.) Similarly, a higher concentration of products tends to be associated with a lower reaction rate. Use the partial pressure of reactants in a gaseous state as a measure of their concentration. Temperature Usually, an increase in temperature is accompanied by an increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10Â °C increase in temperature. Once the temperature reaches a certain point, some of the chemical species may be altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop. Medium or State of Matter The rate of a chemical reaction depends on the medium in which the reaction occurs. It may make a difference whether a medium is aqueous or organic; polar or nonpolar; or liquid, solid, or gaseous. Reactions involving liquids and especially solids depend on the available surface area. For solids, the shape and size of the reactants make a big difference in the reaction rate. Presence of Catalysts and Competitors Catalysts (e.g., enzymes) lower the activation energy of a chemical reaction and increase the rate of a chemical reaction without being consumed in the process. Catalysts work by increasing the frequency of collisions between reactants, altering the orientation of reactants so that more collisions are effective, reducing intramolecular bonding within reactant molecules, or donating electron density to the reactants. The presence of a catalyst helps a reaction to proceed more quickly to equilibrium. Aside from catalysts, other chemical species can affect a reaction. The quantity of hydrogen ions (the pH of aqueous solutions) can alter a reaction rate. Other chemical species may compete for a reactant or alter orientation, bonding, electron density, etc., thereby decreasing the rate of a reaction. Pressure Increasing the pressure of a reaction improves the likelihood reactants will interact with each other, thus increasing the rate of the reaction. As you would expect, this factor is important for reactions involving gases, and not a significant factor with liquids and solids. Mixing Mixing reactants increases their ability to interact, thus increasing the rate of a chemical reaction. Summary of Factors The chart below is a summary of the main factors that influence the reaction rate. There is typically a maximum effect, after which changing a factor will have no effect or will slow a reaction. For example, increasing temperature past a certain point may denature reactants or cause them to undergo a completely different chemical reaction. Factor Affect on Reaction Rate temperature increasing temperature increases reaction rate pressure increasing pressure increases reaction rate concentration in a solution, increasing the amount of reactants increases the reaction rate state of matter gases react more readily than liquids, which react more readily than solids catalysts a catalyst lowers activation energy, increasing reaction rate mixing mixing reactants improves reaction rate