Chemical reactions transform substances at varying speeds. Understanding and controlling this speed, known as the reaction rate, is important. Scientists use a “rate law” to describe how reactant concentrations affect this speed. Catalysts are special substances introduced to influence reaction speed.
Understanding Catalysts and Reaction Rate Laws
A catalyst increases the rate of a chemical reaction without being consumed in the overall process; it remains chemically unchanged and can be reused. Catalysts enable reactions to occur more efficiently, often at lower temperatures or pressures.
A reaction rate law is an experimentally determined equation describing how a reaction’s rate depends on reactant concentrations. This expression typically takes the form “rate = k[A]^x[B]^y,” where ‘k’ is the rate constant, and [A] and [B] are reactant concentrations. The exponents ‘x’ and ‘y’ are reaction orders, indicating how strongly the rate is affected by each reactant’s concentration.
How Catalysts Influence Reaction Speed
Catalysts speed up reactions by providing an alternative pathway with a lower activation energy, the minimum energy required for a reaction. By lowering this energy barrier, more reactant molecules possess the necessary energy to transform into products.
The catalyst participates in elementary steps within the reaction mechanism, forming temporary bonds with reactants. This interaction facilitates the breaking and forming of chemical bonds, making it easier for reactants to convert into products.
Catalyst Inclusion in the Rate Law
A catalyst can appear in the experimentally determined rate law if its concentration directly influences the rate of the slowest step in the reaction mechanism. The rate law is derived from the reaction mechanism, which details the series of elementary steps a reaction undergoes. If the catalyst is involved in the rate-determining step, then its concentration will affect the overall reaction rate.
For example, if a reaction mechanism includes a slow step where a reactant interacts with the catalyst to form an intermediate, then the concentration of that catalyst will be a factor in the rate law. In such cases, increasing the catalyst’s concentration would typically increase the reaction rate. However, if the catalyst is not involved in the rate-determining step, or if its concentration is so high that the reaction sites it provides are saturated, it might not explicitly appear in the rate law.
When a catalyst is present, it primarily affects the value of the rate constant, ‘k’, in the rate law. This is because the catalyst changes the reaction pathway and lowers the activation energy, which directly impacts ‘k’. Consequently, the rate law for a catalyzed reaction is often different from that of an uncatalyzed reaction, sometimes including additional terms involving the catalyst’s concentration or altered reaction orders.
Distinguishing Catalysts from Reactants and Intermediates
Understanding the roles of different chemical species in a reaction is important for clarifying the function of a catalyst.
Reactants are the starting materials that are consumed during a chemical reaction, and their concentrations decrease as the reaction proceeds. They are always present in the overall balanced chemical equation.
Intermediates are species formed in one elementary step of a multi-step reaction mechanism and then consumed in a subsequent step. They are transient, meaning they typically exist for a very short time and do not appear in the overall balanced chemical equation. Unlike reactants, intermediates are not present at the beginning or end of the overall reaction.
Catalysts, in contrast to both reactants and intermediates, participate in the reaction mechanism but are regenerated by the end of the process. They are not consumed and do not appear in the overall balanced equation. The unique characteristic of a catalyst is its ability to accelerate a reaction by providing an alternative pathway while remaining chemically unchanged, allowing it to continue facilitating the reaction.