Catalysts: Their Role In Rate Laws And Chemical Reactions

Catalysts are substances that speed up chemical reactions without being consumed during the process. They can be in the same phase as the reactants, known as homogeneous catalysts, or in a distinct phase, known as heterogeneous catalysts. While catalysts play a crucial role in influencing reaction rates, they do not appear in the overall chemical equation. This is because catalysts do not change the stoichiometry of the reaction, even though they may be involved in multiple steps of the reaction mechanism. The rate law, which predicts the relationship between reaction rate and reactant concentrations, is experimentally determined and does not always explicitly include the catalyst. However, the effect of the catalyst is reflected in the rate constant, which represents the dependence of the reaction rate on factors like catalyst concentration, temperature, and pressure.

Catalysts are selective

The selectivity of a catalyst is influenced by its structure and composition. For example, platinum-based catalysts with a specific surface structure have been shown to preferentially promote the formation of the cis isomer during alkene cis-trans isomerisation conversions. Similarly, the nature of the catalyst plays a crucial role in determining selectivity in hydrocarbon dehydrogenation reactions on metal surfaces. Transition metals, such as nickel and platinum, promote alpha-, beta-, and gamma-H eliminations with varying relative efficiencies, which is a key factor in determining the selectivity of the reaction.

The control of selectivity in catalysis is of utmost importance in the design of new catalytic processes. Higher selectivity can lead to more efficient and environmentally friendly processes by reducing the waste of reactants, eliminating the need for costly separation procedures, and decreasing the formation of toxic by-products. However, achieving precise control over selectivity is challenging due to the complex nature of catalytic reactions and the limited understanding of the underlying chemical principles.

The external stimuli used to activate or switch a catalyst can also impact its selectivity. These stimuli include changes in temperature, pH, light, electric fields, or the introduction of chemical agents. Inhibitors, also known as "negative catalysts," can further modify selectivity by deactivating catalysts or removing reaction intermediates. For instance, in the hydrogenation of alkynes to alkenes, a palladium (Pd) catalyst can be partially "poisoned" with lead(II) acetate (Pb(CH3CO2)2) to prevent the further hydrogenation of the alkene to alkane.

Catalysts speed up reactions

Catalysts are substances that speed up the rate of a chemical reaction without being consumed during the reaction. They achieve this by lowering the activation energy required for the reaction to occur, which, in turn, increases the rate constant at the same temperature relative to an uncatalyzed reaction. Catalysts can be in the same phase as the chemical reactants (homogeneous catalysts) or in a distinct phase (heterogeneous catalysts).

The rate of a chemical reaction at a constant temperature depends on the concentrations of the substances involved. While catalysts influence the reaction rate, they do not appear in the balanced overall chemical equation. This is because the dependence of the reaction on catalyst concentration is generally complex and not needed unless optimized or used in excess. However, the concentration of active particles on the surface of a heterogeneous catalyst should be included in the rate equation.

The effect of a catalyst is to alter the mechanism of the reaction, providing a different route to the products. This can make the reaction multi-step and more complicated, with new barriers and intermediates. For example, Pt metal acts as a catalyst for reactions involving hydrogen or oxygen gas. The gas molecules break their bonds and form atomic species that are weakly bonded to the Pt, allowing them to react with other molecules and leave the surface, returning the Pt metal to its original state.

In summary, catalysts speed up reactions by lowering the activation energy, changing the mechanism of the reaction, and providing a different route to the products. While they influence the reaction rate, catalysts do not typically appear in the overall rate law or balanced chemical equation, except for the concentration of active particles on the surface of a heterogeneous catalyst.

Catalysts don't appear in balanced overall chemical equations

A catalyst is a substance that speeds up a chemical reaction by reducing the activation energy required for the reaction to occur. Catalysts are not consumed or changed in the reaction. They appear on the reactant side initially and later on the product side, remaining chemically the same.

A balanced overall chemical equation is a simplified representation of a chemical reaction. It illustrates only the substances that are consumed to form the products. In these equations, only the substances that undergo a permanent change are included. Since catalysts do not undergo a permanent change, they are not included in the overall equation.

Net chemical equations are critical as they focus on the actual change happening in the reaction and ignore any spectator species, such as catalysts. By understanding the net chemical equations, chemists can better focus on the core transformation occurring, which is essential in both experimental and industrial chemistry.

In a gross or overall equation, the catalyst is usually included. For example, if a catalyst 'C' speeds up a reaction between 'A' and 'B' to form a product 'AB', the equation would look like this: A + B + C → AB + C. In this equation, the amount of 'C' remains unchanged at the beginning and end of the reaction.

In a net or balanced equation, only reactants that are directly involved in the formation of products are included. Since catalysts merely speed up the reaction but are not consumed or altered, they do not appear in the net chemical equation.

Catalysts alter the mechanism of a reaction

A catalyst is a substance that speeds up a chemical reaction or lowers the temperature or pressure required to initiate one. Importantly, a catalyst is not consumed during the reaction and does not appear in the overall chemical reaction equation. However, it can appear in the steps of a reaction mechanism, altering the mechanism in a significant way.

Catalysts can either be in the same phase as the chemical reactants (homogeneous catalysts) or in a distinct phase (heterogeneous catalysts). In the case of heterogeneous catalysts, they are usually not included in the rate equation as their concentration in the reaction mixture is zero. However, the concentration of active particles on the surface of the heterogeneous catalyst should be included in the rate equation.

Catalysts work by providing a means for reactant molecules to break bonds and then form temporary bonds with the catalyst. This allows the catalyst to "help" the chemistry by arranging the reactants in favourable geometries and weakening bonds that need to break. Some catalysts change during the course of the reaction but are returned to their original state at the end.

The Department of Energy (DOE) is actively involved in researching and designing new catalysts and understanding how to make chemical reactions more efficient and targeted. This research is helping to advance solar fuels and develop methods for transforming discarded plastic into new products.

Catalysts lower activation energy

Catalysts are substances that speed up the rate of a chemical reaction without being consumed during the reaction. They do not appear in the balanced overall chemical equation but can influence the reaction rate. The rate law is experimentally determined and can be used to predict the relationship between the rate of a reaction and the concentrations of reactants.

The rate equation is generally established based on empirical evidence to establish a relationship between the rate and reactant concentration. The dependence on catalyst concentration is usually complex and often not needed. Catalysts can either be in the same phase as the chemical reactants or in a distinct phase. Catalysts in the same phase are called homogeneous catalysts, while those in different phases are called heterogeneous catalysts. For example, Pt metal is a heterogeneous catalyst for reactions involving hydrogen gas or oxygen gas.

The effect of a catalyst is that it lowers the activation energy for a reaction, allowing reactant molecules to break bonds and form temporary bonds with the catalyst. This mechanism alters the reaction pathway, introducing new barriers and intermediates. However, the highest barrier is significantly lower than the previous largest barrier, facilitating the reaction. The presence of a catalyst does not change the chemical equation but affects the rate constant, K, which represents the effect of factors such as temperature, pressure, or catalysts on the rate of reaction.

While catalysts do not appear in the overall chemical equation, they can be included in the rate equation as a constant. This is because the catalyst concentration remains constant unless more or less catalyst is added, altering the reaction conditions. The inclusion of a catalyst in the rate equation can make it more complex, especially for multi-step reactions. However, the concentration of active particles on the surface of a heterogeneous catalyst should be considered in the rate equation.

Frequently asked questions