Chemistry Chapter 12: Catalysts and Reactions

Catalysts and Reaction Rates

• Unstable intermediates form between a reactant and a catalyst, facilitating the reaction process.
• Adsorption refers to the accumulation of one substance at the surface of another, crucial in catalytic processes.
• Finely divided or porous catalysts significantly increase the reaction rate due to greater surface area for reactions.

Catalytic Poisons

• Catalytic poisons block active sites on catalysts, reducing their effectiveness and lowering the reaction rate.
• Tetraethyl lead is a common example of a catalytic poison.

Elements in Catalytic Converters

• Platinum, palladium, and rhodium are the key elements that make up catalytic converters, essential for reducing automobile emissions.

Temperature and Reaction Dynamics

• Increasing temperature results in an increased rate of reaction due to higher kinetic energy of particles.
• There is an exponential relationship between temperature and reaction rate, indicating that small temperature increases can lead to large changes in rate.

Measuring Reaction Rates

• The instantaneous rate of reaction is determined by drawing a tangent to the concentration vs. time curve at a specific point.
• Concentration is directly proportional to the rate of reaction; higher concentrations lead to faster reaction rates.

Role of Catalysts

• Catalysts provide active sites for reactions according to the surface adsorption theory without altering themselves chemically.
• A catalyst is defined as a substance that alters the reaction rate but remains chemically unchanged at the end of a reaction.

Types of Catalysis

• Heterogeneous catalysis occurs when the catalyst phase differs from the reactants, while homogeneous catalysis involves the catalyst being in the same phase.
• Autocatalysis refers to reactions where one of the products acts as a catalyst, exemplified by the standardization of KMnO₄.

Activation Energy and Effective Collisions

• Activation energy is the minimum energy required for colliding particles to react.
• An effective collision requires sufficient activation energy and results in product formation.

Factors Affecting Reaction Rates

• Increased concentration of reactants leads to an increased reaction rate.
• The instantaneous rate of a reaction is defined as the change in concentration of a reactant or product per unit time at a specific point.
• Manganese dioxide serves as a catalyst in the decomposition of hydrogen peroxide, enhancing the reaction rate.

Heat Change and Reaction Types

• The sign of ΔH indicates the type of reaction: negative ΔH signifies an exothermic reaction (heat released), while positive ΔH indicates an endothermic reaction (heat absorbed).

Altering Activation Energy

• The activation energy of a reaction can be reduced by adding a catalyst, providing an alternative pathway for the reaction.

Real-World Applications

• Catalytic converters in cars exemplify the surface adsorption theory by transforming harmful pollutants into less harmful products.
• The average rate of reaction can be calculated by dividing the total products produced by the total time.

Effects of Pressure

• Increasing pressure boosts the reaction rate, applicable only to gaseous reactions.

Methods to Increase Reaction Rates

• Options to enhance the reaction rate include increasing temperature, pressure (for gases), surface area, adding a catalyst, or raising reactant concentration.