Question

Determine which of the following statements about catalysts are true. If the statement is false, explain why. (a) A heterogeneous catalyst works by binding one or more of the molecules undergoing reaction to the surface of the catalyst. (b) Enzymes are naturally occurring proteins that serve as catalysts in biological systems. (c) The equilibrium constant for a reaction is greater in the presence of a catalyst. (d) A catalyst changes the pathway of a reaction in such a way that the reaction becomes more exothermic.

Short answer

Statements (a) and (b) are true: a heterogeneous catalyst binds molecules on its surface, and enzymes are biological catalysts. Statements (c) and (d) are false: a catalyst does not affect the equilibrium constant, and it does not alter the exothermicity of a reaction.

Step-by-step solution

Analyzing Statement a

Check if the statement is true: A heterogeneous catalyst works by binding one or more of the molecules undergoing reaction to the surface of the catalyst. Heterogeneous catalysts operate by adsorbing reactants onto the catalyst’s surface, where the reaction takes place. This statement is indeed accurate, as one of the methods by which catalysts can operate involves the adherence of reactants to the catalyst's surface.

Analyzing Statement b

Validate the statement: Enzymes are naturally occurring proteins that serve as catalysts in biological systems. This statement is true. Enzymes are proteins that act as biological catalysts, speeding up reactions in living organisms without being consumed in the process.

Analyzing Statement c

Examine if the equilibrium constant changes: The equilibrium constant for a reaction is greater in the presence of a catalyst. This statement is false. A catalyst speeds up both the forward and reverse reactions equally without altering the equilibrium constant or the position of the equilibrium; it only affects the rate at which equilibrium is achieved.

Analyzing Statement d

Determine the truthfulness of the statement: A catalyst changes the pathway of a reaction in such a way that the reaction becomes more exothermic. This statement is false. While a catalyst does provide an alternative pathway with a lower activation energy, it does not change the overall thermodynamics of the reaction; the enthalpy change (or exothermicity) of the reaction remains the same.

Key concepts

Heterogeneous Catalysts

Heterogeneous catalysts are crucial players in the chemistry arena, affecting reactions by bringing together reactants and their surfaces. Imagine a busy metro station, where trains (reactants) meet, exchange passengers (atoms), and depart in new directions. That’s what a heterogeneous catalyst does — it's like a station, providing a platform where reactants can meet and react more readily.

These catalysts are ‘heterogeneous’ because they exist in a different phase than the reactants. A common example is a solid catalyst with gas reactants. The catalyst's surface acts as an anchor, binding molecules and lowering the activation energy needed for the reaction to occur. This, in turn, speeds up the reaction without the catalyst itself undergoing any permanent change.

For students, visualizing reactions on a molecular level can be challenging. So, an effective analogy, like comparing catalysts to a meeting point, helps to demystify the mechanism by which heterogeneous catalysts work — facilitating encounters between reactant molecules.

Enzymatic Catalysts

Enzymes, nature's own meticulous catalysts, are similar to a master key designed to fit perfectly into a specific lock. Just as a key opens a lock, enzymes bind to specific substrates and accelerate biological reactions.

Enzymes are proteins that play a pivotal role in speeding up reactions in living organisms — acting as organic catalysts. Intriguingly, like a master key being used again and again, enzymes are not used up in the reactions they catalyze. Their particular shape allows them to bind with specific substrates, forming an enzyme-substrate complex, which then undergoes a chemical reaction to form a product. This entire process occurs rapidly and efficiently, all thanks to the unique three-dimensional structure of the enzyme.

For a more in-depth comprehension, it's useful for students to explore how enzyme structure relates to function and how this sophisticated system is central to life's processes. Knowing that enzymes can be inhibited or activated by other molecules adds another layer of appreciation for the complexity of biological systems.

Equilibrium Constant

Think of a see-saw perfectly balanced, despite kids hopping on and off. This state of dynamic balance resembles chemical equilibrium in reactions — the rate of the forward reaction is equal to the rate of the reverse reaction. The equilibrium constant (\(K_{eq}\)) is a number that expresses this balance, a snapshot of the condition of the reaction when it's in this state.

The presence of a catalyst might mislead some into thinking that the numerical value of the equilibrium constant would change. However, a catalyst doesn’t favor either the forward or reverse reaction; it accelerates both equally. As a result, the equilibrium constant remains unchanged. What the catalyst does do is help the system reach equilibrium more quickly.

Students may often confuse the effects of a catalyst on reaction rate with changes in equilibrium position. It's essential to clarify that the equilibrium constant is only affected by changes in temperature, not by catalysts or concentrations of reactants or products.

Reaction Pathway

Envision a hiker finding a shortcut on a mountain trail. The destination and the height of the mountain haven’t changed, but the path to reach the peak is now different, possibly easier. Similarly, a catalyst provides a reaction with an alternative pathway, one that requires less energy to traverse.

A catalyst lowers the activation energy needed for a reaction to proceed, in essence creating a 'shortcut' in the reaction pathway. It doesn’t change the 'height of the mountain', meaning it doesn't alter the initial or final energy states of the reactants and products; the overall energy change of the reaction (enthalpy change) remains constant.

For students grappling with this concept, it's important to distinguish between kinetics and thermodynamics. A catalyst affects kinetics (the reaction rate), by providing a more efficient pathway, but doesn't alter thermodynamics — the energies of the reactants and products. Understanding reaction pathways with the help of these metaphors serves as a guiding tool for visual learners to comprehend complex chemical processes.