What is the primary difference between a catalyst and a reactant in terms of mass?

A reactant's mass decreases as it is converted into products, whereas a catalyst's mass remains constant throughout the reaction because it is not consumed.

How does a catalyst affect the activation energy and the enthalpy change ($\Delta H$) of a reaction?

A catalyst lowers the activation energy by providing an alternative pathway, but it has no effect on the enthalpy change (the energy difference between reactants and products).

When comparing two different catalysts on a gas volume vs. time graph, which one is more effective?

The more effective catalyst will produce a steeper initial gradient on the graph, indicating a higher volume of gas produced in a shorter amount of time.

What is a common error when starting the measurement of gas production in a catalyst practical?

Failing to replace the bung immediately after adding the catalyst leads to gas escaping, which results in an underestimation of the initial reaction rate.

Why is it incorrect to say that a catalyst 'does not participate' in a reaction?

Catalysts do participate by forming intermediate complexes or providing surfaces for reactants to bind to; they are simply regenerated in their original form by the end of the process.

What happens to the results if the temperature of the hydrogen peroxide is not controlled between trials of different catalysts?

Temperature also affects the rate of reaction; if it varies, you cannot determine if the change in rate was caused by the catalyst or the temperature change, making the test unfair.

Define 'Activation Energy' ($E_a$).

Activation energy is the minimum amount of kinetic energy that colliding particles must possess for a chemical reaction to occur.

What is the chemical equation for the catalytic decomposition of hydrogen peroxide?

The equation is $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$. The catalyst is usually written above the arrow as it is not a reactant.

What is the definition of a catalyst?

A catalyst is a substance that increases the rate of a chemical reaction without being used up or permanently changed by the reaction.

How does collision theory explain the effect of a catalyst?

By lowering the activation energy, a catalyst ensures that a greater proportion of collisions have energy $\ge E_a$, leading to a higher frequency of successful collisions.

Practical: Effect of Catalysts on Rate of Reaction | Pearson Edexcel IGCSE Chemistry

7. Physical Chemistry: Part 2

Practical: Effect of Catalysts on Rate of Reaction

Summary

This practical investigation explores how the presence and type of a catalyst influence the speed of a chemical reaction, typically using the decomposition of hydrogen peroxide as a model. By measuring the volume of oxygen gas produced over a fixed time interval, students can quantitatively compare the efficiency of various catalytic substances and understand the underlying principle of lowering activation energy.

1. Definition & Core Concepts

Reaction profile diagram showing the energy levels of reactants and products, with two curves illustrating the higher activation energy of an uncatalyzed reaction versus the lower activation energy of a catalyzed reaction.

2. Underlying Principles

According to Collision Theory, for a reaction to occur, particles must collide with sufficient energy and the correct orientation. A catalyst increases the frequency of 'successful' collisions by lowering the energy barrier that particles must overcome to react.

Because the catalyst provides a lower energy pathway, a significantly larger proportion of the reactant particles possess kinetic energy equal to or greater than the new, lower activation energy ( $E_{a,cat}$ ). This leads to a higher frequency of effective collisions per second.

It is important to note that a catalyst does not change the enthalpy change ( $\Delta H$ ) of the reaction. The energy difference between the reactants and the products remains identical regardless of whether a catalyst is used or which specific catalyst is chosen.

3. Methods & Techniques

The standard experimental setup involves a conical flask containing the liquid reactant, connected via a delivery tube to an inverted measuring cylinder filled with water in a trough. This method, known as downward displacement of water, allows for the precise measurement of the volume of gas evolved.

To ensure a fair comparison, several control variables must be strictly maintained: the volume and concentration of the hydrogen peroxide, the temperature of the solution, and the mass of the catalyst added. Only the type of catalyst (the independent variable) should change between trials.

The procedure requires adding the solid catalyst to the flask and immediately sealing it with a bung. The volume of gas is then recorded at regular time intervals (e.g., every 30 seconds) or after a fixed total duration to calculate the initial rate of reaction.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Practical: Effect of Catalysts on Rate of Reaction

Summary

1. Definition & Core Concepts

A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently changed by the process. In this practical, the decomposition of hydrogen peroxide ( $2H_2O_2 \rightarrow 2H_2O + O_2$ ) is used because it occurs very slowly at room temperature but accelerates significantly in the presence of specific metal oxides.

The rate of reaction in this context is defined as the change in the volume of gas produced per unit of time. By comparing the volume of oxygen collected when using different solids, one can determine which substance acts as the most effective catalyst for this specific chemical change.

The activation energy ( $E_a$ ) is the minimum energy required for reactant particles to collide successfully and form products. Catalysts function specifically by providing an alternative reaction pathway that requires a lower $E_a$ than the uncatalyzed route.

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

It is vital to distinguish between a reactant and a catalyst. While both affect the outcome of a process, a reactant is chemically transformed into a product, whereas a catalyst can be recovered chemically unchanged at the end of the reaction.

Feature	Reactant	Catalyst
Mass Change	Decreases as it is consumed	Remains constant from start to finish
Role in Equation	Appears as a starting material	Often written above the reaction arrow
Mechanism	Forms new chemical bonds in products	Provides an alternative pathway/surface

Different catalysts have varying levels of specificity and efficiency. For example, Manganese(IV) oxide might be highly effective for peroxide decomposition, while Copper(II) oxide might show little to no catalytic activity for the same reaction.

5. Exam Strategy & Tips

When analyzing graphs of gas volume over time, always look at the initial gradient. A steeper slope at the start indicates a faster rate of reaction and therefore a more effective catalyst.

If an exam question asks how to prove a substance is a catalyst, you must describe two steps: first, show it increases the reaction rate, and second, filter, dry, and re-weigh the substance at the end to prove its mass has not changed.

Always check the units on the axes of provided graphs. Rates are typically measured in $cm^3/s$ or $cm^3/min$ ; ensure your calculations for gradient ( $\frac{\text{change in y}}{\text{change in x}}$ ) match these units.

6. Common Pitfalls & Misconceptions

A common misconception is that catalysts 'do not take part' in the reaction. In reality, they interact with reactants (often by providing a surface for adsorption), but they are regenerated by the end of the cycle.

Students often forget to account for the gas already in the delivery tube. To minimize errors, the bung must be replaced as quickly as possible after adding the catalyst, or a divided flask (Ghirardelli flask) can be used to mix reactants after the system is sealed.

Another error is assuming that adding more catalyst will indefinitely increase the rate. While increasing the surface area of a catalyst helps, the rate eventually becomes limited by the concentration of the reactants themselves.