Rate of reaction

Q: Explain why increasing the concentration of hydrochloric acid increases the rate of reaction with magnesium.

There are more $HCl$ particles per unit volume (higher concentration). This leads to a higher frequency of collisions between reacting particles.

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What is Rate of Reaction?

Rate of reaction measures how quickly reactants are converted into products. It can be defined as the change in quantity of a reactant or product per unit time.

Mathematically, for a gas produced:
\text{Rate} = \frac{\text{Volume of gas produced}}{\text{Time taken}}

For a solid reactant losing mass:
\text{Rate} = \frac{\text{Mass lost}}{\text{Time taken}}

Key Principle: The rate is fastest at the start of the reaction (when reactant concentration is highest) and decreases as the reaction proceeds, eventually reaching zero when a reactant is completely used up.

Activation Energy (E_a)

Activation energy (E_a) is the minimum kinetic energy that colliding particles must possess in order to react successfully. It is the energy barrier that must be overcome for bonds to break and new bonds to form.

Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed or used up at the end of the reaction. It works by providing an alternative reaction pathway with a lower activation energy (E_a).

Factors Affecting Rate (Collision Theory)

To understand why rate changes, we use Collision Theory. For a reaction to occur, particles must:

Collide with each other.
Have sufficient energy (\ge E_a) to react.

The table below explains how each factor affects the rate by altering these collision conditions.

Factor Changed	Effect on Particles (Collision Theory)	Result on Rate
Concentration of solutions	More particles per unit volume. This leads to a higher frequency of collisions between reacting particles.	Increases rate.
Pressure of gases	More particles per unit volume (gas is compressed). This is effectively increasing the gas concentration, leading to a higher frequency of collisions.	Increases rate.
Surface area of solids	More exposed particles available for collision. This leads to a higher frequency of collisions between the solid and the surrounding fluid.	Increases rate (if powdered/granular vs. large lump).
Temperature	Two effects: Particles move faster \rightarrowHigher frequency of collisions. More importantly, particles have higher kinetic energy. A significantly larger proportion of particles now have energy ≥ E_a. This leads to more successful collisions.	Increases rate (significantly).
Adding a catalyst	Provides an alternative pathway with a lower activation energy (E_a). More particles now have sufficient energy to react, even at the same temperature.	Increases rate.

Note on Pressure: Students often confuse pressure with volume. Remember: Increasing pressure squeezes gas particles closer together, effectively increasing their concentration. Therefore, the explanation for pressure is identical to that for concentration.

Practical Methods for Investigating Rate

There are three main core methods to measure rate. You must be able to describe the apparatus and method for each.

Method	Apparatus & Setup	What is Measured?	Suitable For
Change in Mass	The mass of the system decreases over time as gas escapes.		Reactions that produce an escaping gas (e.g., CaCO_3 + HCl \rightarrow CO_2).
Volume of Gas	Conical flask connected via delivery tube to a gas syringe OR an inverted measuring cylinder in a water trough (downward displacement of water).	The volume of gas collected increases over time.	Reactions producing any gas that is not highly soluble in water.
Precipitation (Disappearing Cross)	Conical flask placed on a piece of paper with a black cross drawn underneath. The reaction occurs inside the flask.	The time taken for the cross to become invisible due to cloudiness.	Reactions that form an insoluble solid (precipitate) in solution (e.g., Sodium thiosulfate + Hydrochloric acid).

Why use a balance for mass loss? The mass of the reactants inside the flask decreases because the product gas leaves the system. The rate is calculated by the gradient of the mass-loss graph.

Interpreting Rate Graphs

Graphs typically plot Quantity (y-axis) vs. Time (x-axis). The quantity can be Volume of gas, Mass lost, or Concentration remaining.

Key Features to Identify:

Gradient (Slope): The gradient represents the rate of reaction. Steeper gradient = faster rate.
Initial Gradient: The steepest part of the curve at t=0. This is the initial rate, which is the fastest because reactant concentration is highest.
Horizontal Section (Plateau): The line becomes horizontal when the reaction stops. This indicates that a reactant has been completely used up (limiting reagent). The final value on the y-axis tells you the total amount of product formed or reactant consumed.

Comparing Graphs:

If a factor increases rate (e.g., higher temp), the new line will be steeper initially but will reach the same final plateau (if reactant amounts are unchanged).
If a reactant amount is changed, the final plateau height will change.

Worked Example: Interpreting a Mass-Loss Graph

Scenario: Calcium carbonate reacts with hydrochloric acid. The flask is on a balance.
CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)

Question: Explain why the rate of reaction decreases as the reaction progresses.

Answer Structure:

State the change in concentration: As the reaction proceeds, the concentration of hydrochloric acid (HCl) particles decreases because they are being used up.
Link to collisions: This means there are fewer HCl particles per unit volume.
Result: The frequency of successful collisions between CaCO_3 and HCl particles decreases.
Conclusion: Therefore, the rate of reaction slows down.

⚠︎ Confusing Rate with Time

Mistake: Saying 'The rate increases because the time decreases.'

Correction: While true that higher rate means shorter time, examiners require you to explain the mechanism. You must mention particles, collisions, or concentration. Always say: 'The rate increases because there are more frequent collisions,' not just 'it happens faster.'

⚠︎ Catalysts and Enzymes

Mistake: Saying 'The catalyst is used up in the reaction' or 'The catalyst provides energy to the reaction.'

Correction: A catalyst is unchanged at the end of the reaction. It does not provide energy; it lowers the activation energy (E_a) required for the reaction to proceed.

⚠︎ Pressure vs. Volume

Mistake: Describing pressure changes in terms of 'volume' without linking it to particle density.

Correction: When asked about pressure, explicitly state: 'Increasing pressure increases the concentration of gas particles (more particles per unit volume).' This is the critical link to collision theory.

Explaining Temperature Effects

When to use: When asked to explain why increasing temperature increases the rate.

Why examiners accept this: Examiners look for two distinct points: frequency of collisions AND energy of collisions. Mentioning only one is often insufficient for full marks.

Correct Phrasing: 'Increasing temperature increases the kinetic energy of the particles. This means:

Particles collide more frequently.
More importantly, a larger proportion of particles have kinetic energy greater than or equal to the activation energy (E_a), leading to more successful collisions.'

Example: 'The rate increases because more particles possess the minimum energy required to break bonds.'

Describing Graph Gradients

When to use: When asked to describe how the rate changes based on a graph.

Why examiners accept this: 'Rate' is synonymous with 'gradient' or 'steepness' in graphical analysis. Using these terms shows precise understanding.

Correct Phrasing: 'The gradient of the curve becomes less steep as time progresses, indicating that the rate of reaction is decreasing.'

Example: 'At t=10s, the gradient is steeper than at t=50s, so the initial rate is faster.'

Evaluating Practical Methods

When to use: When asked to evaluate a method (e.g., 'Why is a gas syringe better than downward displacement of water?').

Why examiners accept this: Evaluation requires identifying limitations or advantages. For gases, solubility and apparatus friction are key.

Correct Phrasing: 'A gas syringe is more accurate because it avoids errors caused by the gas dissolving in water (if using downward displacement) or the friction of water in the tube. However, large volumes of gas may push the plunger out with force, causing leaks.'

Example: 'Using a balance is suitable for CO_2 because it is dense and escapes easily, but unsuitable for H_2 because the mass loss is very small and hard to measure accurately.'

Past Paper Style Questions

State two ways in which a catalyst increases the rate of a reaction.

It provides an alternative pathway with lower activation energy (E_a). 2. It is not used up / remains chemically unchanged at the end.

Explain why increasing the concentration of hydrochloric acid increases the rate of reaction with magnesium.

There are more HCl particles per unit volume (higher concentration). This leads to a higher frequency of collisions between reacting particles.

Describe how you would investigate the effect of temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid.

Place a conical flask on a piece of paper with a black cross underneath. 2. Add sodium thiosulfate and hydrochloric acid. 3. Start a stopwatch immediately. 4. Look down through the solution at the cross. 5. Stop the timer when the cross is no longer visible due to cloudiness.

The graph shows the volume of gas produced over time. Explain why the line becomes horizontal.

One of the reactants has been completely used up (limiting reagent). No more product can be formed.

Why does increasing the pressure of a gaseous reactant increase the rate of reaction?

Increasing pressure decreases the volume, which increases the number of gas particles per unit volume (concentration). This leads to more frequent collisions between particles.