Topic 6.2: Rate of Reaction Revision Notes
1. Overview
The rate of reaction measures how quickly reactants are converted into products over a specific period of time. Understanding these rates is vital in industrial chemistry to maximize production efficiency and ensure safety by controlling the speed of chemical processes.
Key Definitions
- Rate of Reaction: The change in concentration, volume, or mass of a reactant or product per unit time.
- Catalyst: A substance that increases the rate of a chemical reaction but remains chemically unchanged at the end of the reaction.
- Enzyme: A biological catalyst (usually a protein) that speeds up metabolic reactions.
- Activation Energy ($E_a$): The minimum energy that colliding particles must possess for a reaction to occur.
- Frequency of Collisions: The number of collisions occurring between particles per unit of time.
Core Content
Factors Affecting the Rate of Reaction
The speed of a chemical reaction can be changed by five main factors:
- Concentration of solutions: Increasing concentration increases the rate.
- Pressure of gases: Increasing pressure increases the rate.
- Surface area of solids: Decreasing the particle size (increasing surface area) increases the rate.
- Temperature: Increasing temperature increases the rate.
- Catalysts: Adding a catalyst increases the rate.
Investigating Rates of Reaction
There are two primary practical methods used to measure rate:
Method A: Measuring the volume of gas evolved Using a gas syringe to collect gas over time.
- Example: Magnesium reacting with Hydrochloric Acid.
- Word Equation: magnesium + hydrochloric acid → magnesium chloride + hydrogen
- Symbol Equation: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$
Method B: Measuring the change in mass Placing the reaction vessel on a digital balance; mass decreases as gas escapes.
- Example: Calcium carbonate reacting with Nitric Acid.
- Word Equation: calcium carbonate + nitric acid → calcium nitrate + water + carbon dioxide
- Symbol Equation: $CaCO_3(s) + 2HNO_3(aq) \rightarrow Ca(NO_3)_2(aq) + H_2O(l) + CO_2(g)$
A conical flask on a mass balance with a cotton wool plug in the neck, or a conical flask connected via a delivery tube to a gas syringe.
Interpreting Rate Graphs
- Steepness of gradient: A steeper curve indicates a faster rate.
- Plateau: When the curve goes flat, the reaction has stopped (one reactant is used up).
- Final Volume: The total amount of product formed depends on the amount of limiting reactant, not the rate.
Extended Content (Extended Only)
Collision Theory
For a reaction to occur, particles must collide with energy greater than or equal to the activation energy ($E_a$).
- Concentration & Pressure: Increasing these increases the number of particles per unit volume. This leads to a higher frequency of collisions, increasing the rate.
- Surface Area: Breaking a solid into smaller pieces exposes more particles. This increases the frequency of collisions between reactant particles.
- Temperature:
- Particles gain more kinetic energy and move faster, leading to more frequent collisions.
- Crucially, a much higher proportion of particles now have energy $\geq E_a$, leading to a higher frequency of successful collisions.
- Catalysts: A catalyst provides an alternative reaction pathway with a lower activation energy ($E_a$). Therefore, more particles have sufficient energy to react upon collision.
An energy level diagram/reaction profile showing two pathways—one with a high hump (uncatalysed) and one with a lower hump (catalysed), both starting and ending at the same levels.
Evaluating Practical Methods
- Mass Loss: Most accurate for heavy gases like $CO_2(g)$. Not suitable for $H_2(g)$ as it is too light to produce significant mass changes on standard balances.
- Gas Collection: Good for all gases, but if the gas is soluble (like $SO_2$), some may dissolve in the water if collected over a trough, leading to inaccurate results.
Key Equations
1. Average Rate of Reaction $$\text{Rate} = \frac{\text{Change in mass or volume}}{\text{Time taken}}$$
- Change: measured in $g$ or $cm^3$
- Time: measured in $s$ or $min$
- Units: $g/s$ or $cm^3/s$
2. Gradient (from a graph) $$\text{Gradient} = \frac{\Delta y}{\Delta x}$$
- Used to find the rate at a specific point in time (draw a tangent).
Common Mistakes to Avoid
- ❌ Wrong: Saying "Higher temperature makes particles vibrate more."
- ✓ Right: Temperature increases the kinetic energy and speed of particles in fluids (liquids/gases).
- ❌ Wrong: Saying "Catalysts take part in the reaction and are used up."
- ✓ Right: Catalysts may react intermediate steps, but they are chemically unchanged and their mass is the same at the start and end.
- ❌ Wrong: Confusing "more collisions" with "more frequent collisions."
- ✓ Right: Always use the word frequency or rate of collisions (collisions per second).
Exam Tips
- Command Word "Explain": If a question asks you to "Explain" using collision theory, you must mention energy and frequency. Mentioning only "more collisions" will often lose marks.
- Typical Values: Be prepared to handle numerical data like $113.0g$ (mass) or $5.2 cm^3/s$ (rate). Always check your units.
- Contexts: Exam questions often use the "Marble Chip" ($CaCO_3$) experiment or the decomposition of hydrogen peroxide ($H_2O_2$):
- $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$
- Graphing: If a catalyst is added, the curve should be steeper but must end at the same horizontal level (if the amount of reactants is unchanged).