Chemistry Study Notes: How Fast Does It Go? Monitoring Reaction Rates

Hey everyone! Welcome to a super important topic in Chemistry: figuring out how fast reactions happen. Think about it – some reactions, like an explosion, are over in a flash. Others, like the rusting of an iron gate, take years. As chemists, we need to measure, control, and understand these speeds, or reaction rates.

In these notes, we'll explore the clever ways we can "watch" a reaction as it happens. We'll become chemical detectives, gathering clues to plot the story of a reaction from start to finish. Why is this useful? It helps in everything from manufacturing medicines efficiently to baking the perfect cake! Let's get started.


First things first: What ARE we measuring?

To monitor a reaction, we need to track a property that changes over time. Imagine you're making popcorn. You could monitor the reaction by counting how many kernels pop every 10 seconds. In chemistry, we do the same thing, but with measurable chemical properties.

The key is to choose a property that is easy to measure and is directly related to the amount of reactant being used up or product being formed.

Quick Review: What is Reaction Rate?

Reaction Rate is the change in the concentration (or amount) of a reactant or a product per unit of time. Basically, it's the 'speed' of the reaction.


Choosing Your Detective Tool: Which Method to Use?

Not every method works for every reaction. You need to look at the reaction equation and see what's changing. Here’s a simple guide:

  • IF your reaction produces a gas...
    ...THEN you can measure the volume of gas produced.

  • IF your reaction involves an acid or an alkali getting used up or produced...
    ...THEN you can use titration to measure changes in concentration.

  • IF your reaction is very exothermic (releases a lot of heat) or endothermic (absorbs a lot of heat)...
    ...THEN you might be able to monitor the change in temperature.

  • IF your reaction produces a gas that escapes and has a different mass from the reactants...
    ...THEN you can measure the change in mass of the system.

  • IF one of your reactants or products has a distinct colour...
    ...THEN you can measure the change in colour intensity.

Let's dive into the main methods you need to know!



Method 1: Measuring Gas Volume

When to use it?

This is your go-to method for any reaction that produces a gas. A classic example is a metal reacting with an acid.

Example: $$ \text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} $$

Here, we can track the progress by measuring the volume of hydrogen gas (H₂) produced over time.

How to do it: Step-by-Step

There are two common setups for this:

Setup A: Using a Gas Syringe
  1. Set up your apparatus: a flask for the reaction, connected by a tube to a gas syringe.
  2. Add one reactant (e.g., the magnesium strip) to the flask. Add the second reactant (e.g., the HCl) and immediately start a stopwatch and put the stopper on the flask.
  3. Record the volume of gas collected in the syringe at regular time intervals (e.g., every 15 seconds).
  4. Keep recording until the volume no longer changes – this means the reaction has stopped.
Setup B: Displacement of Water
  1. This is similar, but instead of a syringe, you collect the gas in an inverted measuring cylinder filled with water in a trough.
  2. As the gas bubbles into the measuring cylinder, it pushes the water down (displaces it).
  3. You record the volume of gas on the measuring cylinder scale over time.
Watch Out! Common Mistakes
  • Leaky Apparatus: If the stopper isn't tight or the connections are loose, gas will escape, and your results will be inaccurate.
  • Soluble Gas: The displacement of water method doesn't work for gases that dissolve in water, like ammonia (NH₃) or hydrogen chloride (HCl). The gas syringe is better for these.
  • Delayed Start: Make sure you start the stopwatch the moment the reactants mix!

The Result: Your Graph

You'll plot a graph of Volume of gas (cm³) on the y-axis against Time (s) on the x-axis. It will typically look like a curve that is steep at the beginning and flattens out at the end.

Key Takeaway

Measuring gas volume is a great, continuous way to monitor reactions that produce a gas. The gas syringe is generally the most reliable method.



Method 2: Titrimetric Analysis (by Quenching)

When to use it?

This method is perfect when you need to track the concentration of a substance, especially an acid or an alkali, in a solution.

Example: The hydrolysis of an ester with an alkali. $$ \text{CH}_3\text{COOC}_2\text{H}_5\text{(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{C}_2\text{H}_5\text{OH(aq)} $$

We can monitor the reaction by measuring how the concentration of the reactant, sodium hydroxide (NaOH), decreases over time.

The Big Challenge and The Clever Solution

The Challenge: How can you measure the concentration of something while it's still reacting? By the time you do your titration, the concentration will have changed!

The Solution: Quenching! To "quench" a reaction means to stop it suddenly. Think of it like hitting the 'pause' button on the chemical reaction. We can do this by:

  • Rapid Cooling: Dunking the sample in an ice bath. Low temperatures slow down most reactions dramatically.
  • Dilution: Adding a large amount of cold water can also slow it down.
  • Chemical Quenching: Adding another chemical that instantly stops one of the reactants. (This is a more advanced technique).

How to do it: Step-by-Step

  1. Mix the reactants in a flask and start a stopwatch.
  2. At regular time intervals (e.g., every 2 minutes), use a pipette to withdraw a small, known volume (a sample or 'aliquot') from the reaction mixture.
  3. Immediately transfer this sample to another flask and quench it (e.g., place it in an ice bath).
  4. Once quenched, you can take your time to titrate this sample. For our example, you would titrate the remaining NaOH against a standard acid (like HCl).
  5. Repeat this process of sampling, quenching, and titrating until the titration results don't change anymore.

The Result: Your Graph

You'll plot Concentration of reactant (mol dm⁻³) on the y-axis against Time (min) on the x-axis. This graph will show a downward curve as the reactant gets used up.

Key Takeaway

Titrimetric analysis is a precise method for tracking concentration changes. The key step is to 'quench' (stop) the reaction in samples taken at different times before you titrate them.



Method 3: Measuring Temperature Change

When to use it?

This is a more specialised method. It only works for reactions that produce or absorb a significant amount of heat (strongly exothermic or endothermic). You are essentially measuring the rate of heat production/absorption.

Example: Neutralisation of a strong acid and a strong alkali. $$ \text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{Heat} $$

How to do it: Step-by-Step

  1. Conduct the reaction in an insulated container, like a polystyrene cup, to minimise heat loss to the surroundings.
  2. Place a thermometer or a digital temperature probe into the mixture.
  3. Mix the reactants and start the stopwatch immediately.
  4. Record the temperature at regular intervals until it stops changing or starts to drop (due to cooling).
Watch Out! The Big Limitation

Heat loss is a major problem! No insulation is perfect, so the reaction vessel will always be losing heat to the room. This makes it difficult to get highly accurate, quantitative data for a rate graph. This method is often better for simply showing a reaction is happening quickly or slowly, rather than for detailed rate calculations.

The Result: Your Graph

You'll plot Temperature (°C) on the y-axis against Time (s) on the x-axis. For an exothermic reaction, you'll see the temperature rise and then level off.

Key Takeaway

Monitoring temperature change is a simple way to follow a very energetic reaction, but it's less accurate than other methods for rate calculations due to unavoidable heat loss.



From Data to Answers: Interpreting Your Graph

Collecting the data is only half the battle! Now you need to plot it and understand what the graph is telling you. No matter which method you used, the graph of 'Amount of Product' vs. 'Time' will have a similar shape.

Why the shape?

  • Steepest at the start (t=0): The concentration of reactants is highest, so the particles collide most frequently, and the reaction is fastest.
  • Becomes less steep over time: As reactants are used up, their concentration decreases, leading to fewer collisions and a slower reaction rate.
  • Flattens out (gradient = 0): The reaction has stopped because at least one of the reactants has been completely used up.

Finding the Rate: Average vs. Instantaneous

Don't worry, this is easier than it sounds. Think about a car journey.

  • Average Rate: This is like your average speed for the whole trip. It's the total change divided by the total time.
    $$ \text{Average Rate} = \frac{\text{Total change in quantity measured}}{\text{Total time taken}} $$

  • Instantaneous Rate: This is like looking at the car's speedometer at one specific moment. It's the rate at a single point in time. To find it on a graph:
    1. Choose the time point you are interested in.
    2. Use a ruler to draw a tangent to the curve at that exact point (a straight line that just touches the curve).
    3. Calculate the gradient (slope) of that tangent (change in y / change in x). The gradient of the tangent IS the instantaneous rate!
Did you know?

The initial rate of reaction is the instantaneous rate at time t=0. This is always the fastest point of the reaction and is often what chemists are most interested in comparing between experiments!


Final Summary: You've Got This!

  • Monitoring a reaction means measuring a change over time.
  • The best method depends on the reaction: look for changes in gas volume, concentration (titration), or sometimes temperature, mass, or colour.
  • Quenching (stopping the reaction) is essential for the titration method.
  • A graph of your results tells the story of the reaction's speed.
  • The gradient (slope) of the graph tells you the rate: the steeper the slope, the faster the reaction.
  • The rate is fastest at the start and slows down as reactants are used up.

Practice choosing the right method for different reactions and drawing tangents on graphs, and you'll be a master of reaction rates in no time!