Welcome to Required Practical 7!

In Chemistry, it’s not just about what happens during a reaction, but how fast it happens. This chapter focuses on Required Practical 7 (RP7), where you will learn the two main ways to measure the rate of a reaction: the Initial Rate Method and Continuous Monitoring. Don't worry if kinetics seems a bit "fast-paced" at first—we’ll break down the techniques step-by-step.

What are we trying to achieve?

The goal of RP7 is to gather data that allows us to determine the order of reaction with respect to a reactant. By the end of this practical, you should be able to use your data to link back to the rate equation: \( \text{Rate} = k[A]^m[B]^n \). This practical specifically tests your ability to use different apparatus and techniques (known as AT a, k, and l in your syllabus).


Method 1: The Initial Rate Method (The "Clock Reaction")

The initial rate is the speed of the reaction at the very moment it starts (\( t = 0 \)). At this point, the concentrations of the reactants are exactly what you measured out before mixing them.

How it works

In a "Clock Reaction," you measure how long it takes for a specific, visible change to occur. This might be a color change or a small amount of precipitate forming. Because we are looking at a very small part of the reaction right at the start, we assume the rate is constant during that short time.

The Process

  1. Prepare several versions of the same reaction, but vary the concentration of one reactant while keeping others constant.
  2. Start a timer the moment the reactants are mixed.
  3. Stop the timer when the visible change occurs (e.g., the solution turns blue-black in an Iodine Clock reaction).
  4. Calculate the relative rate using the formula: \( \text{Rate} \propto \frac{1}{t} \).

Quick Tip: We use \( \frac{1}{t} \) as a simplified measure of rate because the "amount of reaction" that happened to cause the color change is the same every time. Therefore, the only variable is the time (\( t \)).

Common Pitfalls to Avoid

  • Temperature fluctuations: Even a small change in temperature can drastically change the rate (as seen in RP3). Use a water bath to keep everything constant.
  • Human error: Reaction times are fast! Having the same person stop the timer each time improves consistency.

Method 2: Continuous Monitoring

While the initial rate method only looks at the start, continuous monitoring tracks the reaction from start to finish. This gives us a "movie" of the reaction rather than just a "snapshot."

Techniques for Monitoring

Depending on the reaction, you can monitor different physical properties:

  • Gas Volume: If a gas is produced (like \( H_2 \) or \( CO_2 \)), use a gas syringe to measure the volume at regular intervals (e.g., every 20 seconds).
  • Mass Loss: If a heavy gas like \( CO_2 \) escapes, place the reaction on a digital balance and record the mass dropping over time.
  • Colorimetry: If the reaction changes color, a colorimeter can measure the absorbance of light. This is very accurate for reactions involving transition metals or iodine.

Graphing the Data

Once you have your data, you plot a graph of concentration (or volume/mass) on the y-axis against time on the x-axis.
To find the rate at any specific time, you must draw a tangent to the curve and calculate its gradient:

\( \text{Gradient (Rate)} = \frac{\Delta y}{\Delta x} \)

Did you know? To find the initial rate from a continuous monitoring graph, you simply draw the tangent at \( t = 0 \).


Linking Practical to Theory: Finding the Order

Once you have the rates for different concentrations, you can figure out the order of reaction (\( 0 \), \( 1 \), or \( 2 \)):

  • Zero Order (0): If you double the concentration, the rate stays the same.
  • First Order (1): If you double the concentration, the rate doubles.
  • Second Order (2): If you double the concentration, the rate quadruples (\( 2^2 = 4 \)).

Note: For more details on calculating these, refer to Section 3.1.9: Rate Equations.


Safety and Accuracy (AT k)

Practical work in AQA Chemistry always requires a focus on safety. In RP7, you may be using acids, oxidising agents, or flammable chemicals.

  • Corrosive substances: Wear safety goggles and lab coats to protect against splashes from acids or alkalis.
  • Toxic chemicals: Some clock reactions use chemicals like iodine or specific indicators. Ensure the room is well-ventilated.
  • Precision: Use a volumetric pipette or burette instead of a measuring cylinder to measure reactant volumes. This reduces the percentage uncertainty in your concentration.

Summary Checklist

Key Takeaways:

  • The Initial Rate Method measures the time taken for a fixed change at the very start of a reaction.
  • Continuous Monitoring involves taking measurements (volume, mass, absorbance) throughout the entire reaction.
  • Rate is the gradient of a concentration-time graph.
  • Tangents are essential for finding the rate at specific points on a curve.
  • Temperature must be controlled as it is a control variable that affects the rate constant \( k \).

Quick Review: If a reaction takes 50 seconds to change color in a clock reaction, the relative rate is \( \frac{1}{50} = 0.02 \text{ s}^{-1} \). If you double the concentration and the time drops to 25 seconds, the new rate is \( 0.04 \text{ s}^{-1} \). Since the rate doubled when the concentration doubled, it is a first-order reaction!


Next in this section: Required Practical 8, where we look at the EMF of electrochemical cells.