Interpreting Data
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
Understanding Rate of Reaction Graphs Rate of reaction graphs show how the amount of product (or reactant) changes over time. Understanding how to read and interpret these graphs is essential for analyzing experimental data.
Most rate graphs show: X-axis: Time (seconds, minutes) Y-axis: Amount of product formed (volume of gas in cm³, mass in g) OR amount of reactant remaining Key Features of Rate Graphs Feature What It Shows What It Means Steep gradient at start Rapid increase in product Fast reaction - high concentration of reactants Gradient decreases over time Product forms more slowly Reaction slowing down - reactants being used up Curve becomes horizontal (plateau) No more product formed Reaction complete - at least one reactant used up Height of plateau Total amount of product Determined by limiting reactant Steeper curve overall Reaches plateau faster Faster reaction rate Comparing Rates from Graphs You can compare reaction rates by looking at the gradient (steepness) of the curves.
Rate of reaction = Gradient of the curve Gradient = Rise / Run = Change in y / Change in x Comparing different conditions: Steeper gradient = faster reaction Gentler gradient = slower reaction The initial gradient (at t=0) is called the initial rate Calculating Rate from a Graph Method 1: Draw a tangent to the curve Choose a point on the curve (e.g., at t = 0 for initial rate) Draw a straight line that just touches the curve at that point (tangent) Make a large right-angled triangle using the tangent line Calculate: Rate = Rise ÷ Run Example: If tangent line shows: 60 cm³ in 30 seconds Rate = 60 ÷ 30 = 2 cm³/s Method 2: Calculate average rate Average rate = Total product formed ÷ Total time Example: 80 cm³ gas in 200 seconds Average rate = 80 ÷ 200 = 0.4 cm³/s Effect of Concentration on Graphs When concentration increases: Initial gradient is STEEPER (faster initial rate) Reaction reaches plateau FASTER If reactant being changed is in EXCESS, plateau height is THE SAME If reactant being changed is LIMITING, plateau height is HIGHER Example: If you double the concentration of acid reacting with marble chips (acid in excess), the curve is steeper but reaches the same final volume (limited by marble chips).
Effect of Temperature on Graphs When temperature increases: Initial gradient is MUCH STEEPER (much faster initial rate) Reaction reaches plateau MUCH FASTER Plateau height is THE SAME (same amount of reactants) Key observation: Higher temperature curves are steeper but reach the SAME final height (because you haven't changed the amount of reactants).
Effect of Surface Area on Graphs When surface area increases (e.g., powder vs lumps): Initial gradient is STEEPER for powder (faster initial rate) Powder reaches plateau FASTER than lumps Plateau height is THE SAME (same mass of reactant used) Important: If you use the SAME MASS in different forms (5g powder vs 5g lumps), they produce the same total amount of product but at different rates.
Effect of Catalyst on Graphs When a catalyst is added: Initial gradient is MUCH STEEPER (much faster rate) Reaction reaches plateau MUCH FASTER Plateau height is THE SAME (catalyst doesn't change amounts) Remember: Catalysts speed up reactions but don't change the amount of product formed - only HOW FAST it's formed.
Interpreting Plateau Heights The height where the graph levels off tells you about limiting reactants: Observation Explanation Same plateau height Same amount of limiting reactant in both experiments Higher plateau More limiting reactant OR limiting reactant has changed Lower plateau Less limiting reactant available Example: If you react 5g marble chips with 50 cm³ acid (acid in excess), you'll get the same final CO₂ volume whether you use powder or lumps - because you're limited by the 5g of marble chips.
Analyzing Multiple Curves on One Graph When comparing multiple experiments on one graph: Steepest curve = fastest reaction (highest temperature/concentration/surface area, or catalyst present) Gentlest curve = slowest reaction (lowest temperature/concentration/surface area, no catalyst) Same final height = same amount of limiting reactant Different final heights = different amounts of limiting reactant Common Graph Analysis Questions Q: How can you tell which reaction is faster?
A: The one with the steeper gradient / the one that reaches the plateau first. Q: Why do all the curves reach the same maximum? A: Because the same amount of limiting reactant was used in each experiment, so the same amount of product can be formed.
Q: Why does the curve level off? A: Because at least one reactant has been completely used up, so the reaction stops. Q: At what point is the reaction fastest? A: At the start (t = 0), when the concentration of reactants is highest.
This is shown by the steepest gradient being at the beginning. Q: When has the reaction finished? A: When the curve becomes horizontal (gradient = 0), showing no more product is being formed. Reading Values from Tables You …
Interactive revision notes, videos and practice questions load below.