Rate of reaction common mistakes

Common mistakes

• Misidentifying the rate unit

  Students often write the rate of reaction as g s⁻¹ or mol s⁻¹ but forget to include the correct unit for the quantity used (e.g. g s⁻¹ for mass change, cm³ s⁻¹ for volume change, or mol s⁻¹ for moles).

  Fix itAlways pair the rate with the appropriate unit that matches the measured quantity: if you are using mass, use g s⁻¹; if volume, use cm³ s⁻¹; if moles, use mol s⁻¹. Write the unit as a fraction (e.g., g s⁻¹) rather than a product (g s).

• Confusing Rate Calculation

  Students often confuse the formula for calculating the mean rate of reaction, mistakenly using the time taken as the numerator instead of the quantity of reactant used.

  Fix itRemember that the mean rate of reaction is calculated by dividing the quantity of reactant used by the time taken. Ensure you set up the formula correctly as mean rate = quantity of reactant used / time.

• Confusing Rate Calculation

  Students often confuse the formula for calculating the mean rate of reaction by using the quantity of reactant instead of the quantity of product formed.

  Fix itAlways ensure to use the quantity of product formed divided by the time taken when calculating the mean rate of reaction.

• Incorrect Units for Rate of Reaction

  Students often use incorrect units when calculating the rate of reaction, such as using grams instead of g.

  Fix itAlways ensure to express the rate of reaction in the correct units, such as g, c, or mol/s, depending on the context of the calculation.

• Misinterpreting Graphs

  Students often misinterpret the steepness of a graph, thinking a steeper graph indicates a slower reaction rate.

  Fix itRemember that a steeper graph gradient actually indicates a faster reaction rate. Always link the gradient to the rate of reaction.

• Misinterpreting Tangents

  Students often incorrectly draw tangents that do not touch the curve at the point of interest, leading to inaccurate rate estimates.

  Fix itEnsure the tangent line touches the curve at the specific time point and is drawn so that it represents the slope accurately at that moment.

• Misunderstanding Graph Gradients

  Students often confuse the steepness of the graph with the amount of product formed, rather than understanding it represents the rate of reaction.

  Fix itFocus on how the steepness of the graph indicates the speed of the reaction, where a steeper gradient means a faster rate of reaction.

• Misunderstanding Reaction Rate Decrease

  Students often think that the rate of reaction decreases because the reactants are used up completely.

  Fix itExplain that the rate usually decreases due to fewer effective collisions between reactant particles as their concentration decreases over time.

• Misunderstanding Graph Gradients

  Students often confuse the steepness of a graph with the amount of product formed, thinking a higher amount always means a faster reaction rate.

  Fix itEmphasize that a steeper gradient specifically indicates a faster rate of reaction, regardless of the total amount of product formed.

• Confusing Rate Calculation

  Students often confuse the calculation of rate by using the wrong quantity (e.g., using mass instead of gas volume or vice versa).

  Fix itEnsure to identify whether you are measuring the change in mass or gas volume and use the appropriate quantity in your rate calculation.