Rate of Reaction - Calcium Carbonate & Hydrochloric Acid (OCR A Level Chemistry A): Revision Note

PAG 9.2: Rate of reaction - calcium carbonate & hydrochloric acid

• At GCSE level, this reaction is a standard experiment to see the effect of changing the surface area of a reactant on the rate of reaction

  • Three sizes of calcium carbonate (marble) are commonly used:

    • Large chips

    • Small chips

    • Powder

• The reaction itself cannot change

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

• However, the application and analysis of the results depends on mathematical skills:

  • Accurate graph plotting

  • Drawing tangents

  • Calculating gradients

• This reaction can be monitored by measuring:

  1. The volume of carbon dioxide gas produced

  2. The mass loss from the reaction vessel

Method 1 - Volume of CO₂ produced

1. Support a gas syringe with a stand, boss and clamp.

2. Add 50 cm³ of dilute hydrochloric acid to a conical flask

3. Loosely connect the gas syringe 

4. Measure 0.40 g of calcium carbonate

5. Add the 0.40 g of calcium carbonate into the conical flask, replace the gas syringe and start the stopwatch

6. Record the volume of gas produced every 10 seconds

   • Continue timing until no more carbon dioxide appears to be given off

• Another method to achieve these results would be the downward displacement of water method

  • This uses an inverted cylinder of water in a water trough

  • This method still records the volume of carbon dioxide produced

Specimen results 1 - Volume of CO₂ produced

• Here is a set of typical results for this experiment:

Graphing the results 1 - Volume of CO₂ produced

Analysis 1 - Volume of CO₂ produced

• The curve of best fit is drawn on the graph

• A tangent can then drawn starting from (0,0) to find the initial rate of reaction

• The gradient of the tangent is calculated

  • This is the rate of reaction

• In the example above, the rate of reaction is:

  • Gradient 3.6 cm³ s^-1 

Method 2 - Mass of the reaction vessel

1. Measure 0.40 g of calcium carbonate into a weighing boat

2. Add 50 cm³ of dilute hydrochloric acid to a conical flask

3. Place the conical flask of hydrochloric acid AND the weighing boat of calcium carbonate onto the balance

4. Measure the combined mass and record this as the t = 0 result

5. Add the 0.40 g of calcium carbonate into the conical flask, replace the weighing boat onto the balance and start the stopwatch

6. Record the mass every 10 seconds

   • Continue timing until no further mass is lost

• It may be necessary to use more dilute concentrations of hydrochloric acid

  • This is because it may not be realistically possible to record the mass every 10 seconds

• A data logger could be connected to the balance (if available) and this can track the mass over time 

Specimen results 2 - Mass of the reaction vessel

• Here is a set of typical results for this experiment:

Graphing the results 2 - Mass of the reaction vessel

Analysis 2 - Mass of the reaction vessel

• The curve of best fit is drawn on the graph

• A tangent can then drawn starting from (0,0) to find the initial rate of reaction

• The gradient of the tangent is calculated

  • This is the rate of reaction

• In the example above, the rate of reaction is:

  • Gradient (a loss of) 5.26 x 10^-3 g s^-1 

• A simpler version of this practical can be done by hitting the TARE / reset button on the balance when the calcium carbonate is added

  • This then records the mass lost and can be used to give a graph with a curve more like the volume of CO₂ produced graph

Practical skills reminder

• This practical develops essential skills in measuring rates of reaction by gas collection or mass loss.

• It also supports:

  • Using a stopwatch and gas syringe or balance to collect data at regular intervals

  • Drawing and analysing a graph to determine the initial rate from a tangent

  • Understanding how experimental method affects the type and accuracy of data collected