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

The Reaction of Calcium Carbonate and 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 shifts more towards the mathematical skills of accurately plotting graphs, drawing tangents and 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, where an upturned measuring cylinder of water is placed 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

Rate of carbon dioxide production in the reaction of calcium carbonate and hydrochloric acid table

Time (s)	10	20	30	40	50	60
Volume of CO2 produced (cm3)	34	57	69	71	72	72

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 because it may not be realistically possible to record the mass every second
• 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

Rate of change in mass in the reaction of calcium carbonate and hydrochloric acid table

Time (s)	0	10	20	30	40	50	60
Mass (g)	302.700	302.650	302.600	302.580	302.568	302.568	302.568

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