Rates of reaction required practical

Aim: To investigate how changing the concentration of a reactant affects the rate of reaction, by measuring the volume of gas produced and by timing how long a solution takes to turn cloudy.

Variables

• Independent: Concentration of hydrochloric acid (gas method) or of sodium thiosulfate solution (cloudiness method)
• Dependent: Volume of gas produced over time, or time taken for the cross to disappear
• Control: Temperature of the solutions; Total volume of solution; Length (mass) of magnesium ribbon, or volume and concentration of acid in the thiosulfate method; Same conical flask and same drawn cross viewed from the same position

Equipment

• Conical flask with a bung and delivery tube
• Gas syringe, or upturned measuring cylinder in a trough of water
• Magnesium ribbon cut into equal lengths
• Hydrochloric acid of different concentrations
• Sodium thiosulfate solution of different concentrations
• Paper with a black cross drawn on it
• Measuring cylinders
• Stopwatch

Method

1. Gas method: measure 50 cm³ of hydrochloric acid into a conical flask and connect the bung and delivery tube to a gas syringe (or an upturned measuring cylinder full of water).
2. Add a 3 cm length of magnesium ribbon, quickly put the bung in and start the stopwatch.
3. Record the volume of gas collected every 10 seconds until no more gas is produced.
4. Repeat with different concentrations of acid, keeping the volume of acid and length of magnesium the same.
5. Cloudiness method: put a conical flask on a piece of paper with a black cross drawn on it.
6. Add 10 cm³ of sodium thiosulfate solution, then add 10 cm³ of dilute hydrochloric acid, swirl and start the stopwatch.
7. Look down through the flask from above and stop the clock when the cross can no longer be seen.
8. Repeat using sodium thiosulfate diluted with water to give different concentrations, keeping the total volume the same, and repeat each concentration three times.

Safety

• Wear eye protection: hydrochloric acid is an irritant.
• The thiosulfate reaction produces sulfur dioxide, which can trigger asthma; work in a ventilated room and pour the mixture away promptly as directed.
• Hydrogen is flammable, so keep flames away from the gas method.
• Do not let the gas syringe plunger be forced out; use small pieces of magnesium.

Results and calculations

For the gas method, record volume against time and plot a graph of volume of gas (y-axis) against time (x-axis) for each concentration; the steeper the curve, the faster the rate. For the cloudiness method, record the time for the cross to disappear, calculate the mean time and use rate ≈ 1 ÷ time. Mean rate = quantity of product formed ÷ time taken (for example cm³/s).

Common mistakes and improvements

• Gas escaping before the bung is inserted; attach the bung as quickly as possible, or keep the magnesium separate in the flask until the bung is in place.
• Different people judging the end point of the cross differently; have the same person watch every run and use the same cross and lighting.
• Temperature changing between runs, which also affects rate; carry out all runs at room temperature or use a water bath.
• Reaction time errors when starting and stopping the stopwatch; repeat each run and calculate a mean, or use a data logger with a light sensor.

Exam tips

• Use collision theory: a higher concentration means more particles in the same volume, so there are more frequent collisions and a faster rate.
• Calculate a mean rate from a graph or data, and on the Higher tier find the rate at a point by drawing a tangent and calculating its gradient.
• Interpret curves: the reaction is fastest at the start (steepest part) and stops when the line goes flat because a reactant is used up.
• Explain why the same person should judge the cross disappearing and why this end point is subjective.
• Know the units of rate, such as cm³/s, g/s or mol/s.