Measuring Rates of Reaction (OCR Gateway GCSE Chemistry: Combined Science)

Investigate the Effect of Surface Area

Practical Skills Development

• Use of appropriate apparatus to make and record a range of measurements accurately, including mass, time, temperature, and volume of liquids and gases
• Making and recording of appropriate observations during chemical reactions including changes in temperature and the measurement of rates of reaction by a variety of methods such as production of gas and colour change

Aim

• To investigate the effect of surface area on the rate of an acid/carbonate reaction

Health & Safety Aspects

• Safety glasses should be worn throughout the preparation
• Hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin

Materials

• Marble chips of three different sizes
• 2.0 mol/dm³ hydrochloric acid
• Measuring cylinder
• Conical flask
• Top-pan balance capable of weighing to 2 d.p.
• Stop watch

Set up for investigating the effect of surface area on a reaction rate

Practical tip

• Try to get the mass of marble chips in each run as close to 2.0 g as possible so the mass loss will be easier to plot on a graph

Method

1. Weigh out 2.0 g of marble chips on a piece of paper or weighing boat
2. Use the measuring cylinder to transfer 25 cm³ of dilute hydrochloric acid into the conical flask
3. Place the flask on the balance and quickly add the marble chips and add a loose plug of cotton wool to the neck of the flask
4. Start the stop watch and record the mass every 30 s for a total of 5 minutes
5. Repeat the experiment for each size of marble chip
6. Draw a line graph of mass loss(x) against time (y) and determine the initial rate of reaction by drawing a tangent to the curve for each size of marble chip

Results

Investigating the Effect of Surface Area Results

Conclusion

• The smaller the size of the chips, the larger the surface area and faster the rate of reaction
• The faster the rate of reaction the greater the mass loss in the same interval of time

Investigate the Effect of Concentration

Aim

• To investigate the effect of changing concentration on the rate of reaction by measuring the formation of a precipitate

Health & Safety Aspects

• Safety glasses should be worn throughout the preparation
• Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
• The reaction between sodium thiosulfate and hydrochloric acid produces sulfur dioxide which is toxic if inhaled
  • This can be particularly problematic for asthmatics so ensure the practical is carried out in a well ventilated area

Materials

• 40 g/dm³ sodium thiosulfate solution
• 1.0 mol/dm³ dilute hydrochloric acid
• Conical flask (100 cm³)
• Black cross on paper
• White paper or white tile
• Stopwatch or timer

Diagram showing the apparatus needed to investigate reaction rate in a precipitation reaction

Practical Tip:

• The same person should be the one recording the time taken for the cross to disappear as this measurement is open to bias as it is based on human perception

Method:

1. Measure 50 cm³ of Na₂S₂O₃ solution into a flask
2. Measure 5 cm³ of dilute HCl into a measuring cylinder
3. Draw a dark cross on a piece of white paper and put it underneath the flask
4. Add the acid into the flask and immediately start the stopwatch
5. Solid sulfur is formed which precipitates in solution, turning cloudy:
   • Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O
6. Look down at the cross and stop the timing when the cross can no longer be seen
7. Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)

Results

• Record your results in a suitable table, eg:

Investigating the effect of concentration of a reaction rate

Conclusion:

• A graph can be plotted of the results with time on the x-axis and the concentration of Na₂S₂O₃ on the y-axis
• With an increase in the concentration of a solution, the rate of reaction will increase so the time for the cross to disappear decreases
• As there are more reactant particles in a given volume, collisions occur more frequently, increasing the rate of reaction