Measuring Rates of Reaction (OCR GCSE Chemistry A (Gateway)) : Revision Note
Investigate the Effect of Surface Area
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
Weigh out 2.0 g of marble chips on a piece of paper or weighing boat
Use the measuring cylinder to transfer 25 cm3 of dilute hydrochloric acid into the conical flask
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
Start the stop watch and record the mass every 30 s for a total of 5 minutes
Repeat the experiment for each size of marble chip
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
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:
Measure 50 cm3 of Na2S2O3 solution into a flask
Measure 5 cm3 of dilute HCl into a measuring cylinder
Draw a dark cross on a piece of white paper and put it underneath the flask
Add the acid into the flask and immediately start the stopwatch
Solid sulfur is formed which precipitates in solution, turning cloudy:
Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O
Look down at the cross and stop the timing when the cross can no longer be seen
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
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