7a - rpa Flashcards
Investigating the effect of surface area on rate of reaction
You can use the reaction of marble chips (a form of calcium carbonate) with dilute hydrochloric acid to investigate the effect of surface area on the rate of a reaction
The equation for the reaction of marble chips with dilute hydrochloric acid
2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O+ CO2(g)
This reaction gives off carbon dioxide gas, so you can follow the rate of the reaction by measuring the volume of gas produced using a gas syringe.
method for reaction of marble chips with dilute hydrochloric acid
Use a measuring cylinder to measure 100 cm³ of dilute hydrochloric acid and add it to a conical flask.
Weigh out 2 g of marble chips using a mass balance. Add the chips to the acid and quickly attach an empty gas syringe to the flask. Start the stopwatch.
Take readings of the gas volume at regular intervals (e.g. every 30 seconds) until the volume hasn’t changed for three readings in a row.
Put the results in a table. Plot a graph with time on the x-axis and volume of gas produced on the y-axis.
Use a pestle and mortar to lightly crush some more marble chips and repeat the experiment using these. Make sure you use exactly the same volume and concentration of acid, and exactly the same mass of marble chips. This will ensure your experiment is a fair test - - see page 10.
Repeat the experiment again with the same mass of powdered chalk.
results of reaction of marble chips with dilute hydrochloric acid
Changing the size of the marble chips changes the surface area of the solid reactant. Smaller particles have a bigger surface area to volume ratio than larger particles - so as you go from large chips to crushed chips to powdered chalk, the surface area to volume ratio of the calcium carbonate increases.
this diagram is an example of the type of thing you would expect to see if you carried out the experiment and plotted your results.
Lines 1 to 3 on the diagram show that the finer the particles are (and therefore the greater the surface area of the solid reactant), the sooner the reaction finishes and so the faster the reaction.
Investigating the effect of concentration on rate in the reaction of marble chips with dilute hydrochloric acid
The reaction between marble chips and hydrochloric acid is also good for measuring how changing the reactant concentration affects reaction rate.
You can measure the effect of concentration on rate by following the same method described on the previous page. However, this time you repeat the experiment with exactly the same mass and surface area of marble chips and exactly the same volume of acid, but using different concentrations of acid. Figure 4 shows the results you might expect to get. Lines 1 to 3 in Figure 4 show that a higher concentration gives a faster reaction, with the reaction finishing sooner.
Investigating the effects of temperature on rate
You can see how temperature affects reaction rate by looking at the reaction between sodium thiosulfate and hydrochloric acid. Sodium thiosulfate and hydrochloric acid are both colourless solutions. They react together to form a yellow precipitate of sulfur, so the reaction mixture will become more turbid (cloudy) as the reaction continues:
2HCI (aq) + Na2S2O3(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l)
This experiment involves looking through the reaction solution at a black cross and timing how long it takes to disappear.
Investigating the effects of temperature on rate - method
Start by measuring out 50 cm³ of dilute sodium thiosulfate solution and 10 cm³ of dilute hydrochloric acid. Use a water bath to gently heat both solutions to the same desired temperature before you mix them.
Mix the solutions in a conical flask. Place the flask on a piece of paper with a black cross drawn on it and start the stopwatch.
Now watch the black cross through the cloudy, yellow sulphur and time how long it takes to disappear. Record your results in a table.
Repeat the reaction a few more times, but each time heat the solutions of hydrochloric acid and sodium thiosulfate to a different temperature - e.g., you could carry out the reaction at 20 °C, 30 °C, 40 °C and 50 °C. Repeat the reaction three times at each temperature and calculate the meantime taken for the cross to disappear. This will make your results more precise.
Investigating the effects of temperature on rate - Control variables
The depth of the solutions must be kept the same, so make sure you use the same reaction flask each time you repeat the experiment. Also, different people might think the cross has disappeared at slightly different times, so the same person should observe the cross each time, to make it a fair test.
Investigating the effects of temperature on rate - The results
You can plot the time taken for the mark to disappear against the temperature of the reacting solutions.
If you look at Figure 2, you can see that the reactions that happened at lower temperatures took longer to obscure the mark, whereas the reactions happening at higher temperatures finished sooner. So increasing the temperature increases the rate of the reaction.
