THE EFFECT OF CONCENTRATION ON REACTION RATE
Concentration describes how much of a particular substance there is in a specific volume of solution. □ A dilute solution has only a little of the substance in a lot of water. □ A concentrated solution has a lot of the substance in less water. If we take 100cm3 beakers of dilute acid and concentrated acid, there will be fewer acid particles in the beaker of dilute acid than in the beaker with concentrated acid, even though the volumes of the two solutions are the same. In chemistry, we count particles in terms of formula masses, so we describe concentration in terms of the number of formula masses (moles) of the substance there are in every cubic decimetre (litre) of solution, written as moles.dm-3 or M for short.
USING A 'CLOCK REACTION' TO MEASURE RATE
A 'clock reaction' is a bit like a race. In a race the competitors are timed over a fixed distance. The shorter the time, the quicker the competitor ran. In a chemical reaction we can time how long it takes for a fixed amount of reaction to take place. Again, the shorter the time, the quicker the reaction, and so the faster the rate.
Speed = fixed distance
Reaction rate = fixed amount of reaction
From this we can see that the reaction rate is proportional to l ÷ time.
THE DISAPPEARING CROSS
The reaction used in this experiment is between dilute hydrochloric acid and sodium thiosulphate (formula Na2S2O3). You can see from the chemical equation below that one of the products is sulphur, which does not dissolve in water.
As the reaction proceeds, a fine precipitate of sulphur forms, which makes the water go cloudy. If the reaction is carried out in a beaker standing on a piece of paper marked with an X, the precipitate eventually becomes thick enough to stop the X from being seen. We can time how long it takes for the X to disappear for different concentrations of the sodium thiosulphate solution. This will show us how the reaction rate changes with concentration.
□ two 100cm3 beakers
□ two 250cm3 beakers
□ one large (100cm3) measuring cylinder
□ one small (10cm3) measuring cylinder
□ 3 teat pipettes
□ stop clock
□ paper marked with a pencil X
□ glass stirring rod
□ 2M hydrochloric acid is corrosive. Wear eye protection at all times and rinse any spillages with plenty of water.
□ Sulphur dioxide is an irritant gas. Avoid breathing in the fumes whilst watching the X on the paper. Leave the tap running to flush the contents of the beaker down the sink.
Label one of the 250cm3 beakers ‘THIO’ and collect in it about 160cm3 of 0.15M sodium thiosulphate solution.
Label the other 250cm3 beaker ‘WATER’ and collect in it about 120cm3 of distilled water.
Label one of the 100cm3 beakers ‘ACID’ and collect in it about 30cm3 of 2M hydrochloric acid.
Use the large (100cm3) measuring cylinder to measure out 50cm3 of sodium thiosulphate and pour this into the remaining EMPTY 100cm3 beaker and stand it on the paper marked with an X.
Use the small (10cm3) measuring cylinder to measure out 5cm3 of 2M hydrochloric acid. Quickly pour the acid into the beaker standing on the cross and start the clock. Then stir the beaker once using the glass rod.
Look down at the X through the solution in the beaker. As soon as the X disappears completely, stop the clock and record the time taken in seconds in the results table.
Rinse out the beaker well and dry it. Reset the clock. Repeat the experiment with different concentrations of sodium thiosulphate - make these up according to the instructions given in the results table. Remember to use the large cylinder for thiosulphate and water and keep the small one for the hydrochloric acid.
When you have finished, clear all the used apparatus to the...
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