Experimental Design (Cambridge O Level Chemistry)

A student investigated the effect of temperature on the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid:

Na₂S₂O₃ (aq) + 2HCl (aq)  → 2NaCl (aq) + S (s) + SO₂ (g) + H₂O (l)

The reaction between these two substances produces a precipitate which makes the mixture turn cloudy. 

A student timed how long it took until the cross could no longer be seen through the precipitate, shown in Figure 1.

They then calculated the rate of reaction and repeated the experiment at different temperatures. 

Figure 1 

Name the product that made the mixture go cloudy.

Suggest two control variables for the investigation that would ensure valid results are obtained.

Define the terms:

From the products formed in part a) identify:

Rock salt is a mixture of sand and salt. Salt dissolves in water while sand does not. 

A group of students separated rock salt. 

They used the following method.

1. Add the rock salt to a beaker.

2. Add 250 cm³ of water.

3. Leave without disturbing to allow the sand to settle to the bottom.

4. After a while, filter the salt water into an evaporating dish.

5. Heat the evaporating dish with a Bunsen burner until salt crystals begin to form.

Identify the residue and filtrate in step 4. 

Suggest one safety precaution the students should take in step 5.

Another student separated water from salty water using the apparatus in the diagram below.

By referring to points 1 and 2 in the diagram, describe how this technique works.

Identify the residue in the experiment being carried out in part (d).

A student investigated the effect of the size of calcium carbonate lumps, CaCO₃, on the rate of reaction with hydrochloric acid, HCl, using the following method:

1. Put 50 cm³ of hydrochloric acid into a conical flask 
2. Add 12 g of large calcium carbonate lumps into the flask
3. Attach the gas syringe
4. Measure the volume of gas produced every 30 seconds for 180 seconds 
5. Repeat steps 1 - 4 using 12 g of small calcium carbonate lumps.
6. The number of moles of gas for each volume was calculated.

The results for large calcium carbonate lumps are shown below.

Table 1

The student had already plotted the data for small calcium carbonate lumps.

  
Plot the data for the large calcium carbonate lumps and draw a line of best fit.

Determine the mean rate of reaction for small calcium carbonate lumps between 35 seconds and 90 seconds. 

Give the unit. 

Use the graph in part (a)

What conclusion can be made about the rate of reaction of small calcium carbonate lumps compared to large calcium carbonate lumps?

Give one reason for your answer. 

Complete and balance the equation for the reaction between calcium carbonate and hydrochloric acid.

State one advantage and two disadvantages of using a gas syringe as opposed to a upturned measuring cylinder when collecting gas.

A student investigated what happened when sodium thiosulphate dissolved in water.

   Experiment 1

By using a measuring cylinder, 20 cm³ of distilled water was poured into a polystyrene cup.
Use the thermometer diagram to record the temperature of the water in the table.

1 g of powdered sodium thiosulphate was added to the cup and the mixture stirred with a thermometer. Use the thermometer diagram to record the temperature of the solution.

   Experiment 2

Experiment 1 was repeated using 2 g of powdered sodium thiosulphate. Record the temperature in the table.

   Experiments 3, 4 and 5

Experiment 1 was repeated using 3 g, 4 g and 5 g of powdered sodium thiosulphate respectively. Record the temperatures in the table.

What type of chemical reaction occurs when sodium thiosulphate dissolves in water?

Explain how the temperature changes would differ in the experiments if 40 cm³ of water were used.

Suggest one change you could make to the apparatus used in the experiments to obtain more accurate results.

Ammonia is produced when aqueous sodium hydroxide is warmed with ammonium sulfate. Ammonia is less dense than air and very soluble in water. The apparatus below was used to prepare a sample of dry ammonia gas.

Name substance C.

Name substance D.

What necessary piece of equipment is missing in the diagram?

Suggest why concentrated sulfuric acid should not be used to dry ammonia.

There are two other mistakes in the apparatus shown in the diagram. Identify and explain these mistakes.

mistake 1 ..........................................................................................................................
explanation .......................................................................................................................
mistake 2 ..........................................................................................................................
explanation .......................................................................................................................