Practical 1

Water Displacement

Experimental Instructions

You will investigate the reaction between magnesium and a dilute acid by measuring the volume of gas produced.

Read all the instructions carefully before starting the experiment.

Instructions

D is 2.00 mol/dm3 acid.

E is magnesium.

0. Using an electronic balance, measure the mass of the magnesium ribbon provided to you.

1. Fig. 1 shows how the apparatus is arranged.

2. Place a boiling tube into the test-tube rack.

3. Use the 25 cm3 measuring cylinder to measure 15.0 cm3 of the acid, D, and carefully pour it into the boiling tube. Record the actual volume of acid you measured.

4. Use the same 25 cm3 measuring cylinder to measure 5.0 cm3 of deionised water.

Carefully add it to the acid in the boiling tube. Mix the acid and water using a glass rod.

5. Add tap water to the tub to half fill the tub (if this has not been done for you).

6. Fill the 100 cm3 measuring cylinder completely with tap water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.

7. Remove the paper towel and clamp the inverted measuring cylinder so that the open end is in the water.

8. Check that the bung fits the boiling tube. Place the end of the delivery tube into the inverted 100 cm3 measuring cylinder.

9. Use a dropper to introduce small amounts of air from the open end of the inverted measuring cylinder to adjust the water level until it reaches the 10 cm3 mark.

10. You may fold or curl the piece of E so that it can easily be dropped into the boiling tube without getting stuck on the sides.

11. Carefully remove the bung from the boiling tube, drop E into the acid and quickly insert the bung into the boiling tube. Make sure that the only way gas can escape the boiling tube is through the delivery tube.

12. The experiment is complete when the volume of gas collected is no longer changing.

Complete Table 1 to show your results.

Key skill 1: Setting up retort stand

Key skill 2: Measuring Cylinder

When measuring liquids with a measuring cylinder,

Preventing parallax error: The correct volume is when we read the volume where the turning point of the meniscus is. (lowest point of the U)
To get an accurate reading, we often first pour slight extra liquid into the measuring cylinder. Then remove excess liquid with a plastic dropper.

Key skill 3: Inverting measuring cylinder

Discussion question:

What is the piece of paper towel used for? Are there other ways apart from using a piece of paper towel?

Key skill 4: Introducing air into the inverted measuring cylinder

Discussion question:

How much should the original volume of gas in the measuring cylinder be?
Step 9 of the instructions says to introduce small amount of air until the water level reaches the 10 cm3 mark. Must it be 10 cm3? Why or why not?

Key skill 5: Inserting rubber bung

Discussion question

How can we quickly tell if the rubber bung is working correctly (preventing gas from escaping from the boiling tube?)

Results

Fill in the table accordingly. For the mass of magnesium ribbon, fill in the units as well.

Volume of gas collected

To find the volume of gas collected, we need to record

the original volume (e.g. 10.3 ml)
the final volume (e.g. 40.4 ml)

Then, the volume of gas collected = 51.4 - 10.3 = 41.1

Actual Questions

The 'Actual Questions' on Page 2 refers to the actual questions used in the O-level practical examinations.

State which reactant, the acid or magnesium, is the limiting reactant. Explain your answer using observations from the experiment. [1]

(You can google for this question)
What is a reactant?
What are the substances reacting in this experiment?
Which of the reactant is completely used up and which is not completely used up? The reactant that is completely used up is the limiting reactant, while the reactant that is left over is the excess reactant.

A student measures the volume of gas produced every 10 seconds during a reaction between magnesium and 0.50 mol/dm3 acid until the reaction is complete. The student's results are shown in Table 2. The acid is the limiting reactant in this reaction.

(i) Plot a graph of the volume of gas collected against time on the grid.

Draw a line of best fit using all of the plotted points. [4]

(ii) Use your graph to calculate the average rate of gas production for the first 25 s of the reaction.

Give your answer to three significant figures.

Show all working.

average rate of gas production for the first 25 s = ______________________________[2]

"Plot a graph of the volume of gas collected against time on the grid."

It is always plot y against x.
Hence, volume of gas collected = y-axis, and time = x-axis.

There are 12 big boxes for y-axis, and 10 big boxes for x-axis. ("One big box" = The box coloured blue on the left)
Based on the data give, you need to choose a suitable scale that can fit the graph paper.
In this case, it is pretty straight forward, and you can do 10 cm^3 per big box. (example on the right)

Using ChatGPT to help you

You can actually get ChatGPT to help you plot the graph. This can serve as a guide. While you cannot submit the graph plotted by ChatGPT, you can tell how your graph should roughly look like.

Similarly, ChatGPT is not a perfect tool yet. From the graph ChatGPT plotted, the "line of best fit" drawn connects each consecutive point with a straight line. This is actually WRONG. For your graph, you need to draw a smooth curve that connects all the points. (Preferrably with curved ruler/french curve)

Limitations of ChatGPT

(iii) Describe how the rate of reaction changes during the reaction. Refer to your graph in your answer. [2]

To be gone through in class

(iv) The same student carries out a second experiment using the same acid with a different concentration of 0.25mol/dm3. All other conditions are the same and the reaction is complete by 90 s.

Sketch a line on the graph to show the results you would predict for this experiment.

Label this line X. [2]

What changed in this experiment? (Concentration)
Did the concentration increase or decrease compared to the original? By how many times? (Decreased by 2x)
Concentration affects the speed of reaction. (The lower the concentration, the slower the speed of reaction. If concentration decreases by 2x, then speed of reaction should decrease by 2x)
What affects the volume of gas collected? Is the volume of gas collected in the two experiments different or the same? (Volume of acid used stays the same because it says 'all other conditions are the same', and since concentration is halved, the actual amount of acid is halved. These means that the total volume of gas collected at 90s is also halved.

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