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Introduction

The alkali metals are the most reactive of metals, and they are placed in Group I of the Periodic Table. You are to simulate the reactions of sodium and potassium with water, and to investigate the properties of the products.

Task 1: Reactions of the alkali metals


Reactions of the Alkali Metals

  1. Open Yenka file Model 1.
  2. Select a lump of sodium and place it in the simulation. Select the sample of sodium and view the atomic animation. Describe what you see, and explain the bonding in sodium.
    Answer
    Regular arrangement (lattice) of sodium ions/atoms.
    Each sodium atom loses its outer electron, forming a sea of delocalised electrons. The lattice of positive ions is attracted to the sea of electrons by strong electrostatic forces.
  3. Add the sample of sodium to the water bath and write down your observations. Select the water bath in order to record the maximum temperature reached. Use the reaction details to write down the equation for the reaction occurring.
    Answer
    Bubbles, orange flame, 38 °C
    2Na + 2H2O → H2 + 2NaOH
  4. Allow the reaction to finish, then filter the solution into a clean beaker. Select the filtrate and observe the atom viewer. Describe what you see.
    Answer
    Sodium ions and hydroxide ions in constant random motion.
  5. Describe the bonding in solid sodium hydroxide.
    Answer
    Ionic – lattice of alternating positive sodium ions and negative hydroxide ions, held together by strong electrostatic forces
  6. Add some universal indicator to the solution, record the colour and use the appropriate chart to estimate the pH. What does this tell you about sodium hydroxide solution?
    Answer
    Deep blue, pH 14
    Strong alkali.
  7. Undo the addition by pressing Ctrl and Z at the same time. Now test the solution with phenolphthalein. Undo and test with thymol blue. Record your results in the table.
    Colour with phenolphthalein Colour with thymol blue
    Answer
    pink
    Answer
    blue
  8. Reload the simulation (F5) but this time add a lump of potassium instead of the sodium. Record your observations. Select the water bath in order to record the maximum temperature reached. Use the reaction details to write down the equation for the reaction occurring.
    Answer
    Bubbles, flames, 30 °C
    2K + 2H2O → H2 + 2KOH
  9. Test the pH of the solution using universal indicator.
    Answer
    14
  10. Both the metals are in the same group of the Periodic Table. With reference to the electronic structure of sodium and potassium, explain why the elements are placed in the same group of the Periodic Table.
    Answer
    Both have one electron in their outer shell.
  11. To demonstrate the reactivity of potassium, open a new simulation and try adding a small sample to some hydrochloric acid. This should only be carried out in the simulator as it is far too dangerous to be attempted in real life. Record your observations.
    Answer
    Explodes

Summary

Sodium and potassium exhibit very similar properties and undergo similar reactions since they are in the same group of the Periodic Table. There is a trend within the group, with potassium being more reactive than sodium. This trend can be explained in terms of the electronic structures of the elements. Both metals react violently with water, producing alkaline solutions and releasing hydrogen .

Task 2: A titration using potassium hydroxide

  1. Open Yenka file Model 2. The simulation contains the product of your reaction of potassium with water. It also contains a burette holding 50 cm3 of 1 M hydrochloric acid. The potassium hydroxide solution is an alkali so will react with the acid. What sort of reaction will occur? Predict the products of the reaction.
    Answer
    Neutralisation, producing potassium chloride and water.
  2. Add the thymol blue indicator to the potassium hydroxide solution. Use the tap to add the acid slowly to the alkali (about 1 cm3 every 2 seconds) until 24 cm3 have been added. Pause the simulation. Record what happens to the pH graph.
    Answer
    Very little change to start with, but starts to fall more steeply over last few cm3.
  3. Now add the acid much more slowly and restart the simulation. When the pH changes very rapidly and drops to 7, pause the simulation and read the volume of 1.0 M acid added. Record the colour of the indicator.
    Answer
    25.65 cm3, solution goes yellow.
  4. Sketch the shape of the pH graph you have produced.
    Answer


    Shape of pH curve

  5. Use the volume of acid added to calculate the number of moles of acid added (moles = concentration x volume / 1000).
    Answer
    moles of acid = 0.02556
  6. This is equal to the number of moles of alkali, and therefore equals the number of moles of potassium added at the start of the experiment. Remember that you added 1 g of potassium. Use this information to work out the relative atomic mass of potassium (moles = mass / relative atomic mass).
    Answer
    0.02556 = 1.0 / Relative Atomic Mass
    so Ar = 1.0 / 0.02556 = 39

Summary

Titration is a technique that allows the addition of carefully controlled volumes of substances to one another. By titrating with a known concentration of acid, it is possible to work out the amount of alkali present in a solution formed from a known mass of potassium, and hence calculate the relative atomic mass of the alkali metal.

Teacher Summary

  • Pupils need to know about electronic arrangement and the Periodic Table in order to answer questions.
  • Note that the reaction between alkali metals and acids should never be attempted.
  • The titration simulation is suitable for all pupils, but the use of the data to work out the Relative Atomic Mass of potassium is only suitable for the more advanced pupils.