Acid-Base Properties of Period 3 Oxides Explained

Acid-Base Properties of Period 3 Oxides Explained

Research Question

How do the acid-base properties of the oxides of Period 3 elements change across the period, from sodium to sulfur?

Hypothesis

As you move from left to right across Period 3, the oxides will transition from basic to acidic. Sodium oxide and magnesium oxide will form alkaline solutions, aluminium oxide will be amphoteric (reacting with both acids and bases), and silicon dioxide, phosphorus(V) oxide, and sulfur(VI) oxide will form acidic solutions. This trend is due to the increasing electronegativity of the elements across the period, which affects the type of bonding in the oxide and its behaviour in water.

Variables

  • Independent Variable: The Period 3 element oxides (Na2O, MgO, Al2O3, SiO2, P4O10, SO3).
  • Dependent Variable: The pH of the solution formed by the reaction of the oxide with water, or the reactivity of the oxide with an acid and a base.
  • Controlled Variables: The mass of each oxide, the volume of water, and the temperature of the solutions.

Materials

  • Small samples of the following oxides: sodium oxide (Na2​O), magnesium oxide (MgO), aluminium oxide (Al2​O3​), silicon dioxide (SiO2​), phosphorus(V) oxide (P4​O10​), and sulfur(VI) oxide (SO3​).
  • Deionized water.
  • Universal indicator solution or pH meter.
  • Beakers or test tubes.
  • Aqueous hydrochloric acid (HCl) and aqueous sodium hydroxide (NaOH) solutions.
  • Bunsen burner and heat-resistant mat (for certain reactions).
  • Spatulas, stirring rods, and safety goggles.

Safety Precautions

  • Wear safety goggles at all times.
  • Na2​O and P4​O10​ react vigorously and exothermically with water. Add them to water in small quantities and with great care.
  • SO3​ is highly corrosive and should be handled in a fume hood.
  • Handle strong acids and bases with caution.

Procedure

  1. Preparation: Place a small, equal amount of each oxide into separate test tubes.
  2. Reaction with Water:
    • Carefully add 5 mL of deionized water to the test tube containing Na2​O. Stir and observe. Note any colour changes with universal indicator or measure the pH.
    • Repeat this process for MgO and Al2​O3​. Note that the reactions will be less vigorous. MgO is sparingly soluble and Al2​O3​ is insoluble, so the pH will be measured of the supernatant liquid.
    • For SiO2​, stir with water and note that no reaction occurs.
    • For P4​O10​ carefully add to water in a fume hood, stir, and measure the pH.
    • For SO3​ carefully add to water in a fume hood, stir, and measure the pH. SO3 can be made in situ burning sulfur in oxygen. Burn a small mass of sulfur on a deflagrating spoon in a gas cylinder of oxygen. Add 50ml water containing universal indicator to the gas jar and swirl to dissolve the gas.
  3. Reaction with Acid and Base (for insoluble oxides):
    • Place a small amount of Al2​O3​ in two separate test tubes. Add HCl to one and NaOH to the other. Gently heat both test tubes with a Bunsen burner and observe if the Al2​O3​ dissolves.
    • Repeat this for SiO2​, noting that it will only react with the strong base, NaOH.

Data Collection and Analysis

  • Qualitative Observations: Record the solubility of each oxide in water and the visual observations of the reactions.
  • Quantitative Data: Create a table to record the pH of the resulting solution for each oxide.
  • Analysis: Compare the pH values to the position of the element in Period 3. Analyze how the bonding (ionic vs. covalent) relates to the acid-base nature. Note the amphoteric nature of aluminum oxide by its reactivity with both an acid and a base.

Conclusion and Evaluation

Based on your data, write a conclusion that answers the research question. Discuss whether your hypothesis was supported. Evaluate the experiment, including sources of error and limitations (e.g., the insolubility of certain oxides, the vigor of some reactions). Suggest ways to improve the investigation, such as using a different method to test the acid-base nature of insoluble oxides. This investigation should demonstrate a clear understanding of periodicity, bonding, and experimental design.

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