Experiment 10 Analysis of an Aluminum Zinc Alloy: A Lab Guide

In an aluminum-zinc alloy experiment, the goal is to determine the percentage composition of each metal by measuring the volume of hydrogen gas produced when the alloy reacts with a strong acid. The reaction works because zinc reacts with hydrochloric acid to produce hydrogen gas, while aluminum does not react under the same conditions. This difference allows you to calculate the mass of zinc in the sample and, by subtraction, the mass of aluminum.

Understanding the Chemical Basis

The core of this experiment is a selective reaction. When you add an aluminum-zinc alloy to hydrochloric acid, only the zinc reacts. The balanced chemical equation is:

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

Aluminum does not react with hydrochloric acid under normal conditions because it is protected by a thin, adherent oxide layer. This passive layer prevents the acid from reaching the metal surface. The hydrogen gas produced comes entirely from the zinc component of the alloy.

Why Aluminum Does Not React

Aluminum is a reactive metal, but its immediate reaction with oxygen in the air forms a tough, transparent layer of aluminum oxide (Al₂O₃). This oxide is chemically inert to many acids, including dilute hydrochloric acid. The oxide layer is only a few nanometers thick but is highly effective at protecting the underlying metal. In this experiment, you rely on this property to isolate the zinc reaction.

Experimental Procedure

You will need a sample of the aluminum-zinc alloy, a known volume of hydrochloric acid (typically 1 M or 2 M), a gas collection apparatus (such as an inverted graduated cylinder or a eudiometer), and a balance. The procedure follows these steps:

1. Weigh the alloy sample accurately (around 0.1–0.2 grams is typical).
2. Set up the gas collection apparatus, ensuring it is completely filled with water and inverted in a water bath.
3. Add the alloy to the acid in a reaction flask. Quickly seal the flask and connect it to the gas collection tube.
4. Allow the reaction to proceed until no more gas bubbles appear (usually 5–10 minutes).
5. Record the final volume of hydrogen gas collected. Correct this volume to standard temperature and pressure (STP) using the ideal gas law.

Calculating the Zinc Mass

The volume of hydrogen gas at STP is directly proportional to the moles of zinc that reacted. Using the molar volume of a gas at STP (22.4 L/mol), you can convert the gas volume to moles of H₂. From the balanced equation, one mole of H₂ comes from one mole of Zn. Multiply the moles of Zn by its molar mass (65.38 g/mol) to find the mass of zinc in the sample.

Example calculation: If you collected 40.0 mL of H₂ at STP:

• Moles H₂ = 0.0400 L / 22.4 L/mol = 0.00179 mol
• Mass Zn = 0.00179 mol × 65.38 g/mol = 0.117 g

Determining the Aluminum Mass

Once you know the mass of zinc, subtract it from the total alloy mass to get the mass of aluminum. Then divide each metal’s mass by the total mass and multiply by 100% to find the percentage composition.

Sources of Error and Troubleshooting

Several factors can affect the accuracy of your results. The most common issues include:

• Incomplete reaction: If the acid is too dilute or the alloy pieces are too large, the reaction may stop before all the zinc has reacted. Use fine filings or small chips, and ensure the acid is in excess.
• Gas leakage: Any leak in the apparatus will cause a low gas volume reading. Check all connections and the stopper seal.
• Temperature and pressure corrections: Failing to correct the gas volume to STP can introduce significant error. Record the room temperature and atmospheric pressure at the time of the experiment.
• Oxide layer interference: If the aluminum oxide layer is damaged or if the alloy has been freshly filed, some aluminum may react, producing extra hydrogen. Use a sample with an intact oxide layer.

Correcting Gas Volume to STP

The combined gas law (P₁V₁/T₁ = P₂V₂/T₂) lets you convert the measured volume to the volume it would occupy at 0°C and 1 atm. You need three measurements from the lab: the room temperature (in Kelvin), the atmospheric pressure (from a barometer), and the vapor pressure of water at that temperature (from a reference table). The partial pressure of hydrogen gas is the atmospheric pressure minus the water vapor pressure.

Worked example: Suppose you collect 48.2 mL of gas at 22.0°C and 755.2 mmHg. The vapor pressure of water at 22.0°C is 19.8 mmHg.

• Partial pressure of H₂ = 755.2 mmHg − 19.8 mmHg = 735.4 mmHg
• Convert to atm: 735.4 mmHg ÷ 760 mmHg/atm = 0.9676 atm
• Temperature in Kelvin: 22.0°C + 273.15 = 295.15 K
• Apply the combined gas law: V₂ = (P₁V₁T₂) / (P₂T₁) = (0.9676 atm × 48.2 mL × 273.15 K) / (1 atm × 295.15 K) = 43.2 mL at STP

This corrected volume gives you a more accurate mole count. Without this correction, the calculated zinc mass could be off by 5–10%, depending on lab conditions.

Checking the Oxide Layer Assumption

A subtle but important point is that the aluminum oxide layer must be intact for the experiment to work. If you file, scratch, or sand the alloy sample before adding it to the acid, you may expose fresh aluminum metal. Exposed aluminum can react with hydrochloric acid to produce hydrogen gas:

2Al (s) + 6HCl (aq) → 2AlCl₃ (aq) + 3H₂ (g)

This side reaction would inflate the hydrogen volume and lead to an overestimate of the zinc content. To avoid this, handle the alloy pieces gently and do not abrade the surface. If the alloy comes as a powder or fine filings, the oxide layer may already be compromised, and the experiment will not work reliably.

Key Takeaways

• The experiment relies on the selective reaction of zinc with hydrochloric acid; aluminum does not react due to its protective oxide layer.
• The volume of hydrogen gas produced is directly proportional to the mass of zinc in the alloy sample.
• Correct the gas volume to STP using the ideal gas law before performing stoichiometric calculations.
• Common errors include incomplete reaction, gas leakage, and failure to correct for temperature and pressure.
• Repeat the measurement with multiple samples to improve accuracy and identify systematic errors.