Question

Describe a series of simple chemical reactions that you could use to determine whether a particular metal sample is "aluminum 2S" (99.2\% Al) or "magnalium" (70\% Al, 30\% Mg). You are permitted to destroy the metal sample in the testing.

Short answer

Observe reactions with copper(II) chloride and sulfuric acid to identify if the metal is 'aluminum 2S' or 'magnalium'. Quantify aluminium content using a spectrophotometer. If evolution of heat is more and the solution turns blue, it indicates presence of aluminum. If significant gas evolution is observed but aluminum content is less than 99.2%, it is likely 'magnalium'.

Step-by-step solution

Test for Aluminum

In the first step, a sample of the metal needs to be taken and mixed with a solution of copper(II) chloride (\( CuCl_2 \)). When aluminum reacts with copper(II) chloride, it forms aluminum chloride and copper. The reaction occurs rapidly generating a lot of heat and turning the solution blue. If the evolution of heat is prolific and the solution turns blue, the metal contains aluminum.

Quantify Aluminum Content

To differentiate between 'aluminum 2S' and 'magnalium', the amount of aluminum in the sample needs to be quantified. You can use a spectrophotometer to measure the amount of copper formed in the reaction. The amount of copper is directly proportional to the amount of aluminum in the sample. If the aluminium concentration is about 99.2%, it's 'aluminum 2S'.

Test for Magnesium

In the third step, the sample is reacted with a solution of sulfuric acid (\( H_2SO_4 \)). Magnesium reacts with sulfuric acid to produce hydrogen gas and magnesium sulfate. If sufficient amounts of gas are produced it suggests the presence of magnesium. If the evolution of this gas is observed but the quantity of aluminum was less than 99.2%, the metal is likely 'magnalium'.

Key concepts

Aluminum 2S Identification

Identifying Aluminum 2S is a fundamental task in material science, particularly when precision in metal composition is required for engineering applications. The process begins with a simple reaction test where the sample is mixed with copper(II) chloride (CuCl_2). The ensuing reaction, characterized by significant heat evolution and a blue coloration as copper metal is formed, indicates the presence of aluminum due to its high reactivity with the chloride.

Distinguishing Aluminum 2S

Once aluminum presence is confirmed, the specific determination of Aluminum 2S requires quantifying the aluminum content. Since 2S refers to a composition with approximately 99.2% aluminum, precision is key. The intensity of the reaction and comparison with known standards can provide initial clues, but for accurate quantification, a spectrophotometer can be utilized to measure the copper produced in the reaction. By linking the amount of copper to the amount of aluminum, we can infer whether the sample is indeed Aluminum 2S—if nearly all the sample reacts to produce copper, it's a strong indication of the high purity aluminum.

Magnalium Differentiation

Magnalium is an alloy composed of 70% aluminum and 30% magnesium, presenting a unique challenge for identification due to its mixed composition. After establishing the presence of aluminum as described in the aluminum identification section, we focus on magnesium detection.

Reacting with Sulfuric Acid

To ferret out the presence of magnesium, the piece de resistance is the addition of sulfuric acid (H2SO4). When magnesium encounters sulfuric acid, it produces hydrogen gas and magnesium sulfate—a reaction that is not only indicative of magnesium but also quantifiable. The evolution of gas bubbles and the specific rate of this reaction can help determine the amount of magnesium present. If both metals are detected but not in the purity expected of Aluminum 2S, the sample is most likely magnalium. This differentiation is essential, especially since magnalium has distinct properties from pure aluminum or aluminum alloys with higher aluminum content, such as enhanced strength and reduced weight.

Spectrophotometer Use in Chemistry

Spectrophotometry is an analytical method that is harnessed to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution.

The Role of Spectrophotometry

In the context of metal identification, spectrophotometry is invaluable. It offers a non-destructive means to quantify the concentration of elements in a sample based on their light absorption characteristics. When used in the test for Aluminum 2S, for instance, it measures the amount of light absorbed by the copper formed in the reaction with copper(II) chloride. The amount of light absorbed correlates with the concentration of copper, and consequently, aluminum.

By using this method, not only is it possible to confirm the presence of specific metals, but also to deduce the precise percentages of aluminum and magnesium in a magnalium sample. This allows not only for differentiation between Aluminum 2S and magnalium but enables precise quality control in manufacturing processes where material properties are critical.