Question

Bronze is a solid solution of \(\mathrm{Cu}(\mathrm{s})\) and \(\mathrm{Sn}(\mathrm{s})\) ; solutions of metals like this that are solids are called alloys. There is a range of compositions over which the solution is considered a bronze. Bronzes are stronger and harder than either copper or tin alone. (a) \(\mathrm{A} 100.0\) -g sample of a certain bronze is 90.0\(\%\) copper by mass and 10.0\(\%\) tin. Which metal can be called the solvent, and which the solute? (b) Based on part (a), calculate the concentration of the solute metal in the alloy in units of molarity, assuming a density of 7.9 \(\mathrm{g} / \mathrm{cm}^{3}\) . (c) Suggest a reaction that you could do to remove all the tin from this bronze to leave a pure copper sample. Justify your reasoning.

Short answer

In the bronze alloy, copper is the solvent and tin is the solute. The concentration of tin in the alloy is approximately 6.65M. To remove tin from the bronze, one possible reaction is to treat it with nitric acid, which selectively dissolves tin to produce soluble tin nitrate while leaving copper behind. The reaction is: \(\mathrm{Sn(s) + 4HNO_3(aq) \rightarrow Sn(NO_3)_4(aq) + 2NO_2(g) + 2H_2O(l)}\). This reaction is chosen as it forms a soluble tin compound that can be easily separated from the copper.

Step-by-step solution

(a) Identifying the Solvent and Solute

In a mixture, the component with the larger proportion is the solvent while the other is the solute. It's given that the alloy is 90.0% copper and 10.0% tin by mass. Since the proportion of copper is higher, copper is the solvent and tin is the solute in this alloy.

(b) Calculating the Concentration of the Solute

To calculate the concentration of tin, the solute, in this bronze alloy in molarity, we need to follow these steps: 1. Determine the mass of tin in the sample. 2. Convert the mass of tin to moles, using its molar mass. 3. Determine the volume of the sample. 4. Calculate the concentration of tin in the alloy in molarity. 1. Mass of tin: Given that the alloy sample is 100.0g and 10.0% of it is tin, the mass of tin in the sample is: \(10.0 \% \times 100.0 \mathrm{g} = 10 \mathrm{g}\) 2. Moles of tin: The molar mass of tin (\(\mathrm{Sn}\)) is approximately 118.71 g/mol. So, the moles of tin in the sample are: \(\frac{10 \mathrm{g}}{118.71 \mathrm{g/mol}} \approx 0.0842 \mathrm{mol}\) 3. Volume of the sample: Given that the density of the alloy is 7.9g/cm³, and the mass of the sample is 100g, the volume of the sample is: \(\frac{100 \mathrm{g}}{7.9 \mathrm{g/cm³}} \approx 12.66 \mathrm{cm³}\) 4. Concentration of tin in the alloy in molarity: Converting the volume of the sample from cm³ to L, we get: \(\frac{12.66 \mathrm{cm³}}{1000} = 0.01266 \mathrm{L}\) Now, we can calculate the concentration of tin in the alloy: \(\frac{0.0842 \mathrm{mol}}{0.01266 \mathrm{L}} \approx 6.65 \mathrm{M}\) The concentration of tin in the alloy is approximately 6.65M.

(c) Suggesting a Reaction to Remove Tin and Justification

One possible reaction to remove tin from the bronze alloy is to treat it with nitric acid, which can selectively dissolve tin to produce soluble tin nitrate while leaving copper behind. The reaction is as follows: \(\mathrm{Sn(s) + 4HNO_3(aq) \rightarrow Sn(NO_3)_4(aq) + 2NO_2(g) + 2H_2O(l)}\) The reason for choosing nitric acid is that it reacts with tin to form a soluble compound, tin nitrate, which can be easily separated from the copper. Copper, on the other hand, is relatively unreactive under the same conditions, especially if it forms a protective oxide layer on its surface. After removing the tin from the alloy, the remaining copper can be purified further through an electrorefining process.

Key concepts

Alloys

Alloys are fascinating mixtures composed of two or more elements where the primary component is generally a metal. Here's what you need to know:

- The most common reason to create an alloy is to enhance the properties of a metal, such as its strength, hardness, or resistance to corrosion. For instance, bronze, an alloy of copper and tin, is much stronger and harder than either copper or tin alone.
- The concept of solvent and solute can also apply in alloys. In our example of bronze, copper makes up the larger percentage of the mix, making it the solvent, while tin, being present at a smaller percentage, acts as the solute.

Alloys are crucial in numerous applications, from construction and electronics to jewelry making. They're specifically designed to exhibit superior properties for various applications.

Molarity

Molarity is a measure of the concentration of a solute in a solution, expressed in moles per liter (mol/L). It's a widely used measure for solutions in chemistry. Here's how it applies to the problem:

- To find the molarity of tin in the bronze alloy, we calculate how many moles of tin are in a given volume of the alloy.
- In our exercise, we determined that there are approximately 0.0842 moles of tin in a sample, and the volume of the sample is about 0.01266 liters. The molarity is thus calculated to be about 6.65 M.

This concentration tells us how much tin is really present in the sample, a useful piece of information especially if we're considering chemical reactions or alloy properties. Having such an accurate measure allows chemists and engineers to predict behaviors and reactions of the alloy with different substances.

Chemical Reactions in Metallurgy

Chemical reactions are fundamental in metallurgy, a field concerned with the extraction and processing of metals. In our context, we look at how we might remove tin from bronze. Here's the general idea:

- By using specific chemical reactions, such as treating bronze with nitric acid ( HNO_3 ), we can selectively dissolve one component. Tin reacts with nitric acid to form tin nitrate, a soluble compound which can be separated from the solid copper.
- This technique shows how controlled reactions can alter the composition of an alloy, allowing us to obtain a more pure metal form.

These kinds of reactions are critical in processes like refining, purification, and even in recycling of metals. Each reaction is tailored to the specific properties of the metals and alloys involved, ensuring precision and efficiency in transforming raw materials into useful products.