Problem 7 - Chemistry The Central Science In Si Units

Question

The labels have fallen off three bottles containing powdered samples of metals; one contains zinc, one lead, and the other platinum. You have three solutions at your disposal: \(1 \mathrm{M}\) sodium nitrate, \(1 M\) nitric acid, and \(1 M\) nickel nitrate. How could you use these solutions to determine the identities of each metal powder?

Step-by-Step Solution

Answer

React nitric acid with the samples: zinc will react, the least reactive (no reaction) is platinum, and the other is lead.

Step 1: Understanding Metal Reactivity

Zinc, lead, and platinum have different reactivity levels with acids and nitrates. Zinc is the most reactive and will react with nitric acid ( NO_3 ) to produce zinc nitrate and hydrogen gas. Lead is less reactive and can react slightly with nitric acid to form lead nitrate, but no reaction with sodium nitrate and nickel nitrate. Platinum is the least reactive and will not react with any of these solutions.

Step 2: Testing with Nitric Acid

Add a small amount of nitric acid ( NO_3 ) to each metal powder. Observe reactions. Zinc will react vigorously, producing gas and forming zinc nitrate. If a metal shows this reaction, it is zinc.

Step 3: Testing with Sodium Nitrate and Nickel Nitrate

Use the sodium nitrate and nickel nitrate solutions to test the remaining metals. Lead does not react with either of these solutions, while platinum will also show no reaction to any solution due to its low reactivity. Thus, after identifying zinc with nitric acid, if two metals show no reaction with sodium or nickel nitrate, they must be lead and platinum.

Step 4: Confirming Metal Identities

After observing the reactions, identify the metals: the one that reacts strongly with nitric acid is zinc, the one that doesn't react with any solution after identifying zinc is platinum. The remaining one is lead.

Key Concepts

Understanding Zinc Reactivity

Zinc's reactivity is a powerful tool in distinguishing it from other metals like lead and platinum. It's one of the more reactive metals, especially when it contacts acids like nitric acid. When you place zinc in nitric acid, a vigorous reaction occurs. This process releases hydrogen gas, visibly bubbling as the zinc converts into zinc nitrate.

• This reactivity makes zinc stand out quickly due to the pronounced gas formation.
• The gas you see is actually hydrogen, a clear sign that the reaction is happening as expected.
• This reactivity is not only limited to nitric acid; zinc will generally show reactivity with several other substances as well.

If you're trying to identify zinc among a group of metals, nitric acid is an excellent reagent because of these visible, vigorous reactions. In a setup where you have a mystery powder, if you see bubbling and hear fizzing after adding nitric acid, there's a good chance you've found zinc.

Analyzing Lead Reactivity

Lead displays a middling reactivity, which means it doesn't react as vigorously as zinc, but it's not entirely inert either. Lead can slightly react with nitric acid to form lead nitrate but won't show the same intense bubbling reaction you see with zinc.

• Due to this subdued reaction, observing lead’s interaction with nitric acid might not be as dramatic.
• The reaction primarily results in the formation of lead nitrate, which remains in solution without the release of gas.
• Importantly, lead does not readily react with solutions like sodium nitrate and nickel nitrate.

Thus, when working with unidentified metal powders, you should expect a quiet response from lead when exposed to nitric acid. If after testing with stronger acids you notice no bubbling, this could suggest the presence of lead, especially if other high-reactivity metals like zinc have been ruled out.

Investigating Platinum Reactivity

Platinum is a particularly interesting metal because of its low reactivity. This quality makes it very useful for applications where corrosion resistance is key, but it also means that in a chemical context, platinum doesn't react with many common solutions.

• Regardless of whether you use nitric acid, sodium nitrate, or nickel nitrate, platinum will generally show no observable chemical reaction.
• This lack of reaction is a crucial identifying feature when working with mixed metal powders.
• It's this inertness that allows platinum to be used in settings that might degrade other metals.

In scenarios where you have unknown metal samples, if you find a metal that remains unchanged after exposure to a variety of solutions, it's very likely platinum. This unique attribute of resistance to chemical attack under normal laboratory conditions is what sets platinum apart from its chemically more active peers like zinc and lead.