Understanding the Reaction of Copper with Dilute Sulphuric Acid

The question asks what happens when pieces of copper metal are placed in dilute sulphuric acid. To answer this, we need to consider the reactivity of copper compared to hydrogen and the properties of dilute sulphuric acid.

Metal Reactivity and Acid Reactions

When a metal reacts with a dilute acid, it typically displaces hydrogen gas if the metal is more reactive than hydrogen. The reactivity of metals is often represented by the reactivity series. Metals higher in the series are more reactive and can displace hydrogen from dilute acids.

The reactivity series places hydrogen between less reactive metals like copper, silver, and gold, and more reactive metals like zinc, iron, and magnesium. The simplified order showing relevant metals is:

• Potassium (K)
• Sodium (Na)
• Calcium (Ca)
• Magnesium (Mg)
• Aluminium (Al)
• Zinc (Zn)
• Iron (Fe)
• Hydrogen (H)
• Copper (Cu)
• Silver (Ag)
• Gold (Au)

From this series, we can see that copper is placed below hydrogen. This means copper is less reactive than hydrogen.

Copper and Dilute Sulphuric Acid Reaction

Dilute sulphuric acid ($\text{H}_2\text{SO}_4$) behaves like a typical acid, releasing hydrogen ions ($\text{H}^+$) in solution. For a reaction to occur where the metal displaces hydrogen from the acid, the metal must be able to donate electrons to the hydrogen ions, forming hydrogen gas ($\text{H}_2$).

Since copper is less reactive than hydrogen, it cannot displace hydrogen from dilute acids like dilute sulphuric acid. Therefore, no chemical reaction takes place between copper metal and dilute sulphuric acid.

The expected reaction for a metal more reactive than hydrogen (M) with dilute sulphuric acid is:

$\text{M(s)} + \text{H}_2\text{SO}_4\text{(aq, dilute)} \rightarrow \text{MSO}_4\text{(aq)} + \text{H}_2\text{(g)}$

However, for copper (Cu), since it is less reactive than hydrogen, this displacement reaction does not occur:

$\text{Cu(s)} + \text{H}_2\text{SO}_4\text{(aq, dilute)} \rightarrow \text{No Reaction}$

Analyzing the Options

Let's look at the given options based on this understanding:

1. Vigorous reaction occurs and a colourless gas is liberated. This would happen with a metal significantly more reactive than hydrogen, like magnesium or zinc. Copper is not this reactive with dilute acid.

2. No reaction takes place. This aligns with our understanding that copper is less reactive than hydrogen and cannot displace it from dilute sulphuric acid.

3. The mixture slowly turns blue. A blue colour indicates the formation of copper ions ($Cu^{2+}$), typically in a solution like copper sulfate ($CuSO_4$). While copper sulfate solution is blue, it would only form if copper reacted. Copper reacts with hot concentrated sulphuric acid or oxidizing acids like nitric acid to form copper salts, but not with dilute sulphuric acid.

4. A gas with suffocating smell is liberated. A gas with a suffocating smell, like sulfur dioxide ($\text{SO}_2$), is produced when copper reacts with hot concentrated sulphuric acid. In this reaction, the acid itself is reduced. Dilute sulphuric acid does not react in this way with copper.

Based on the reactivity series and the nature of dilute sulphuric acid, the only accurate statement is that no reaction takes place.

Conclusion

When copper metal is put in a beaker containing dilute sulphuric acid, no reaction occurs because copper is less reactive than hydrogen and cannot displace hydrogen from the acid.