Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following metals will not reduce \(\mathrm{H}_{2} \mathrm{O} ?\) (a) Li (b) \(\mathrm{Fe}\) (c) \(\mathrm{Cu}\) (d) \(\mathrm{Ca}\)

Step-by-Step Solution

Answer

Copper (Cu) will not reduce water.

Step 1: Determine Reactivity Series

Identify the reactivity of the metals listed in terms of their ability to react with water. Metals that are higher in the reactivity series can reduce water to produce hydrogen gas.

Step 2: Reactivity Analysis: Li and Ca

Lithium (
Li) and calcium (
Ca) are highly reactive metals. They are capable of reacting with water to produce hydrogen gas, hence they can reduce water.

Step 3: Reactivity Analysis: Fe

Iron (
Fe) is less reactive compared to lithium and calcium, but it can still react with steam (not cold water) to produce hydrogen gas. Therefore, it can reduce water under certain conditions.

Step 4: Reactivity Analysis: Cu

Copper (
Cu) is a low reactivity metal and is below hydrogen in the reactivity series. It does not react with water or steam to produce hydrogen gas.

Step 5: Identify the Metal that Does Not Reduce Water

Since copper (
Cu) cannot react with water to produce hydrogen, it will not reduce
H_2O. The other metals (
Li,
Fe, and
Ca) can react with water or steam to some degree to produce hydrogen gas.

Key Concepts

Metal Reactivity with Water

Understanding how different metals react with water is essential in chemistry. Metals vary in their reactivity, and this determines whether they will interact with water to form hydrogen gas. The reactivity series ranks metals based on their ability to displace hydrogen from water.

Here's how it works: metals at the top of the reactivity series, like lithium (Li) and calcium (Ca), react vigorously with water, even at room temperature. These reactions produce hydrogen gas and metal hydroxides.

• Lithium + Water \( ightarrow\) Lithium Hydroxide + Hydrogen gas
• Calcium + Water \( ightarrow\) Calcium Hydroxide + Hydrogen gas

Less reactive metals, such as iron (Fe), don't react with water at room temperature. However, they react with steam. This is because the higher temperature provides the energy needed for the reaction.

Copper (Cu), a low reactivity metal, does not react with water or even steam. It means it is more stable and remains unaffected in water conditions. Remembering these patterns helps predict metal behavior in chemical reactions.

Hydrogen Production

Hydrogen gas is produced during the reaction of highly reactive metals with water. This type of reaction is characterized by a visible fizzing or bubbling as hydrogen gas is released.

The basic reaction looks like this:

• Metal + Water \( ightarrow\) Metal Hydroxide + Hydrogen gas

Hydrogen production during these reactions is important for various applications. For instance:

• Energy Storage: Hydrogen is a clean fuel source, often used in fuel cells to power vehicles.
• Chemical Synthesis: It's a key reactant in making ammonia and methanol, substances used industrially.

Keep in mind that not all metals can produce hydrogen this way. Metals lower in the reactivity series, such as copper, do not engage in these reactions under normal conditions.
This connection between reactivity and hydrogen production is crucial for many scientific and industrial processes.

Reduction by Metals

Reduction is a chemical process where an element gains electrons, often from a metal. In terms of water ( \(H_2O \)), it involves the transformation into hydrogen gas. When a metal reduces water, it donates electrons to hydrogen ions (from water), turning them into hydrogen gas.

Key points to remember:

• Only metals higher than hydrogen in the reactivity series can reduce water to hydrogen gas.
• Metals like copper, which are below hydrogen, cannot perform this reduction as they are less willing to give up electrons.

Reduction by metals is foundational for understanding many electrochemical reactions, including how batteries and corrosion work. By knowing which metals can perform reductions, we can determine their potential uses in various chemical processes.

Reactivity and reduction capacity help us predict how different metals will behave in reactions, providing insights into their practical applications.