Question

The boiling points of oxygen and argon are \(-183^{\circ} \mathrm{C}\) and \(-189^{\circ} \mathrm{C},\) respectively. Because the boiling points are so similar, argon obtained from the fractional distillation of liquid air is contaminated with oxygen. The following three-step procedure can be used to obtain pure argon from the oxygen-contaminated sample: (1) Excess hydrogen is added to the mixture and then the mixture is ignited. (2) The mixture from step (1) is then passed over hot copper(II) oxide. (3) The mixture from step (2) is passed over a dehydrated zeolite material (see Chapter 21 ). Explain the purpose of each step, writing chemical equations for any reactions that occur.

Short answer

In this three-step purification process, the first step is the combustion of oxygen with hydrogen to form water, removing oxygen from the mixture. In the second step, any residual hydrogen is reacted with copper(II) oxide to form water and copper, removing hydrogen from the mixture. Then the third step is passing the remaining mixture over a dehydrated zeolite material to adsorb water, leaving pure argon.

Step-by-step solution

Understanding Step 1

In the first step, excess hydrogen is added to the mixture and ignited. The purpose of this step is to react oxygen in the mixture with hydrogen to form water. This is a basic combustion reaction in which oxygen reacts with hydrogen to produce water. The reaction equation: \[2H_2(g) + O_2(g) \rightarrow 2H_2O(g)\]

Understanding Step 2

In the second step, the mixture from the previous step is passed over hot copper(II) oxide. The purpose here is to ensure all hydrogen is removed. Any surplus hydrogen from the previous step reacts with copper(II) oxide to produce water and copper. Here is the chemical equation for this reaction: \[H_2(g) + CuO(s) \rightarrow H_2O(g) + Cu(s)\]. In this step, water will be also produced and turned into steam due to the heat.

Understanding Step 3

In the third step, the mixture from step 2 is passed over a dehydrated zeolite material. Zeolite being a porous material, is used because it has the ability to adsorb water. The remaining substance will be pure argon, since oxygen and hydrogen have been removed in the form of water.

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which it transitionsfrom a liquid to a gas. Each element or compound has its own characteristicboiling point. In the case of the fractional distillation of liquid air, understanding the boiling points of gases is crucial. Oxygen boils at \(-183^{\circ} C\) and argon at \(-189^{\circ} C\). These temperatures are very close to each other, which causes a challenge in separating these gases via distillation.

During fractional distillation, gases with lower boiling points evaporate first.However, since the boiling points of oxygen and argon are so similar,it can be difficult to achieve a complete separation. This is why additional steps are needed to purify argon after its initial separation attempt through fractional distillation.

• Boiling point: Temperature where a liquid becomes a gas.
• Fractional distillation separates substances based on boiling points.
• Close boiling points for oxygen and argon complicate separation.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products through the reorganization of atoms. In this problem, the first two steps involve specific chemical reactions to purify argon. The first step involves a combustion reaction where oxygen reacts with hydrogen to form water:

• Combustion Reaction: \[2H_2(g) + O_2(g) \rightarrow 2H_2O(g)\]
  This reaction helps in removing oxygen from the mixture.
• The second step ensures that any excess hydrogen is converted by passing the mixture over copper(II) oxide:\[H_2(g) + CuO(s) \rightarrow H_2O(g) + Cu(s)\]
  This finalizes the removal of hydrogen from the argon.

Both reactions are essential in changing the composition of the mixture to facilitate the further purification of the argon.

Purification Process

Purifying a substance means removing impurities to get a pure product. In the context of obtaining pure argon, this is crucial because of the need to separate it from oxygen and hydrogen, which could not be purposefully separated by fractional distillation alone. In the given procedure, the steps are methodical purification methods.

- **Step 1:** Use the combustion reaction to convert contaminating oxygen into water. - **Step 2:** Remove leftover hydrogen by reacting it with copper(II) oxide, also resulting in water production. - **Step 3:** The mixture is then passed through dehydrated zeolite, which acts as an adsorbent. Zeolite is a porous material that captures and holds onto water, leaving behind only pure argon.
This three-step process meticulously removes the impurities of oxygen and hydrogen, leveraging chemical reactions and adsorption, leading to the desired purification of argon.