Displacement using hydrogen gas. The Mole Concept

In the setup shown, why is hydrogen gas burned at the outlet of the combustion tube during the reduction of copper(II) oxide?

• To ensure an inert atmosphere throughout the tube

• To increase the temperature for faster reduction

• To remove excess hydrogen by converting it to water

• To prevent air from diffusing back into the hot tube

Answer explanation

Burning hydrogen gas at the outlet prevents air from diffusing back into the hot tube, which could lead to unwanted oxidation of copper(II) oxide during the reduction process.

Which role does anhydrous calcium chloride play in the hydrogen delivery line before the combustion tube?

• It dries hydrogen by absorbing water vapor

• It neutralizes acid fumes from the generator

• It removes carbon dioxide from the gas stream

• It catalyzes the reduction of copper(II) oxide

Answer explanation

Anhydrous calcium chloride is used to dry hydrogen by absorbing water vapor, preventing moisture from interfering with combustion processes.

During the reduction of copper(II) oxide by hydrogen, which observation confirms copper metal has formed?

• Appearance of a blue crystalline solid in the tube

• Formation of a black powder coating the porcelain dish

• Change from black solid to reddish-brown metal

• Effervescence inside the combustion tube

Answer explanation

The correct observation is the change from black solid to reddish-brown metal, indicating the reduction of copper(II) oxide to copper metal. The black solid is copper(II) oxide, and the reddish-brown color confirms copper formation.

Match the following balanced ionic equations with their corresponding reactions.

• Reaction of zinc with water to produce zinc hydroxide and hydrogen gas.

• Zn2+ + SO4 2− → ZnSO4

• Formation of zinc sulfate from zinc ions and sulfate ions.

• Zn + 2H+ → Zn2+ + H2

• Reaction of zinc with sulfuric acid to produce zinc sulfate and water.

• Zn + H2SO4 → ZnSO4 + H2O

• Reaction of zinc with hydrogen ions to produce zinc ions and hydrogen gas.

• Zn + 2H2O → Zn(OH)2 + H2

Answer explanation

The correct equation, Zn + 2H+ → Zn2+ + H2, shows zinc reacting with hydrogen ions from sulfuric acid to produce zinc ions and hydrogen gas, accurately representing the reaction taking place.

Copper(II) oxide is reduced by hydrogen according to CuO(s) + H2(g) → Cu(s) + H2O(l). This method is suitable for copper but not for very reactive metals like magnesium because (a)   .

• Copper forms a stable hydride that
• Hydrogen reduces oxides of less rea
• Magnesium oxide reacts too slowly t
• Hydrogen preferentially oxidizes to

Answer explanation

Hydrogen effectively reduces oxides of less reactive metals like copper at moderate heat, while more reactive metals like magnesium require higher energy to reduce their oxides, making this method unsuitable.

A student heats 1.60 g of copper(II) oxide in a stream of dry hydrogen and obtains 1.28 g of copper. What is the empirical formula of the oxide?

• Cu2O because copper atoms double after reduction

• CuO because oxygen mass is 0.32 g

• Cu2O because oxygen mass is 0.32 g

• CuO because hydrogen adds one oxygen atom

Answer explanation

The mass of oxygen can be calculated as 1.60 g (initial copper(II) oxide) - 1.28 g (copper produced) = 0.32 g. The ratio of copper to oxygen is 1.28 g Cu to 0.32 g O, simplifying to 2:1, indicating the empirical formula is CuO.

What is the primary product formed when zinc reacts with dilute hydrochloric acid?

• Zinc chloride and hydrogen gas

• Zinc oxide and water

• Zinc sulfate and oxygen

• Zinc hydroxide and hydrogen gas

Which observation indicates that a metal oxide has been successfully reduced by hydrogen?

• Change from a solid to a liquid state

• Color change from white to black

• Change from a black solid to a shiny metal

• Formation of a gas with a pungent odor

Why is hydrogen gas often used in the reduction of metal oxides in laboratory settings?

• It is a cheap and readily available gas

• It produces no harmful byproducts

• It can reduce a wide range of metal oxides effectively

• It reacts at room temperature without heating