Question

Pennies in the United States now consist of a zinc disk that is coated with a thin layer of copper. If a penny is scratched and then soaked in hydrochloric acid, it is possible to dissolve the zinc disk, leaving only a thin, hollow shell of copper. Write an unbalanced chemical equation to illustrate the reaction of zinc metal with hydrochloric acid, which produces dissolved zinc chloride and evolves gaseous hydrogen.

Short answer

The reaction between zinc metal (Zn) and hydrochloric acid (HCl) produces dissolved zinc chloride (ZnCl₂) and gaseous hydrogen (H₂). The balanced chemical equation for this reaction is: \(Zn + 2HCl \rightarrow ZnCl_2 + H_2\).

Step-by-step solution

Identify the reactants and products

The reactants are zinc metal (Zn) and hydrochloric acid (HCl). The products of the reaction are dissolved zinc chloride (ZnCl₂) and gaseous hydrogen (H₂).

Writing the unbalanced chemical equation

We can write the unbalanced chemical equation by placing an arrow between the reactants and products. Zn + HCl → ZnCl₂ + H₂

Balancing the chemical equation

To balance the chemical equation, we need to make sure there is an equal number of atoms of each element on both sides of the equation. Here, we have one Zn atom on both sides, but there are two Cl atoms in ZnCl₂ compared to one in HCl, and there are two H atoms in H₂ compared to the one in HCl. To balance the equation, we need two HCl molecules for each H₂ molecule and one ZnCl₂ molecule. So, the balanced chemical equation will be as follows. Zn + 2HCl → ZnCl₂ + H₂

Key concepts

Balancing Chemical Equations

Understanding how to balance chemical equations is a cornerstone of studying chemistry. When we look at the reaction between zinc and hydrochloric acid, it's important to first identify the reactants and products, as in the given problem. In an unbalanced equation such as Zn + HCl → ZnCl₂ + H₂, our goal is to ensure the same number of each type of atom appears on both sides of the arrow.

To balance the equation, we look at each element individually. With zinc (Zn), the balance is already there — one atom on each side. For chlorine (Cl), there are two chlorine atoms in zinc chloride (ZnCl₂) on the product side and only one chlorine atom in the hydrochloric acid (HCl) on the reactant side. Hydrogen (H) poses a similar problem, with two hydrogen atoms in the hydrogen gas (H₂) and only one in the hydrochloric acid molecule.

The solution involves doubling the number of HCl molecules, which gives us two chlorine atoms and two hydrogen atoms on the reactant side. Now, the equation reads Zn + 2HCl → ZnCl₂ + H₂. This way, each side of the equation has one Zn atom, two Cl atoms, and two H atoms, and the equation is balanced. This step is crucial for predicting the amounts of reactants needed and the products formed in chemical reactions.

Reactivity of Zinc

Zinc is a metallic element with a fair share of interesting properties, particularly when it comes to its reactivity. When zinc comes into contact with hydrochloric acid, a single-replacement reaction occurs where zinc displaces hydrogen. This is because zinc is more reactive than hydrogen in the electrochemical series.

• Zinc has a tendency to lose electrons and form positive ions, a characteristic of metals known as being 'oxidized.'
• The reaction between zinc and HCl is spontaneous and exothermic, meaning it releases heat.
• Zinc metal dissolves in hydrochloric acid, forming zinc chloride and hydrogen gas, which can be seen as bubbles during the reaction.

Zinc's reactivity is crucial in the context of this textbook exercise. Students should understand that the acid not only reacts with the metal but also effectively dissolves it, producing new substances with different properties while liberating hydrogen gas in the process.

Properties of Hydrochloric Acid

Hydrochloric acid (HCl) is a strong, corrosive acid that is widely used in chemistry as well as in many industrial processes. It is the aqueous solution of hydrogen chloride gas and is known for its distinctive pungent smell.

• As a strong acid, it dissociates completely in water, producing a high concentration of hydrogen ions (H⁺), which gives it a low pH.
• Hydrochloric acid reacts vigorously with metals that are above hydrogen in the reactivity series, such as zinc, to produce metal chlorides and hydrogen gas.
• In the context of the given problem, hydrochloric acid is responsible for reacting with the zinc core of the penny, dissolving it and leaving behind the copper shell.

When hydrochloric acid comes into contact with zinc, the H⁺ ions in the acid are reduced to hydrogen molecules (H₂), while the zinc is oxidized to zinc ions (Zn²⁺), which then pair with the chloride ions (Cl⁻) from the hydrochloric acid to form zinc chloride (ZnCl₂). This reactivity with metals makes hydrochloric acid a very important reagent in the preparation of various metal chlorides.