Question

Consider the following reaction: $$ \mathrm{Zn}(s)+2 \mathrm{HCl}(a q) \rightarrow \mathrm{ZnCl}_{2}(a q)+\mathrm{H}_{2}(g) $$

Short answer

The given chemical reaction is a single replacement reaction between solid zinc (Zn) and aqueous hydrochloric acid (HCl) to produce aqueous zinc chloride (ZnCl₂) and hydrogen gas (H₂). To balance the reaction, we need to ensure there are the same number of atoms of each element on both sides. The balanced reaction is: \( \mathrm{Zn}(s) + 2\mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq) + \mathrm{H}_{2}(g) \) where we have 1 zinc atom, 2 hydrogen atoms, and 2 chlorine atoms on both sides of the reaction.

Step-by-step solution

Understand the Reaction

In this reaction, solid zinc (Zn) reacts with aqueous hydrochloric acid (HCl) to produce aqueous zinc chloride (ZnCl₂) and hydrogen gas (H₂). The symbols "s", "aq", and "g" represent the state of the substances involved: "s" for solid, "aq" for aqueous (dissolved in water), and "g" for gas.

Identify the Type of Reaction

This is a single replacement reaction (also known as a single displacement reaction), in which one element replaces another element in a compound. In our case, zinc (Zn) replaces hydrogen (H) in hydrochloric acid (HCl) to form zinc chloride (ZnCl₂) and hydrogen gas (H₂).

Write the Unbalanced Reaction

Begin by writing the unbalanced reaction as follows: \( \mathrm{Zn}(s) + \mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq) + \mathrm{H}_{2}(g) \)

Balance the Reaction

To balance the reaction, we need to ensure that the number of atoms of each element is the same on both sides of the reaction. In the unbalanced reaction, there are 2 chlorine (Cl) atoms on the right side, but only 1 on the left side. To balance this, we should multiply the number of hydrochloric acid (HCl) molecules by 2 on the left side: \( \mathrm{Zn}(s) + 2\mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq) + \mathrm{H}_{2}(g) \) Now, the reaction is balanced with 1 zinc atom, 2 hydrogen atoms, and 2 chlorine atoms on both sides of the reaction.

Verify the Balanced Reaction

Check if the reaction is balanced by counting the number of atoms of each element on both sides of the reaction: - On the left side: 1 Zn, 2 H, 2 Cl - On the right side: 1 Zn, 2 H, 2 Cl Both sides have the same number of atoms for each element, which confirms that the reaction is balanced. The final balanced reaction is: \( \mathrm{Zn}(s) + 2\mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_{2}(aq) + \mathrm{H}_{2}(g) \)

Key concepts

Single Replacement Reaction

A single replacement reaction, also known as a single displacement reaction, is a type of chemical reaction where one element is replaced by another in a compound. In the provided exercise, zinc (Zn) reacts with hydrochloric acid (HCl), resulting in the release of hydrogen gas (H₂) and the formation of zinc chloride (ZnCl₂).

This reaction is represented by the formula:
\[ \text{A} + \text{BC} \rightarrow \text{AC} + \text{B} \]
where element A replaces element B in the compound BC, producing a new compound AC and the free element B. To identify a single replacement reaction, look for a pattern where a pure element reacts with a compound, causing the elements to switch places.

These reactions typically occur between metals and ionic compounds or between halogens and compounds containing halogens. The activity series of metals and halogens can help predict if a single replacement reaction will occur, where more reactive elements replace less reactive ones.

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry that involves equalizing the number of each type of atom on both sides of a chemical equation. To balance an equation, you'll follow several steps, ensuring that mass and charge are conserved.

Begin by writing the unbalanced equation, as in our zinc and hydrochloric acid reaction:
\[ \text{Zn}(s) + \text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g) \]
This initial step identifies the reactants and products and their respective states. Then, systematically adjust the coefficients—the numbers before the chemical formulas—until there are equal numbers of atoms for each element on both sides. In our example, adjusting the coefficient in front of HCl to 2 yields a balanced equation with equal amounts of Zn, H, and Cl atoms on each side. Remember, coefficients can only be whole numbers, and you cannot change the chemical formulas themselves during the balancing process.

Reaction Stoichiometry

Reaction stoichiometry refers to the quantitative relationship between reactants and products in a chemical reaction. It involves using the balanced equation to calculate the masses, volumes, or moles of substances consumed and produced. Understanding stoichiometry is crucial for predicting the amounts of materials needed and the products formed.

In the zinc and hydrochloric acid reaction:
\[ \text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g) \]
stoichiometry tells us that one mole of zinc reacts with two moles of hydrochloric acid to produce one mole of zinc chloride and one mole of hydrogen gas. Based on these mole ratios, we can derive other relationships such as mass-to-mass or volume-to-volume conversions using molar mass and gas laws, respectively.

Effective stoichiometry also relies on starting with a balanced chemical equation and using the coefficients to convert between reactants and products. It's the cornerstone for much of the quantitative analysis in chemistry, enabling precise predictions and control within chemical processes.