Question

Write balanced equations for each of the following reactions (some of these are analogous to reactions shown in the chapter). (a) Aluminum metal reacts with acids to form hydrogen gas. (b) Steam reacts with magnesium metal to give magnesium oxide and hydrogen. (c) Manganese(IV) oxide is reduced to manganese(II) oxide by hydrogen gas. (d) Calcium hydride reacts with water to generate hydrogen gas.

Short answer

(a) 2 Al + 6 HCl → 2 AlCl₃ + 3 H₂ (b) Mg + H₂O → MgO + H₂ (c) MnO₂ + 2 H₂ → MnO + 2 H₂O (d) CaH₂ + 2 H₂O → Ca(OH)₂ + 2 H₂

Step-by-step solution

(a) Aluminum metal reacts with acids to form hydrogen gas

First, we need to identify the reactants and products involved in the reaction. In this case, the reactants are aluminum metal (Al) and an acid (we'll use hydrochloric acid, HCl, as an example), and the products are a salt (aluminum chloride, AlCl₃) and hydrogen gas (H₂). Now we can write the unbalanced equation: Al + HCl → AlCl₃ + H₂ To balance the equation, note that there are 3 chlorine atoms in the product AlCl₃, so we need 3 HCl molecules for each Al atom. And since 3 moles of HCl will produce 3 moles of H atoms, we need to balance the H₂ gas by having 3/2 moles of H₂. The balanced equation is: 2 Al + 6 HCl → 2 AlCl₃ + 3 H₂

(b) Steam reacts with magnesium metal to give magnesium oxide and hydrogen

The reactants in this reaction are steam (water in its gaseous form, H₂O) and magnesium metal (Mg). The products are magnesium oxide (MgO) and hydrogen gas (H₂). We can write the unbalanced equation as: Mg + H₂O → MgO + H₂ To balance the equation, observe that there are 2 H atoms in both the reactants and products side as well as 1 O atom. So, the equation is already balanced: Mg + H₂O → MgO + H₂

(c) Manganese(IV) oxide is reduced to manganese(II) oxide by hydrogen gas

In this reaction, manganese(IV) oxide (MnO₂) is reduced to manganese(II) oxide (MnO) by hydrogen gas (H₂). The other product of the reaction is water (H₂O). The unbalanced equation can be written as: MnO₂ + H₂ → MnO + H₂O To balance the equation, note that we need 2 H atoms in MnO and H₂O. However, there is only 1 O atom on the product side, so we need to also balance the O atoms by adding an extra H₂O to the product side. Thus, we have: MnO₂ + 2 H₂ → MnO + 2 H₂O

(d) Calcium hydride reacts with water to generate hydrogen gas

In this reaction, calcium hydride (CaH₂) reacts with water (H₂O) to generate hydrogen gas (H₂) and form calcium hydroxide (Ca(OH)₂). The unbalanced equation can be represented as: CaH₂ + H₂O → Ca(OH)₂ + H₂ To balance the equation, observe that we need two O atoms and four H atoms on the product side to balance with the reactant side. Thus, the balanced equation is: CaH₂ + 2 H₂O → Ca(OH)₂ + 2 H₂

Key concepts

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in studying chemistry, allowing us to understand how substances interact during reactions. In a chemical equation, the number of atoms for each element on the reactant side must match the number on the product side. This obeys the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.
One approach to balancing equations is to start by writing the unbalanced equation and then adjust coefficients to balance the individual atoms on both sides. Coefficients are the numbers placed before compounds or elements in an equation to indicate the number of molecules (or atoms) involved. For example, in the reaction between aluminum and hydrochloric acid, the unbalanced equation is:

• Al + HCl → AlCl₃ + H₂

To balance this, we see that aluminum chloride (AlCl₃) contains three chlorine atoms, requiring three HCl molecules for each Al atom. Thus, the balanced equation is:

• 2 Al + 6 HCl → 2 AlCl₃ + 3 H₂

By changing coefficients step-by-step, we ensure that the same number of atoms of each element is present on both sides, achieving a balanced equation.

Types of Chemical Reactions

Chemical reactions are processes in which substances, known as reactants, are transformed into different substances, called products. There are several types of chemical reactions that are commonly encountered in chemistry:

• Synthesis Reactions: Two or more simple substances combine to form a more complex substance. Example: 2 H₂ + O₂ → 2 H₂O.
• Decomposition Reactions: A single compound breaks down into simpler substances. Example: 2 H₂O → 2 H₂ + O₂.
• Single Displacement Reactions: One element replaces another in a compound. Example: Zn + 2 HCl → ZnCl₂ + H₂.
• Double Displacement Reactions: The ions of two compounds exchange places. Example: AgNO₃ + NaCl → AgCl + NaNO₃.
• Combustion Reactions: A substance reacts with oxygen, releasing energy as heat and light. Example: CH₄ + 2 O₂ → CO₂ + 2 H₂O.

Understanding these types helps predict the products of reactions and balance the chemical equations effectively.

Metal and Acid Reactions

Metal and acid reactions are a classic example of single displacement reactions, where a metal reacts with an acid to produce a salt and hydrogen gas. In these reactions, the metal replaces the hydrogen in the acid. The reactivity of metals varies, and not all metals react with acids to release hydrogen gas.
For instance, when aluminum (Al) reacts with hydrochloric acid (HCl), the reaction can be represented as:

• Al + 3 HCl → AlCl₃ + 1.5 H₂

Balancing this equation involves ensuring that the number of atoms remains consistent on both sides, which gives:

• 2 Al + 6 HCl → 2 AlCl₃ + 3 H₂

Other examples include magnesium (Mg) reacting with sulfuric acid (H₂SO₄) to produce magnesium sulfate (MgSO₄) and hydrogen gas:

• Mg + H₂SO₄ → MgSO₄ + H₂

These reactions are not only important in the laboratory but also have industrial applications, such as in the production of hydrogen gas.