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

Step-by-step solution

(1. Aluminum metal reacts with acids to form hydrogen gas)

We'll first write the word equation, which is: Aluminum + Acid → Aluminum salt + Hydrogen gas The general equation is: Al + HX → AlX + H₂ Let's balance the equation (taking an example of Hydrochloric acid): 2Al + 6HCl → 2AlCl₃ + 3H₂ Here, the balanced equation consists of 2 moles of aluminum reacting with 6 moles of hydrochloric acid to produce 2 moles of aluminum chloride and 3 moles of hydrogen gas.

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

The word equation is: Steam + Magnesium → Magnesium oxide + Hydrogen gas The general equation is: H₂O + Mg → MgO + H₂ Now, let's balance the equation: H₂O + Mg → MgO + H₂ There's no need to adjust the coefficients because the equation is already balanced.

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

The word equation is: Manganese (IV) oxide + Hydrogen gas → Manganese (II) oxide + Water The general equation is: MnO₂ + H₂ → MnO + H₂O Now, let's balance the equation: MnO₂ + 2H₂ → MnO + 2H₂O The balanced equation consists of 1 mole of manganese(IV) oxide reacting with 2 moles of hydrogen gas to produce 1 mole of manganese(II) oxide and 2 moles of water.

(4. Calcium hydride reacts with water to generate hydrogen gas)

The word equation is: Calcium hydride + Water → Calcium hydroxide + Hydrogen gas The general equation is: CaH₂ + H₂O → Ca(OH)₂ + H₂ Now, let's balance the equation: CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂ The balanced equation consists of 1 mole of calcium hydride reacting with 2 moles of water, producing 1 mole of calcium hydroxide and 2 moles of hydrogen gas.

Key concepts

Balancing Chemical Equations

Balancing chemical equations is an essential skill in chemistry. It involves ensuring that the number of atoms for each element is the same on both sides of a chemical reaction. This is based on the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction.
When balancing chemical equations, coefficients are placed in front of chemical formulas to adjust the number of molecules. For example, in the reaction of aluminum with hydrochloric acid, we start with the equation:\[ \text{Al} + \text{HCl} \rightarrow \text{AlCl}_3 + \text{H}_2 \]We adjust to balance:- 2 aluminum atoms on the reactant side balance with 2 on the product.- 6 hydrogen atoms in hydrochloric acid (HCl) give 6, matching those in hydrogen gas (H₂).- This balances the resulting equation: \[ 2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2 \]Balancing chemical equations ensures the reaction adheres to fundamental chemical laws and the stoichiometry is accurate.

Chemical Equation Writing

Chemical equation writing converts verbal descriptions of chemical reactions into a standardized format. This involves representing substances with chemical formulas and indicating the reactants and products.
For example, consider the reaction of steam with magnesium:- Verbal description: "Steam reacts with magnesium to produce magnesium oxide and hydrogen gas."- Chemical equation: \[ \text{H}_2\text{O} + \text{Mg} \rightarrow \text{MgO} + \text{H}_2 \]
When writing chemical equations, it's important to identify all reactants and products and use correct formulas. Additionally, equations are typically unbalanced initially and require balancing, which might already be naturally balanced, as in the magnesium and steam equation. Each component, such as water (H₂O), magnesium (Mg), etc., must be correctly represented to maintain the integrity of the reaction.

Chemical Reactants and Products

In every chemical reaction, substances called *reactants* interact to form new substances known as *products*. Understanding these is crucial as it's the foundation of a chemical reaction.
For example, when calcium hydride reacts with water:- **Reactants**: Calcium hydride (CaH₂) and water (H₂O)- **Products**: Calcium hydroxide (Ca(OH)₂) and hydrogen gas (H₂) The word equation might be straightforward: "Calcium hydride + Water → Calcium hydroxide + Hydrogen gas." However, the balanced chemical equation must reflect stoichiometry:\[ \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \]
Reactants are transformed through breaking and forming of bonds within the reaction. Products are the outcome of these transformations, showcasing new properties. Knowing how to identify and balance reactants and products helps in predicting the yield and nature of chemical reactions.