Question

Complete and balance each of the following combination reactions: (a) \(\mathrm{Zn}+\mathrm{P} \rightarrow\) (b) \(\mathrm{Al}+\mathrm{S} \rightarrow\)

Short answer

(a) \( 3 \mathrm{Zn} + 2 \mathrm{P} \rightarrow \mathrm{Zn}_3\mathrm{P}_2 \); (b) \( 2 \mathrm{Al} + 3 \mathrm{S} \rightarrow \mathrm{Al}_2\mathrm{S}_3 \).

Step-by-step solution

Determine the Products

In a combination reaction, two or more elements combine to form a compound. For (a) Zinc (Zn) reacts with Phosphorus (P) to form Zinc Phosphide. For (b), Aluminum (Al) reacts with Sulfur (S) to form Aluminum Sulfide.

Write the Chemical Formulas of Products

The compounds formed are: For Zinc Phosphide: The formula is \( \mathrm{Zn}_3\mathrm{P}_2 \) because Zinc has a valency of 2 and Phosphorus has a valency of 3.For Aluminum Sulfide: The formula is \( \mathrm{Al}_2\mathrm{S}_3 \) because Aluminum has a valency of 3 and Sulfur has a valency of 2.

Write Unbalanced Equations

Write the unbalanced chemical equations:(a) \( \mathrm{Zn} + \mathrm{P} \rightarrow \mathrm{Zn}_3\mathrm{P}_2 \)(b) \( \mathrm{Al} + \mathrm{S} \rightarrow \mathrm{Al}_2\mathrm{S}_3 \)

Balance Equation (a)

To balance \( \mathrm{Zn} + \mathrm{P} \rightarrow \mathrm{Zn}_3\mathrm{P}_2 \), 3 Zn are needed to match 3 Zn in the product, and 2 P are needed to match 2 P in the product:Balanced equation (a): \( 3 \mathrm{Zn} + 2 \mathrm{P} \rightarrow \mathrm{Zn}_3\mathrm{P}_2 \)

Balance Equation (b)

To balance \( \mathrm{Al} + \mathrm{S} \rightarrow \mathrm{Al}_2\mathrm{S}_3 \), 2 Al are needed for 2 Al in the product, and 3 S are needed to match 3 S in the product:Balanced equation (b): \( 2 \mathrm{Al} + 3 \mathrm{S} \rightarrow \mathrm{Al}_2\mathrm{S}_3 \)

Key concepts

Combination Reaction

A combination reaction, often known as a synthesis reaction, is one where two or more simple substances combine to form a more complex compound. These reactions are typically straightforward  simply take two elements or compounds and let them form a new single substance. A typical example of a combination reaction is the reaction between hydrogen and oxygen to form water.
In our specific exercise, we have Zinc (Zn) combining with Phosphorus (P) to form Zinc Phosphide,
and Aluminum (Al) combining with Sulfur (S) to create Aluminum Sulfide.

• This type of reaction is highlighted by the nature of its simplicity and the direction towards order from chaos,
  as multiple reactant species become one.
• The outcome is generally an ionic or covalent compound, depending on the nature of the reactants.

Understanding combination reactions helps you predict the product of a reaction simply by knowing the reactants.

Balancing Chemical Equations

Balancing chemical equations is crucial in chemistry because it follows the Law of Conservation of Mass  matter is neither created nor destroyed. Therefore, the number of atoms for each element must be the same on both sides of the equation.
During balancing, coefficients are adjusted in front of chemical formulas to ensure this equality. To balance:

• Start with the given unbalanced equation. For Zinc and Phosphorus: \( \mathrm{Zn} + \mathrm{P} \rightarrow \mathrm{Zn}_3\mathrm{P}_2 \).
• Count and compare the number of atoms for each element on both sides. Three zincs and two phosphorus atoms must appear equally.
  This results in the balanced equation: \( 3\mathrm{Zn} + 2\mathrm{P} \rightarrow \mathrm{Zn}_3\mathrm{P}_2 \).
• Repeat the process for Aluminum and Sulfur: Starting with \( \mathrm{Al} + \mathrm{S} \rightarrow \mathrm{Al}_2\mathrm{S}_3 \), you will need two aluminum and three sulfurs, producing the balanced equation \( 2\mathrm{Al} + 3\mathrm{S} \rightarrow \mathrm{Al}_2\mathrm{S}_3 \).

Balancing equations ensures accurate representation of chemical processes, allowing us to predict the amounts necessary and produced in reactions reliably.

Valency of Elements

The valency of an element describes its ability to combine with other atoms  specifically, the number of electrons it tends to gain, lose, or share to achieve a full shell of electrons, mimicking the electronic configuration of a noble gas.
Knowing an element's valency aids in writing chemical formulas for compounds.Take Zinc and Phosphorus, for instance:

• Zinc typically shows a valency of +2, meaning it tends to lose two electrons.
  Phosphorus often has a valency of -3 as it gains three electrons.
• When these elements combine, the need to balance charges influences their ratios.
  The formula \( \mathrm{Zn}_3\mathrm{P}_2 \) reflects a balance: 3 Zinc atoms contribute a total charge of +6, compensating for the total -6 charge from 2 Phosphorus atoms.

Similarly for Aluminum and Sulfur:

• Aluminum has a valency of +3, preferring to lose three electrons.
  Sulfur, with a valency of -2, tends to gain two electrons.
• The compound \( \mathrm{Al}_2\mathrm{S}_3 \) balances the charges: two Aluminum atoms giving +6, matching with three Sulfur atoms bringing a -6 total charge.

Understanding valency ensures you can predict the structure and formulas of chemical compounds correctly.