Question

Complete and balance the following equations: (a) \(\mathrm{Rb}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) (b) \(\mathrm{Cs}_{2} \mathrm{O}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) (c) \(\mathrm{P}_{2} \mathrm{O}_{5}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\)

Short answer

(a) Balanced as: 2Rb + 2H₂O → 2RbOH + H₂; (b) Balanced as: Cs₂O + H₂O → 2CsOH; (c) Balanced as: P₂O₅ + 3H₂O → 2H₃PO₄.

Step-by-step solution

Identify the reaction type for equation (a)

In equation (a), we have Rubidium (Rb), a metal, reacting with water (H₂O). This is a single replacement reaction where Rubidium will replace hydrogen in water to form Rubidium hydroxide (RbOH) and hydrogen gas (H₂).

Write the unbalanced equation for (a)

The unbalanced chemical equation is: \[ \mathrm{Rb}(s) + \mathrm{H}_2 \mathrm{O}(l) \longrightarrow \mathrm{RbOH}(aq) + \mathrm{H}_2(g) \] Each reactant and product should be included in the equation.

Balance the equation for (a)

First, balance the Rb by having an equal number of atoms on either side of the reaction. Then, balance the oxygen and hydrogen:- Since we need 2 Rb atoms to balance the hydroxide ions produced in the products, we rewrite:\[ 2\mathrm{Rb}(s) + 2\mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{RbOH}(aq) + \mathrm{H}_2(g) \] Thus, the equation is balanced.

Identify the reaction type for equation (b)

In equation (b), Cesium oxide (Cs₂O) reacts with water. This reaction will form Cesium hydroxide (CsOH), a synthesis reaction.

Write the unbalanced equation for (b)

The unbalanced chemical equation is: \[ \mathrm{Cs}_2\mathrm{O}(s) + \mathrm{H}_2\mathrm{O}(l) \longrightarrow \mathrm{CsOH}(aq) \]

Balance the equation for (b)

To balance CsOH on the products' side, adjust the number of CsOH produced:\[ \mathrm{Cs}_2\mathrm{O}(s) + \mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{CsOH}(aq) \] Thus, the equation is balanced.

Identify the reaction type for equation (c)

The reaction involves Diphosphorus pentoxide (P₂O₅) with water, which is a combination reaction forming phosphoric acid (H₃PO₄).

Write the unbalanced equation for (c)

The unbalanced chemical equation is: \[ \mathrm{P}_2\mathrm{O}_5(s) + \mathrm{H}_2\mathrm{O}(l) \longrightarrow \mathrm{H}_3\mathrm{PO}_4(aq) \]

Balance the equation for (c)

Balancing phosphorus first, then hydrogen and oxygen:- To balance 2 phosphorus in P₂O₅, we need 2 H₃PO₄, so have:\[ \mathrm{P}_2\mathrm{O}_5(s) + 3\mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{H}_3\mathrm{PO}_4(aq) \] The equation is balanced now.

Key concepts

Single Replacement Reaction

In a single replacement reaction, one element replaces another element in a compound. This typically occurs between a more reactive metal and a less reactive one. For instance, consider when Rubidium (Rb) reacts with water (H₂O). Here, Rubidium, as a very reactive metal, replaces hydrogen in water. This results in the formation of Rubidium hydroxide (RbOH) and hydrogen gas (H₂).

The generic formula for a single replacement reaction can be written as:

• A + BC → AC + B

Where A is a single element, BC is a compound, AC is a new compound, and B is the replaced element.

Balancing such a reaction requires ensuring that the number of each type of atom is the same on both sides of the equation. In our example, the balanced equation is:

• 2Rb(s) + 2H₂O(l) → 2RbOH(aq) + H₂(g)

This balanced equation reflects the stoichiometry of the reaction, showing how the atoms are conserved.

Synthesis Reaction

A synthesis reaction, also known as a combination reaction, occurs when two or more simple substances combine to form a more complex product. This is a relatively straightforward reaction type and is common in chemical synthesis.

In our example with Cesium oxide ( Cs₂O ) reacting with water, the products form a single new compound: Cesium hydroxide ( CsOH ). Here, two elements or simple compounds essentially "synthesize" into something new and generally more complex.

• The reaction can be generalized by the formula: A + B → AB

To balance the synthesis reaction, you need to make sure the same number of each type of atom is present on both sides of the equation. For Cesium oxide with water, the balanced chemical equation becomes:

• Cs₂O(s) + H₂O(l) → 2CsOH(aq)

This balance ensures that atoms of cesium, oxygen, and hydrogen remain the same before and after the reaction takes place.

Combination Reaction

Combination reactions, a type of synthesis reaction, involve the joining of two elements or substances resulting in a single compound. This type of reaction is a foundational chemical process and can be observed in many different chemical contexts.

Consider the reaction of Diphosphorus pentoxide ( P₂O₅ ) with water, resulting in the formation of phosphoric acid ( H₃PO₄ ). This is a classic example of a combination reaction and follows the pattern where two different substances form a new compound.

• The general formula is: X + Y → XY

Balancing combination reactions requires careful attention to the number of atoms of each element on both sides. In this case, balancing involves ensuring that phosphorus, hydrogen, and oxygen are conserved, resulting in the balanced equation:

• P₂O₅(s) + 3H₂O(l) → 2H₃PO₄(aq)

This balanced format reflects the actual stoichiometry of the chemical reaction, thereby reinforcing the principle of the conservation of mass.