Question

Write a balanced chemical equation for each of the following double- replacement reactions: (a) Aqueous solutions of tin(II) chloride and potassium sulfide react to give a tin(II) sulfide precipitate and aqueous potassium chloride. (b) Aqueous solutions of cobalt(II) nitrate and sodium chromate react to give a cobalt(II) chromate precipitate and aqueous sodium nitrate.

Short answer

(a) \( \text{SnCl}_2(\text{aq}) + \text{K}_2\text{S}(\text{aq}) \rightarrow \text{SnS}(s) + 2\text{KCl}(\text{aq}) \); (b) \( \text{Co(NO}_3)_2(\text{aq}) + \text{Na}_2\text{CrO}_4(\text{aq}) \rightarrow \text{CoCrO}_4(s) + 2\text{NaNO}_3(\text{aq}) \)."

Step-by-step solution

Write Reactants' Formulas for Reaction (a)

Identify the reactants involved. Tin(II) chloride is written as \( \text{SnCl}_2 \) and potassium sulfide is written as \( \text{K}_2\text{S} \). Both are in aqueous form, so we have \( \text{SnCl}_2(\text{aq}) + \text{K}_2\text{S}(\text{aq}) \).

Write Products' Formulas for Reaction (a)

For the products, tin(II) sulfide is denoted as \( \text{SnS} \) and precipitates out, while potassium chloride remains in solution as \( \text{KCl} \). Thus, the products are \( \text{SnS}(s) + \text{KCl}(\text{aq}) \).

Balance Reaction for Reaction (a)

Balance the atoms on each side: \( \text{SnCl}_2(\text{aq}) + \text{K}_2\text{S}(\text{aq}) \rightarrow \text{SnS}(s) + 2\text{KCl}(\text{aq}) \). This balanced equation ensures equal numbers of each type of atom on both sides.

Write Reactants' Formulas for Reaction (b)

Identify the reactants. Cobalt(II) nitrate is \( \text{Co(NO}_3)_2 \) and sodium chromate is \( \text{Na}_2\text{CrO}_4 \), both in aqueous form: \( \text{Co(NO}_3)_2(\text{aq}) + \text{Na}_2\text{CrO}_4(\text{aq}) \).

Write Products' Formulas for Reaction (b)

Identify the products. Cobalt(II) chromate precipitates and is \( \text{CoCrO}_4 \), and the remaining product is sodium nitrate \( \text{NaNO}_3 \) in aqueous form: \( \text{CoCrO}_4(s) + \text{NaNO}_3(\text{aq}) \).

Balance Reaction for Reaction (b)

Balance the equation to keep the number of atoms equal on both sides: \( \text{Co(NO}_3)_2(\text{aq}) + \text{Na}_2\text{CrO}_4(\text{aq}) \rightarrow \text{CoCrO}_4(s) + 2\text{NaNO}_3(\text{aq}) \).

Key concepts

Double Replacement Reactions

In chemistry, a double replacement reaction, also known as a double displacement or metathesis reaction, occurs when parts of two ionic compounds are exchanged to form two new compounds. The general formula is: \( AB + CD \rightarrow AD + CB \).
In these reactions, typically, ions from two compounds switch places. Here, one of the products is usually a precipitate, a gas, or water. The key significance is that these reactions typically involve soluble ionic compounds.
For example, when aqueous solutions of tin(II) chloride \( \text{(SnCl}_2) \) and potassium sulfide \( \text{(K}_2\text{S)} \) are mixed, tin(II) sulfide \( \text{(SnS)} \) precipitates, while potassium chloride \( \text{(KCl)} \) remains dissolved in water. Similarly, for cobalt(II) nitrate \( \text{(Co(NO}_3)_2)} \) and sodium chromate \( \text{(Na}_2\text{CrO}_4)} \), cobalt(II) chromate \( \text{(CoCrO}_4)} \) precipitates, illustrating a typical double replacement reaction.

• The first step is to identify the reactants and predict the possible products.
• Next, determine if any of the products precipitate out or remain gaseous, which confirms a successful double replacement.

Balancing Chemical Equations

Balancing chemical equations is a necessary step to ensure that chemical reactions obey the law of conservation of mass. This law states that matter cannot be created or destroyed. Therefore, a balanced equation indicates that the same number of each type of atom appears on both sides of the reaction arrow.
To balance an equation, first write the number of atoms of each element present in the reactants and products. Begin by balancing the elements that appear only once on each side. Continue with those that appear multiple times. Least common factors are useful in balancing.
For instance, in the reaction of tin(II) chloride with potassium sulfide:- Each side of the equation must have equal numbers of \( \text{Sn} \), \( \text{Cl} \), \( \text{K} \), and \( \text{S} \) atoms.- Thus, the balanced equation is \( \text{SnCl}_2(\text{aq}) + \text{K}_2\text{S}(\text{aq}) \rightarrow \text{SnS}(s) + 2\text{KCl}(\text{aq}) \).- Similarly, for cobalt(II) nitrate and sodium chromate, the final balanced equation is \( \text{Co(NO}_3)_2(\text{aq}) + \text{Na}_2\text{CrO}_4(\text{aq}) \rightarrow \text{CoCrO}_4(s) + 2\text{NaNO}_3(\text{aq}) \).Remember, practice is key to mastering the art of balancing equations! It helps to visualize each element's presence within molecules to ensure accuracy.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions of ionic compounds are mixed, leading to the formation of an insoluble solid called a precipitate. This is a subtype of double replacement reactions where one product is a solid.
A precipitate forms when the product of the solubility parameters of the ions in solution exceeds the ion product constant of that compound in water. This often results in opaque or cloudy solutions as the solid begins to settle out of the liquid.
For example:

• The reaction between \( \text{SnCl}_2 \) and \( \text{K}_2\text{S} \) leads to the formation of tin(II) sulfide \( \text{(SnS)} \), which precipitates out of the solution.
• Similarly, mixing \( \text{Co(NO}_3)_2 \) with \( \text{Na}_2\text{CrO}_4 \) results in cobalt(II) chromate \( \text{(CoCrO}_4)} \) precipitating.

To predict whether a precipitation reaction will occur, refer to solubility rules. These guidelines help identify which combinations of ions will result in precipitates, providing a crucial tool for chemists when exploring reaction pathways.