Question

When a sodium chromate solution and aluminum bromide solution are mixed, a precipitate forms. Complete and balance the equation for this reaction, including the phases of each reactant and product.

Short answer

The balanced chemical equation for the reaction between sodium chromate solution and aluminum bromide solution, including the phases of each reactant and product, is: \(Na_2CrO_4(aq) + AlBr_3(aq) \rightarrow 6NaBr(aq) + Al_2(CrO_4)_3(s)\).

Step-by-step solution

Identify the ions in the reactants

Sodium chromate is made up of sodium (Na⁺) ions and chromate (CrO₄²⁻) ions. Aluminum bromide is made up of aluminum (Al³⁺) ions and bromide (Br⁻) ions.

Predict the products

In a double replacement reaction, the cations and anions from the reactants switch places to form new products. So, the new products are sodium bromide and aluminum chromate. Sodium bromide is formed when sodium (Na⁺) ions combine with bromide (Br⁻) ions, and aluminum chromate is formed when aluminum (Al³⁺) ions combine with chromate (CrO₄²⁻) ions.

Balance the equation

Now, we need to balance the chemical equation. The unbalanced equation is: Na2CrO4 + AlBr3 → NaBr + Al2(CrO4)3 To balance the equation: 1. Balance the sodium atoms: there are 2 sodium atoms on the left side and only 1 on the right side, so we need to place a 2 in front of NaBr. Na2CrO4 + AlBr3 → 2NaBr + Al2(CrO4)3 2. Balance the bromine atoms: there are 3 bromine atoms on the left side and 2 on the right side, so we need to place a 3 in front of NaBr. Na2CrO4 + AlBr3 → 6NaBr + Al2(CrO4)3

Include the phases

According to the solubility rules, sodium bromide (NaBr) is soluble, so it will be in the aqueous phase (aq). Aluminum chromate (Al2(CrO4)3) is the precipitate formed, so it will be in the solid phase (s). Including the phases, the balanced chemical equation is: Na2CrO4(aq) + AlBr3(aq) → 6NaBr(aq) + Al2(CrO4)3(s)

Key concepts

Double Replacement Reactions

Double replacement reactions are a type of chemical reaction where two compounds exchange their components to form two new compounds. This swap occurs between the positive ions (cations) and the negative ions (anions) of the reactants.

In the context of our example where sodium chromate ((Na_2CrO_4)) and aluminum bromide ((AlBr_3)) are the reactants, the sodium ((Na^+)) and aluminum ((Al^{3+})) cations switch places with their respective anion partners. The sodium ion pairs with the bromide ion ((Br^-)), and the aluminum ion pairs with the chromate ion ((CrO_4^{2-})), leading to the formation of new compounds: sodium bromide (NaBr) and aluminum chromate ((Al_2(CrO_4)_3)).

Understanding the nature of double replacement reactions can help predict the products that will form when two ionic compounds react. It’s also worth noting these reactions occur primarily in aqueous (water-based) solutions where the ions are free to move and react.

Solubility Rules

Solubility rules are guidelines that predict the solubility of compounds in water. Solubility itself refers to the maximum amount of a substance that can be dissolved in a solvent at a given temperature. Generally, a substance is considered soluble in water if it dissolves to at least 0.1 moles per liter at room temperature.

These rules state that most nitrate ((NO_3^-)) salts and salts containing the alkali metal ions (like sodium, (Na^+)) are soluble. For our exercise, sodium bromide (NaBr) is soluble in water according to these rules and thus exists in the aqueous state ((aq)). On the other hand, compounds like aluminum chromate ((Al_2(CrO_4)_3)) do not align with the solubility rules and are insoluble or less soluble in water, meaning they would typically form a solid precipitate ((s)).

Grasping these solubility rules can greatly assist in predicting whether a compound will primarily exist as precipitated solid or remain dissolved in an aqueous solution during a chemical reaction.

Precipitation Reactions

Precipitation reactions are a subtype of double replacement reactions where at least one of the new compounds formed is insoluble in water and settles out of the solution as a solid, known as a precipitate. This process occurs when the product of the reaction has a lower solubility than the reactants.

In the reaction between sodium chromate and aluminum bromide, we see aluminum chromate ((Al_2(CrO_4)_3)) is the component that lacks solubility and thus precipitates. The formation of a precipitate is often the driving force behind double replacement reactions, where the insoluble compound triggers the reaction to move forward. By recognizing the possible formation of a precipitate through the solubility rules, one can predict and confirm the occurrence of a precipitation reaction.

Precipitation reactions have numerous applications including the purification of water, the separation of ions in a solution for analytical purposes, and are fundamental in geochemical and biological processes.