Question

If a solution of sodium phosphate (also known as trisodium phosphate, or \(\mathrm{TSP}\) ), \(\mathrm{Na}_{3} \mathrm{PO}_{4}\), is poured into seawater, precipitates of calcium phosphate and magnesium phosphate are formed. (Magnesium and calcium ions are among the principal ions found in seawater.) Write net ionic equations for these reactions.

Short answer

Net ionic equation for calcium phosphate: 3Ca^{2+}(aq) + 2PO_4^{3-}(aq) -> Ca_3(PO_4)_2(s). Net ionic equation for magnesium phosphate: 3Mg^{2+}(aq) + 2PO_4^{3-}(aq) -> Mg_3(PO_4)_2(s).

Step-by-step solution

- Write potential full reaction formulas

First, we hypothesize possible reactions between trisodium phosphate and the calcium and magnesium ions in seawater. Trisodium phosphate (Na_3PO_4) can react with calcium ions (Ca^{2+}) and magnesium ions (Mg^{2+}) to form calcium phosphate (Ca_3(PO_4)_2) and magnesium phosphate (Mg_3(PO_4)_2), respectively.

- Balance the full reaction equations

Balance the equations from Step 1 to maintain the conservation of mass and charge. For calcium phosphate: 3Ca^{2+}(aq) + 2Na_3PO_4(aq) -> Ca_3(PO_4)_2(s) + 6Na^+(aq). For magnesium phosphate: 3Mg^{2+}(aq) + 2Na_3PO_4(aq) -> Mg_3(PO_4)_2(s) + 6Na^+(aq).

- Write the net ionic equations

Now, remove the spectator ions (Na^+) that don't participate in the reaction to form the net ionic equations: For calcium phosphate, 3Ca^{2+}(aq) + 2PO_4^{3-}(aq) -> Ca_3(PO_4)_2(s). For magnesium phosphate, 3Mg^{2+}(aq) + 2PO_4^{3-}(aq) -> Mg_3(PO_4)_2(s).

Key concepts

Trisodium Phosphate Reactions

Trisodium phosphate (ab{Na_3PO_4}), is a versatile chemical used in a variety of cleaning products and also has applications in water treatment. When it reacts with metal cations like calcium (ab{Ca^{2+}}) and magnesium (ab{Mg^{2+}}), present in seawater, it forms respective phosphate salts. These reactions are highly important in managing water hardness and also in removing unwanted ions from solutions. Since ab{Na_3PO_4} readily reacts with multivalent cations, it is a useful agent in forming precipitates that can easily be filtered out from water systems, contributing to water softening and purifying processes. Understanding these reactions is also vital for aquarists, who need to maintain specific water conditions for marine life.

Chemical Equation Balancing

Balancing chemical equations is a fundamental concept in chemistry that aligns with the Law of Conservation of Mass. In the solutions provided, the reactions were balanced to ensure that the number of atoms for each element is the same on both sides of the reaction equation.
Moreover, it is critical to also consider the charge conservation; the overall charge should be equal on both sides of the reaction. Working methodically, as shown in the solution, identifies that coefficients need to be applied to trisodium phosphate and the calcium or magnesium ions to achieve this balance. To further support understanding, it might help to enumerate the atoms on each side of the equation before and after balancing, or use algebraic methods for more complex equations. Ensuring each student grasps this concept is vital for solving advanced chemical problems.

Precipitation Reactions

Precipitation reactions are where dissolved substances react to form one or more solid products. The provided exercise illustrates this type of reaction, as ab{Na_3PO_4} reacts with the divalent cations ab{Ca^{2+}} and ab{Mg^{2+}}, forming solid salts that precipitate out from the solution. Recognizing a precipitation reaction involves predicting product solubility, using solubility rules that guide us on whether a compound will be soluble or insoluble in water. Learning how to write net ionic equations, as demonstrated in the solution, is critical for identifying the ions that directly participate in forming the precipitate.
This knowledge is essential in various applications, from water treatment to materials synthesis, and understanding it can also help avoid undesired precipitates in industrial processes or in maintaining the balance of nutrients in aquariums and natural bodies of water.