Question

A common analysis for the quantity of halide ions \(\left(\mathrm{Cl}^{-}, \mathrm{Br}^{-}, \text {and } \mathrm{I}^{-}\right)\) in a sample is to precipitate and weigh the halide ions as their silver salts. For example, a given sample of seawater can be treated with dilute silver nitrate, \(\mathrm{AgNO}_{3},\) solution to precipitate the halides. The mixture of precipitated silver halides can then be filtered from the solution, dried, and weighed as an indication of the halide content of the original sample. Write the net ionic equations showing the precipitation of halide ions from seawater with silver nitrate.

Short answer

The net ionic equations for the precipitation of halide ions as silver salts in seawater using silver nitrate are: 1) Chloride ion: \[ \ce{Ag+ + Cl^{-} -> AgCl} \] 2) Bromide ion: \[ \ce{Ag+ + Br^{-} -> AgBr} \] 3) Iodide ion: \[ \ce{Ag+ + I^{-} -> AgI} \]

Step-by-step solution

Write the balanced chemical equation for each halide ion reaction with AgNO₃

We need to write three balanced chemical equations, one for each halide ion (Cl⁻, Br⁻, and I⁻) reacting with AgNO₃: 1) Reaction of Cl⁻ with AgNO₃: \[ \ce{AgNO3 + Cl^- -> AgCl + NO3^-} \] 2) Reaction of Br⁻ with AgNO₃: \[ \ce{AgNO3 + Br^- -> AgBr + NO3^-} \] 3) Reaction of I⁻ with AgNO₃: \[ \ce{AgNO3 + I^- -> AgI + NO3^-} \]

Write the net ionic equation for each halide ion reaction by eliminating spectator ions

To write the net ionic equations, we need to remove the spectator ions, which are the ions that remain in the solution and do not participate in the precipitation reaction. In this case, the spectator ion is NO₃⁻. 1) Net ionic equation for precipitation of Cl⁻: \[ \ce{Ag+ + Cl^{-} -> AgCl} \] 2) Net ionic equation for precipitation of Br⁻: \[ \ce{Ag+ + Br^{-} -> AgBr} \] 3) Net ionic equation for precipitation of I⁻: \[ \ce{Ag+ + I^{-} -> AgI} \] Now we have the net ionic equations for the precipitation of halide ions as silver salts in seawater using silver nitrate.

Key concepts

Net Ionic Equations

Understanding net ionic equations is essential for students studying chemistry. These equations show the chemical species that directly participate in a chemical reaction. To arrive at the net ionic equation, you first write a balanced chemical equation, which includes all the reactants and products in the reaction.

In our example with halide ions reacting with silver nitrate, we start by writing the complete balanced equation for each reaction. Then, we identify the spectator ions—these are ions that appear unchanged on both the reactant and product sides of the equation. They don't participate in the actual precipitation reaction and can be left out of the net ionic equation.

When the spectator ions are removed, what's left is the net ionic equation. It highlights the formation of a precipitate, which is a solid product that forms from the reaction of aqueous solution ions. Net ionic equations are valuable as they simplify the reaction down to its essence and help us understand what is truly happening in the beaker.

Silver Nitrate Reactions

Silver nitrate, with the chemical formula \( \mathrm{AgNO}_{3} \), is a common reagent in chemistry, particularly in reactions involving halide ions. When added to a halide-containing solution, like seawater, silver nitrate reacts to form insoluble silver halides. The reaction process involves the exchange of ions—silver ions (Ag+) from the silver nitrate combine with the halide ions (Cl⁻, Br⁻, and I⁻) to form the respective silver salts, AgCl, AgBr, and AgI.

These reactions are significant because they allow for the qualitative and quantitative analysis of halide ions in a sample by precipitating them in the form of silver salts. As a precipitating agent, silver nitrate is an invaluable tool in analytical chemistry. The straightforward reaction results in the halides settling out of the solution as a solid, which can be weighed to determine the quantity of each respective halide ion present in the original solution.

Spectator Ions

In every aqueous chemical reaction, not all ions will contribute to the formation of the product. These uninvolved ions are called spectator ions because they 'watch' the reaction happen without participating directly. In the provided exercise, the nitrate ion (NO3⁻) is such a spectator ion.

Spectator ions are present in both the reactant mixture and the final solution and do not undergo any chemical change. They are important because by identifying and eliminating spectator ions, we can simplify chemical equations to their net ionic form, which demonstrates the actual chemical change.

Understanding spectator ions is crucial because it empowers students to predict the outcome of reactions and write precise net ionic equations, which are more representative of the true chemistry at play. Recognizing these ions helps reinforce the concept that not everything in a solution will influence the outcome of a precipitation reaction or others like it.