Problem 21 - General Chemistry: Principles and Modern Applications

Question

Predict in each case whether a reaction is likely to occur. If so, write a net ionic equation. (a) \(\mathrm{HI}(\mathrm{aq})+\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq}) \longrightarrow\) (b) \(\operatorname{CuSO}_{4}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{CO}_{3}(\mathrm{aq}) \longrightarrow\) (c) \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{Na}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \longrightarrow\)

Step-by-Step Solution

Answer

(a): No reaction. \n (b): Reaction occurs. The balanced net ionic equation: Cu^{2+}(aq) + CO_{3}^{2-}(aq) \longrightarrow CuCO_{3}(s). \n (c): Reaction occurs. The balanced net ionic equation: 3Cu^{2+}(aq) + 2PO_{4}^{3-}(aq) \longrightarrow Cu_{3}(PO_{4})_{2}(s).

Step 1: Case (a): HI(aq) + Zn(NO3)2(aq)

Based on the reactivity series, hydrogen ions (H+) cannot displace zinc (Zn) from its salt. Therefore, no reaction is expected to occur.

Step 2: Case (b): CuSO4(aq) + Na2CO3(aq)

Here, copper sulfate (CuSO4) reacts with sodium carbonate (Na2CO3) to form copper carbonate (CuCO3) and sodium sulfate (Na2SO4). Copper carbonate, being insoluble, will precipitate out of the solution. The net ionic equation for the reaction is: \[ \mathrm{Cu^{2+}(aq) + CO_{3}^{2-}(aq) \longrightarrow CuCO_{3}(s)} \]

Step 3: Case (c): Cu(NO3)2(aq) + Na3PO4(aq)

Copper nitrate (Cu(NO3)2) reacts with sodium phosphate (Na3PO4) to produce copper phosphate (Cu3(PO4)2) which is insoluble and therefore precipitates, and sodium nitrate (NaNO3): \[ 3\mathrm{Cu^{2+}(aq) + 2PO_{4}^{3-}(aq) \longrightarrow Cu_{3}(PO_{4})_{2}(s)} \]. Please notice that the '(s)' notation after the product indicates a solid product, meaning a precipitate was formed in the reaction.

Key Concepts

Chemical Reactions Prediction

Predicting chemical reactions involves understanding the nature of the reactants and the possible products. It is often necessary to consider factors such as solubility, reactivity, and common reaction types.
For example, to predict if a reaction will occur between hydrogen iodide (HI) and zinc nitrate (Zn(NO₃)₂), one must consider if the reactants will form a more stable product. Here, based on the principle that hydrogen ions cannot displace zinc ions from their compound, it becomes apparent that no reaction will take place.

• If the reactants result in the formation of a precipitate, a gas, or a weak electrolyte, a chemical reaction is likely.
• Checking the compatibility of ions to see if they form new compounds is crucial.

Therefore, being familiar with solubility tables and reactivity series can greatly assist in predicting whether or not a chemical reaction will occur.

Precipitation Reactions

Precipitation reactions are a subtype of double displacement reactions. They occur when two aqueous solutions react to form an insoluble solid called a precipitate.
The key is to recognize that certain ions, when combined, will not remain in the aqueous solution but instead form a solid.
A prime example is the reaction between copper sulfate (CuSO₄) and sodium carbonate (Na₂CO₃), which produces copper carbonate (CuCO₃) and sodium sulfate (Na₂SO₄).
In this instance, copper carbonate is insoluble and precipitates out as a solid:
\[ \text{Cu}^{2+}(aq) + \text{CO}_3^{2−}(aq) \longrightarrow \text{CuCO}_3(s) \]

• Look at solubility rules to determine the likelihood of a precipitate.
• Insoluble products will appear as solids along with notation '(s)'.

Understanding these rules can help predict the outcome of mixing different ionic compounds.

Reactivity Series

The reactivity series is a chart of metals arranged by their ability to displace other metals from compounds. It ranks metals from most reactive to least reactive.
It is an invaluable tool for predicting single displacement reactions. When a more reactive metal comes into contact with a compound containing a less reactive metal, it may displace it.
However, as seen in the case of HI and Zn(NO₃)₂, if the metal in the compound is more reactive than the one trying to displace it (here, hydrogen ions trying to displace zinc), no reaction will occur.

• Metals high in the series displace metals lower in the series from their compounds.
• Hydrogen gas can be displaced by metals above it in the series.

By applying the reactivity series, one can systematically predict whether certain reactions, particularly involving metals, will occur.