Problem 53 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write balanced molecular and net ionic equations for the reactions of (a) manganese with dilute sulfuric acid, (b) chromium with hydrobromic acid, (c) tin with hydrochloric acid, (d) aluminum with formic acid, \(\mathrm{HCOOH}\).

Step-by-Step Solution

Answer

The short versions of the balanced molecular and net ionic equations for each reaction are: (a) Mn + H2SO4 -> MnSO4 + H2 Net ionic: Mn + 2H+ -> Mn2+ + H2 (b) 2Cr + 6HBr -> 2CrBr3 + 3H2 Net ionic: 2Cr + 6H+ -> 2Cr3+ + 3H2 (c) Sn + 2HCl -> SnCl2 + H2 Net ionic: Sn + 2H+ -> Sn2+ + H2 (d) 2Al + 6HCOOH -> 2Al(HCOO)3 + 3H2 Net ionic: 2Al + 6H+ -> 2Al3+ + 3H2

Step 1: Write the unbalanced molecular equation

\(Mn + H_2SO_4 \rightarrow MnSO_4 + H_2\)

Step 2: Balance the molecular equation

The equation is already balanced.

Step 3: Write the ionic equation

\(Mn + 2H^+ + SO_4^{2-} \rightarrow Mn^{2+} + SO_4^{2-} + H_2\)

Step 4: Write the net ionic equation

\(Mn + 2H^+ \rightarrow Mn^{2+} + H_2\) (b) Reaction between chromium and hydrobromic acid

Step 1: Write the unbalanced molecular equation

\(Cr + HBr \rightarrow CrBr_3 + H_2\)

Step 2: Balance the molecular equation

\(2Cr + 6HBr \rightarrow 2CrBr_3 + 3H_2\)

Step 3: Write the ionic equation

\(2Cr + 6H^+ + 6Br^- \rightarrow 2Cr^{3+} + 6Br^- + 3H_2\)

Step 4: Write the net ionic equation

\(2Cr + 6H^+ \rightarrow 2Cr^{3+} + 3H_2\) (c) Reaction between tin and hydrochloric acid

Step 1: Write the unbalanced molecular equation

\(Sn + HCl \rightarrow SnCl_2 + H_2\)

Step 2: Balance the molecular equation

\(Sn + 2HCl \rightarrow SnCl_2 + H_2\)

Step 3: Write the ionic equation

\(Sn + 2H^+ + 2Cl^- \rightarrow Sn^{2+} + 2Cl^- + H_2\)

Step 4: Write the net ionic equation

\(Sn + 2H^+ \rightarrow Sn^{2+} + H_2\) (d) Reaction between aluminum and formic acid

Step 1: Write the unbalanced molecular equation

\(Al + HCOOH \rightarrow Al(HCOO)_3 + H_2\)

Step 2: Balance the molecular equation

\(2Al + 6HCOOH \rightarrow 2Al(HCOO)_3 + 3H_2\)

Step 3: Write the ionic equation

\(2Al + 6H^+ + 6HCOO^- \rightarrow 2Al^{3+} + 6HCOO^- + 3H_2\)

Step 4: Write the net ionic equation

\(2Al + 6H^+ \rightarrow 2Al^{3+} + 3H_2\)

Key Concepts

Molecular Equations

Molecular equations are chemical equations that display the reactants and products in their molecular form, rather than in their ionic forms. This type of equation is often the starting point when predicting the outcome of a chemical reaction because it shows every compound involved as it would appear in its original state. For instance, when manganese reacts with dilute sulfuric acid, the molecular equation looks simple: \( Mn + H_2SO_4 \rightarrow MnSO_4 + H_2 \). This formulation allows chemists to see what happens before breaking it down further.
In every molecular equation, physical states are typically noted (such as gas \((g)\), liquid \((l)\), solid \((s)\), or aqueous \((aq)\)). This detail gives context about how the reaction may proceed. Knowing the molecular equation helps in identifying which substances interact to form new substances.

• Manganese with sulfuric acid: \(Mn + H_2SO_4 \rightarrow MnSO_4 + H_2\)
• Chromium with hydrobromic acid: \(Cr + HBr \rightarrow CrBr_3 + H_2\)
• Tin with hydrochloric acid: \(Sn + HCl \rightarrow SnCl_2 + H_2\)
• Aluminum with formic acid: \(Al + HCOOH \rightarrow Al(HCOO)_3 + H_2\)

Balancing Chemical Equations

Balancing chemical equations is a key skill in chemistry, ensuring that the same number of each type of atom appears on both sides of the equation. This reflects the law of conservation of mass where matter, meaning the atoms themselves, is neither created nor destroyed in a chemical reaction. To balance an equation, one must adjust the coefficients – numbers placed before compounds – without altering the chemical formulas of the reactants or products.
For example, in the reaction involving chromium and hydrobromic acid, the unbalanced equation \( Cr + HBr \rightarrow CrBr_3 + H_2 \) is adjusted to \( 2Cr + 6HBr \rightarrow 2CrBr_3 + 3H_2 \), ensuring chromium, bromine, and hydrogen atoms are equal on both sides. Balancing equations is crucial as it provides a detailed picture of what happens during the reaction and is a prerequisite for translating them into ionic or net ionic equations.

Acid-Metal Reactions

Acid-metal reactions are a fascinating type of chemical reaction where acids react with metals to usually produce hydrogen gas and a salt. These reactions can be exothermic, meaning they release energy, and the metal typically gets oxidized while the hydrogen ions in the acid are reduced to form hydrogen gas.
Consider tin reacting with hydrochloric acid: the scientific equation \( Sn + 2HCl \rightarrow SnCl_2 + H_2 \) shows tin forming a chloride salt by displacing hydrogen. During this, tin is oxidized from an oxidation state of 0 in \( Sn \) to \( +2 \) in \( SnCl_2 \), while hydrogen transitions from \( +1 \) in \( HCl \) to 0 in \( H_2 \). Net ionic equations are useful here to focus on the actual species involved in the electron transfer processes, e.g., \( Sn + 2H^+ \rightarrow Sn^{2+} + H_2 \).

• Common features:
  • Release of hydrogen gas \((H_2)\)
  • Formation of salts like chlorides, bromides, sulfates

Understanding acid-metal reactions helps in predicting the products formed and explains the stoichiometry of reactions involving acids and metals.