Problem 53 - Chemistry The Central Science

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, HCOOH.

Step-by-Step Solution

Answer

a) Mn (s) + 2H+ (aq) -> Mn2+ (aq) + H2 (g) b) 2Cr (s) + 6H+ (aq) -> 2Cr3+ (aq) + 3H2 (g) c) Sn (s) + 2H+ (aq) -> Sn2+ (aq) + H2 (g) d) 2Al (s) + 6H+ (aq) -> 2Al3+ (aq) + 3H2 (g)

Step 1: Identify the reactants and their oxidation states

For each reaction, we need to identify the reactants and their possible oxidation states. They are as follows: a) Manganese (Mn): +2 b) Chromium (Cr): +3 c) Tin (Sn): +2 d) Aluminum (Al): +3

Step 2: Write the balanced molecular equations

Now, let's write the balanced molecular equations for each reaction: a) Manganese with sulfuric acid Mn (s) + H2SO4 (aq) -> MnSO4 (aq) + H2 (g) b) Chromium with hydrobromic acid 2Cr (s) + 6HBr (aq) -> 2CrBr3 (aq) + 3H2 (g) c) Tin with hydrochloric acid Sn (s) + 2HCl (aq) -> SnCl2 (aq) + H2 (g) d) Aluminum with formic acid 2Al (s) + 6HCOOH (aq) -> 2Al(HCOO)3 (aq) + 3H2 (g)

Step 3: Write the balanced net ionic equations

Now, we'll eliminate spectator ions from each balanced equation and write the net ionic equations: a) Manganese with sulfuric acid Mn (s) + 2H+ (aq) + SO42- (aq) -> Mn2+ (aq) + SO42- (aq) + H2 (g) Net ionic equation: Mn (s) + 2H+ (aq) -> Mn2+ (aq) + H2 (g) b) Chromium with hydrobromic acid 2Cr (s) + 6H+ (aq) + 6Br- (aq) -> 2Cr3+ (aq) + 6Br- (aq) + 3H2 (g) Net ionic equation: 2Cr (s) + 6H+ (aq) -> 2Cr3+ (aq) + 3H2 (g) c) Tin with hydrochloric acid Sn (s) + 2H+ (aq) + 2Cl- (aq) -> Sn2+ (aq) + 2Cl- (aq) + H2 (g) Net ionic equation: Sn (s) + 2H+ (aq) -> Sn2+ (aq) + H2 (g) d) Aluminum with formic acid 2Al (s) + 6HCOO- (aq) + 6H+ (aq) -> 2Al3+ (aq) + 6HCOO- (aq) + 3H2 (g) Net ionic equation: 2Al (s) + 6H+ (aq) -> 2Al3+ (aq) + 3H2 (g)

Key Concepts

Redox Reactions

Redox reactions, or oxidation-reduction reactions, are processes where the oxidation states of atoms are changed. In these reactions:

• Oxidation involves the loss of electrons, increasing the oxidation state.
• Reduction involves the gain of electrons, decreasing the oxidation state.

In the exercise, metals like manganese and chromium react with various acids. Here’s a breakdown:

• Manganese goes from an oxidation state of 0 in its elemental form to +2 in manganese sulfate, meaning it loses electrons.
• Similarly, chromium, tin, and aluminum also lose electrons, increasing their respective oxidation states during their reactions.

The other component, hydrogen ions (H⁺), gains electrons to form hydrogen gas (H₂), thus undergoing reduction. Recognizing which element loses or gains electrons helps in balancing these reactions and forming net ionic equations.

Stoichiometry

Stoichiometry revolves around the calculation of reactants and products in chemical reactions. It helps us understand the quantitative relationships needed for reactions to occur fully.
In our examples, stoichiometry ensures the chemical equations are balanced, meaning:

• The number of atoms for each element is the same on both sides of the equation.
• The charge is conserved, maintaining neutrality.

To balance a reaction, coefficients are adjusted. For instance:

• In the reaction of tin with hydrochloric acid, one mole of tin reacts with two moles of HCl to form tin (II) chloride and hydrogen gas. This balancing ensures all atoms initially present are accounted for in the products.

Mastery of stoichiometry involves precise adjustments of these coefficients to reflect actual chemical processes, especially in complex reactions.

Molecular Equations

Molecular equations show the complete chemical formulas of reactants and products without indicating ionic species or their dissociation. These equations provide a clear, straightforward view of the reaction.
For example:

• In the reaction of aluminum with formic acid, the molecular equation is written as: 2Al (s) + 6HCOOH (aq) → 2Al(HCOO)₃ (aq) + 3H₂ (g).
• This equation displays aluminum and formic acid as they react to form aluminum formate and hydrogen gas.

Though simplified, molecular equations are invaluable for broad understanding. However, to grasp finer details and eliminate spectator ions, net ionic equations are often derived for deeper insights into the actual chemical changes taking place.