Problem 142 - Chemistry The Molecular Nature of Matter

Question

With which of the following will the weak acid \(\mathrm{HCHO}_{2}\) react? For those with which there is a reaction, write the formulas of the products. (a) \(\mathrm{KOH},\) (b) \(\mathrm{MgO},\) (c) \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

HCHO2 reacts with KOH to form KCHO2 and H2O, and with MgO to form Mg(CHO2)2 and H2O. There is no reaction with NH3.

Step 1 - Identifying Possible Reactions

Analyze the substances given (KOH, MgO, NH3) to determine which can react with the weak acid HCHO2 (formic acid). As a weak acid, HCHO2 can react with strong bases to form salt and water. KOH is a strong base, MgO is a basic oxide and can react with acids, and NH3 is a weak base.

Step 2 - Reaction with KOH

Combine HCHO2 with KOH to form a salt and water. The reaction is: \(\mathrm{HCHO}_{2} + \mathrm{KOH} \rightarrow \mathrm{KCHO}_{2} + \mathrm{H}_{2}\mathrm{O}\).

Step 3 - Reaction with MgO

Combine HCHO2 with MgO to form a salt and water. The reaction is: \(2\mathrm{HCHO}_{2} + \mathrm{MgO} \rightarrow \mathrm{Mg}(\mathrm{CHO}_{2})_{2} + \mathrm{H}_{2}\mathrm{O}\).

Step 4 - Reaction with NH3

Since NH3 is a weak base, it does not readily react with weak acids like HCHO2 under normal conditions, so there is no reaction in this case.

Key Concepts

Acid-Base Reactions

Understanding acid-base reactions is critical to grasping many chemical processes, as these reactions are fundamental in chemistry. An acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton. When an acid and base react, they typically form water and a salt, following the general equation:
\[\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}\]
In the context of our exercise, the weak acid involved is \(\mathrm{HCHO}_{2}\), formic acid. Being a weak acid means it doesn't completely ionize in water. The reaction with a strong base (\(\mathrm{KOH}\)) or a basic oxide (\(\mathrm{MgO}\)) leads to the production of a salt and water. This type of reaction is called a neutralization reaction. However, when two weak reactants, such as a weak acid and a weak base, are mixed, the reaction is less favorable. In the case of \(\mathrm{HCHO}_{2}\) and \(\mathrm{NH}_{3}\), we do not observe a reaction under normal conditions. This concept is fundamental because it establishes the criteria for predicting whether two substances will react in an acid-base manner.

Chemical Reaction Writing

Writing chemical reactions is a skill that involves demonstrating the change of reactants into products using chemical formulas. While writing reactions, it is essential to follow the law of conservation of mass, meaning the number of atoms for each element should remain the same on both sides of the reaction.
In our example, with the focus on \(\mathrm{HCHO}_{2}\) reactions, we saw how to write balanced reactions for acid-base interactions:

• For the reaction with \(\mathrm{KOH}\), we have: \[\mathrm{HCHO}_{2} + \mathrm{KOH} \rightarrow \mathrm{KCHO}_{2} + \mathrm{H}_{2}\mathrm{O}\]
• For the reaction with \(\mathrm{MgO}\), we balance it with stoichiometric coefficients: \[2\mathrm{HCHO}_{2} + \mathrm{MgO} \rightarrow \mathrm{Mg}(\mathrm{CHO}_{2})_{2} + \mathrm{H}_{2}\mathrm{O}\]

It is crucial not just to write the reactants and products, but also to ensure that the coefficients are correct, providing a balanced representation of the reaction.

Weak Acids and Bases

When discussing weak acids and bases, we're referring to their ability to dissociate in water. Weak acids, such as \(\mathrm{HCHO}_{2}\), partially ionize, which means a reversible reaction is established between the un-ionized acid and the ions produced.
The general dissociation reaction for a weak acid can be represented as:
\[\mathrm{HA} \rightleftharpoons \mathrm{H}^{+} + \mathrm{A}^{-}\]
The double arrow indicates a state of equilibrium between the reactants and products, contrasting with the single arrow in strong acid reactions, implying complete ionization.
Weak bases, like \(\mathrm{NH}_{3}\), react similarly by not fully accepting protons. The equilibrium state for weak acids and bases affects their reactivity, yielding fewer ions, and consequently, a weaker reaction with one another in comparison to their strong counterparts. This concept is vital for predicting whether a weak acid will react with other substances and determining the strength and direction of the reaction.