Problem 7 - Chemistry An Atoms First Approach

Question

Consider separate aqueous solutions of HCl and \(\mathrm{H}_{2} \mathrm{SO}_{4}\) with the same molar concentrations. You wish to neutralize an aqueous solution of NaOH. For which acid solution would you need to add more volume (in milliliters) to neutralize the base? a. the HCI solution b. the \(\mathrm{H}_{2} \mathrm{SO}_{4}\) solution

Step-by-Step Solution

Answer

The larger volume would be required for the HCl solution to neutralize the NaOH, since the stoichiometry of the reaction involves a 1:1 mole ratio for HCl and NaOH, whereas H2SO4 has a 2:1 mole ratio with NaOH. Answer: a. the HCl solution.

Step 1: Write the balanced chemical equations for the reactions

Write the balanced chemical equation for the neutralization of NaOH with HCl and with H2SO4. NaOH + HCl -> NaCl + H2O 2NaOH + H2SO4 -> Na2SO4 + 2H2O

Step 2: Calculate the stoichiometry of the reactions

Calculate the molar ratios of NaOH to HCl and NaOH to H2SO4 in the balanced chemical equations: For NaOH + HCl -> NaCl + H2O, the ratio is 1:1. For 2NaOH + H2SO4 -> Na2SO4 + 2H2O, the ratio is 2:1.

Step 3: Compare stoichiometry to determine which solution requires more volume

Compare the stoichiometry calculated in Step 2 to determine which acid solution (HCl or H2SO4) needs a larger volume to neutralize NaOH: A 1 mole of HCl neutralizes 1 mole of NaOH, and 1 mole of H2SO4 neutralizes 2 moles of NaOH. Since the molar concentrations of HCl and H2SO4 solutions are the same, half the volume of H2SO4 solution is needed to neutralize the same amount of NaOH compared to the volume of HCl solution. Therefore, the larger volume would be required for the HCl solution. The answer is: a. the HCl solution

Key Concepts

Chemical Stoichiometry

Chemical stoichiometry involves the calculations of the quantities of reactants and products involved in a chemical reaction using the balanced chemical equation. It's essential for understanding how different molecules interact and how much of each substance is needed to react completely.

In the context of neutralization reactions, where an acid reacts with a base to form water and a salt, stoichiometry allows us to predict the volume of an acid required to neutralize a given amount of base. As seen in the exercise, the balanced equations are key to this understanding.

For instance, when we analyze the neutralization of NaOH with HCl and H2SO4, the stoichiometry reveals different molar ratios. A 1:1 ratio for the HCl reaction means each mole of NaOH needs one mole of HCl to neutralize, whereas the H2SO4 reaction has a 2:1 ratio, requiring only half a mole of H2SO4 for every mole of NaOH.

Thus, understanding stoichiometry not only aids in performing accurate calculations but also helps visualize the actual amount of substances participating in a chemical reaction.

Acid-Base Reactions

Acid-base reactions are a type of chemical reaction that involves the transfer of protons from an acid to a base. The most common product of an acid-base reaction is water, along with a salt that forms from the remaining ions.

In the example given, the HCl and H2SO4 are both acids donating protons (H+ ions) to the base, NaOH. The reaction between HCl and NaOH produces NaCl (table salt) and water, while the reaction between H2SO4 and NaOH produces Na2SO4 and water.

The products depend on how many protons the acid can donate—HCl is monoprotic (donates one proton), and H2SO4 is diprotic (can donate two protons). This difference is critical in determining how much acid is needed to neutralize a given amount of base. Simplifying these concepts helps in understanding how neutralization reactions work and how stoichiometry plays a part in these reactions.

Molarity and Solution Concentration

Molarity is a measure of the concentration of a solution, expressing the amount of a substance in moles per liter of solution. It's represented by the unit M (molar) and is a fundamental concept in chemistry when dealing with solutions.

In our exercise, both the HCl and H2SO4 solutions have the same molarity, which implies that they contain an equal number of moles of acid per liter of solution. This uniformity allows for a straightforward comparison of the volume needed to neutralize a set amount of NaOH. Since molarity is the ratio of moles to volume, we can calculate the volume of acid needed once we know the number of moles required to neutralize the base.

For example, if both acid solutions were 1 M, you would need 1 liter of HCl to neutralize 1 mole of NaOH but only 0.5 liters of H2SO4 for the same amount of NaOH due to the stoichiometry of the compared reactions. Clear explanations of molarity ensure that students can effectively measure and predict the outcomes of their chemical reactions.