Neutralization Reactions and Titration

Neutralization Reactions

• Neutralization reactions involve reacting an acid or base with its counterpart to create a neutral solution that can be safely disposed of.

Neutralization Defined

• Acids and bases react compulsively until one or both are used up.
• If both are completely used up, the resulting solution is neutral.

Chemistry of Neutralization

• Acids and bases cancel each other out when combined.
• For every $OH^-$ ion, an $H^+$ ion will combine to form water ($H_2O$).
• Ideally, you want to add the precise amount of acid or base to ensure both are completely used up, leaving only water.
• The remaining water self-ionizes, but this is a minimal reaction.

Water's Role in Acid-Base Reactions

• The reaction is a one-to-one reaction.
• When an acid such as $HCl$ is added, it reacts to form hydronium ($H_3O^+$) and a conjugate base.
• When a base is added, it dissociates in water to form a conjugate acid and hydroxide ($OH^-$).
• The acid-base reaction fundamentally occurs with the water species, producing hydronium and hydroxide ions.
• The leftovers are the salt.

Salts

• Salt refers to the leftover cation and anion from the acid-base reaction.
• Salts are ionic compounds and exist in water.
• The acid-base reaction always produces water and a salt.

Example Reaction

$H^+ + OH^- \rightarrow H_2O$

• The acid ($H^+$) reacts with the base ($OH^-$) to form water, which is neutral.
• The remaining ions form the salt.

Real-World Application: Antacids

• Stomach acid (hydrochloric acid) can cause heartburn when it's regurgitated into the esophagus.
• Antacids contain weak bases to neutralize the acid.
• The bases in antacids react with the hydrochloric acid in the esophagus, neutralizing it.

Stoichiometry of Neutralization

• The reaction is always one to one.
• If you have five moles of acid, you need five moles of base to neutralize it.

Neutrality and pH

• At neutrality, the pH should be 7.
• This is because the only acid-base activity is the autoionization of water, where the concentrations of $H^+$ and $OH^-$ are both $1 \times 10^{-7}$.

Example Calculation

• Neutralizing 0.5 moles of sodium hydroxide ($NaOH$) with sulfuric acid ($H<em>2SO</em>4$).
• There is one mole of hydroxide ($OH^-$) for every one mole of sodium hydroxide.
• For every one mole of hydroxide, one mole of $H^+$ is needed.
• For every mole of sulfuric acid, there are two moles of $H^+$.
• $0. 5 \, \text{moles of NaOH} \times (1 \, \text{mole of} \, OH^- / 1 \, \text{mole of} \, NaOH) \times (1 \, \text{mole of} \, H^+ / 1 \, \text{mole of} \, OH^-) \times (1 \, \text{mole of} \, H<em>2SO</em>4 / 2 \, \text{moles of} \, H^+) = 0.25 \, \text{moles of} \, H<em>2SO</em>4$

Titration

• Titration is an industrial application of neutralization.

Titration Process

• A known solution is added to an unknown solution until the mixture is neutral.
• By knowing the amount of the known solution added (usually a base), the amount of acid in the unknown solution can be calculated.
• This allows determination of the concentration of the unknown solution.

Titration Principle

• The goal is to achieve perfect neutrality, where moles of acid equal moles of base.
• Overshooting the endpoint compromises the accuracy of the concentration calculation.

Two Main Types of Titration Problems

1. Determining the concentration of an unknown acid or base.
2. Determining the volume needed to neutralize waste.

Equivalence Point

• The equivalence point is the moment when the solution is neutral, and the moles of acid equal the moles of base.

Importance of Accuracy

• Overshooting the equivalence point provides inaccurate information about the unknown concentration.

Indicators

• Indicators or pH probes are used to determine when the solution is neutral.
• Indicators are preferred because they change color at a specific pH and flip quickly, and do not require calibration.

Titration Steps

1. Measure a known volume of the unknown acid.
2. Use a base of known concentration, often sodium hydroxide, loaded into a burette (which measures volume precisely).
3. Add the base dropwise to the unknown acid while looking for a color change (if using an indicator).
4. Repeat the titration multiple times for accuracy.

Observing the Endpoint

• When using phenolphthalein, the solution will change from clear to a pale pink at the endpoint.

Calculations

• Moles of acid divided by volume of acid gives the concentration of the acid.

Example Problem 1

• Neutralizing 100 milliliters containing 0.1 moles of $H^+$ ions using sodium hydroxide at 0.01 moles per milliliter.
• Here, need 0.1 mole of $OH^-$. From the concentration of $NaOH$, you can find you need $10 \, \text{mL}$.

Example Problem 2

• 25 milliliters of $H<em>2SO</em>4$ is neutralized by 18 milliliters of 1 molar sodium hydroxide, and the goal is to determine the concentration.
• $18 \, \text{mL NaOH} \times (1 \, \text{L} / 1000 \, \text{mL}) \times (1 \, \text{mol NaOH} / 1 \, \text{L}) \times (1 \, \text{mol} \, OH^- / 1 \, \text{mol NaOH}) \times (1 \, \text{mol} \, H^+ / 1 \, \text{mol} \, OH^-) \times (1 \, \text{mol} \, H<em>2SO</em>4 / 2 \, \text{mol} \, H^+) = 0.009 \, \text{mol} \, H<em>2SO</em>4$
• Divide by liters to find the molarity. $0.009 \, \text{mol} / 0.025 \, \text{L} = 0.36 \, \text{M}$

Example Problem 3

• How many liters of 2 M $NaOH$ are required to neutralize 100 mL 2 M $HCl$?
• Process is the same, but you calculate for a volume instead of a molarity.

Key Concepts

• Determine the number of moles of acid and set it equal to the number of moles of base needed and solve depending on what the question is asking.