Neutralization and Titrations

Neutralization reaction

Generally, when acids and bases react together they form a salt and water.

The H+ ion and OH− ion combine together to make H2O and the leftover cation and anion form a salt.

Example 1:

2 HCl(aq) + Mg(OH)2(aq) → 2 H2O(ℓ) + MgCl2(aq)

Example 2:

2HNO3(aq) + Ba(OH)2(aq) → 2H2O(ℓ) + Ba(NO3)2(aq)

What is Salt?

Salt is NOT only table salt (NaCl). In Chemistry, salts are ANY ionic compound.

Titration

Titration is a process to find the concentration of either an acid or a base. It uses stoichiometry to determine the molarity of the unknown solution

Titrant - the solution of known concentration

Analyte - the solution of unknown concentration

Indicator - chemical that will show when the acid and base have neutralized each other. Usually the chemical used is phenolphthalein, which turns pink.

Equivalence point - when the concentrations of H+ ions and OH− ions are equal (neutralization)

End point - When the indicator changes color

Titration Curve

A titration curve shows the pH vs the amount of titrant added.

The pH of the equivalence point can be determined by the picking the midpoint where the steepest part of the graph (vertical).

titration curves will look different depending on whether a strong or weak acid is used.

Choosing an indicator

Indicators are chosen based on their range of pH. For strong acids and bases the steepness of the curve allows for a wider range of indicators to be used. But in the case of weak acids and bases indicators where the equivalence point is will determine which indicator to use.

Titration Calculations

We know that at the equivalence point the number of moles of H+ ions and OH− ions are equal. So to figure out the concentration of our unknown we need to figure out the moles of H+ ions and OH− used.

Molarity is given by:

Molarity = moles/liters

So to find the number of moles used in a titration we multiply the volume used in liters

moles = Molarity x Liters

So we can relate concentration and volume of the acid and base in the following way:

Macid ᐧ Vacid = Mbase ᐧ Vbase

To find the concentration of the unknown we only need to know three of the above variables.

Example

Suppose that a titration is performed and 20.70ml of 0.500M NaOH is required to reach the end point when titrated against 15.00ml of HCl of unknown concentration.

Let's use the equation:

Macid ᐧ Vacid = Mbase ᐧ Vbase

Vacid = 15.00ml

Mbase = 0.500M

Vbase = 20.70ml

Therefore:

Macid (15.00ml) = (0.500M)(20.70ml)

Macid = (0.500M)(20.70ml) / (15.00ml)

Macid = 0.690M

Page updated

Google Sites

Report abuse