ACID-BASE CHEMISTRY

Definitions:

Arrhenius

Acids = proton donors (H+)

Bases = hydroxide donors

Bronsted-Lowry

Acids = proton donors

Base = proton acceptors (water can act as a base)

There are two ways of showing an acid reaction:

HCl H+ + Cl-

orHCl + H2O H3O+ + Cl-

Hydronium ion

• To simplify the equations, write H+ , although it is understood that it is always H3O+

What makes an acid “strong”?

When dropped in water, the acid completely dissociates into hydrogen ions and the anion

Ex: HNO3

HClO4

H2SO4

• A weak acid

doesn’t completely dissociate.

It forms an equilibrium condition, with only a small part of the acid solute producing hydrogen ions.

HC2H3O2 H+ + C2H3O2-

• The meaning of pH

Pure water ionizes slightly

H2O H+ + OH-

At 25ºC, [H+] = 1.0 X 10-7 M

(1 out of 10,000,000 molecules)

The pH of pure water is 7.

The pH of a solution that has a

[H+] =1.0 X 10-4 M is 4

What is the relationship between [H+] and pH?

The pH scale

Is a way of expressing [H+]

“p” = - log of

So pH = -log [H+]

If pH = 7.0, then [H+] = ?(The number of significant digits in the concentration is

determined by the number of sig. digs in the pH after the decimal point.)

• Use your calculator to convert hydrogen ion concentration to pH

• What is the pH when [H+] = 4.3 X 10-5?

• On your calculator:

-log 4.3 EE -5 =

What is the pH when [H+] = 9.2 X 10-8?

= 6.1 X 10-3?

To convert pH to hydrogen ion concentration:

• What is [H+] when the pH is 6.32?

• On your calculator:2nd log - 6.32=

When the pH is 7.01?

When the pH is 1.32?

• pH tells you whether a solution is acidic or basic:

When pH=7, it is neutral

When pH > 7, it is basic

When pH < 7 it is acidic

To review..

• What is an acid?

• What is a Base?

• What is the PH scale?

• How do you determine PH from [H+] concentration?

• How do you determine [H+] from PH?

Try these…

• What is the PH of a solution where [H+] 9.2 X 10-8?– 7.03 (Note Sig digs)

– What is the [H+] of a solution with a pH of 5.00

1.00 X 10 -5 (Note Sig digs)

Titration

• When equal amounts of base and acid are mixed together, a salt water solution is created.

HCl + NaOH NaCl + H2O

A technique known as titration can be used to measure the number of moles present, or the concentration of a solution.

The change in pH can be measured, using an indicator, such as phenolphthalein.

• Since the number of moles of base and acid are equal at neutralization, then

ML = MLBase acid

This is also called the equivalence point.

Ex:What is the molarity of a NaOH solution if 252 ml are neutralized by 51.0 ml of a 6.00 M HCl solution?

What is the molarity of a HCl solution if 34.6 mL are needed to neutralize .52 g of KOH solid?

• Finding the concentration of a base or an acid is also known as standardization.

Besides using a solution of known molality to standardize by titration a solution of unknown molarity, a chemist can also use a solid to standardize.

Ex: 0.350 g of an acid with a molar mass of 53.0 g is titrated to neutrality with a 25.2 ml of a NaOH solution. What is the concentration of the NaOH?

Titration of Vinegar

• What is PH

• What is a tritration

• What is an equivalence point?

• How could you determine the molarity of an unknown acid or base?

• Another use of titration:

Determining the molar mass of an acid or base.

• Ex: 26.5 mL of a 0.25M NaOH solution were used to titrate to equivalence a .326 g sample of an acid. What is its molar mass?

• Another example: It takes 40.3 mL of a 0.32 M solution of NaOH to neutralize .544 g of an unknown acid. What is its molar mass?

The Titration challenge..

Today… You will determine the molarity of a base from determining

the molarity of an acid.

• Acid you will be using KHP…• What information will you need to determine the

molarity of NaOH it is reacting with?• Moles of KHP reacted• Moles of NaOH (Needed to completely react

with KHP)• Volume of NaOH reacted

Procedure

• Mass of Khp mass out .30g.

• (Place in Flask)

• Add Distilled water to dissolve all kHp

• Use titration to determine the volume of NaOH needed to neutralize.

• Determine Molarity of NaOH

This can be done with titration using phenol phthalein

• When the phenol phtalein turns to a pink the solution is neutral

• Base will be in the burette… You need to add with a funnel (slowly)

• Turn the stop cock on the burette till you have an even measurement

• Add a drop each time and swirl your flask until it turns pink

• Record final volume of NaOH difference between the amount your started with and ending point

• Molar Mass of kHP=204..23 g/mol