Acids and Bases

Define a Bronsted-Lowry

acid

Proton donor

Define a Bronsted-Lowry

base

Proton acceptor

What ion causes a solution

to be acidic? (2 answers)

Name and formula

H + (hydrogen ion) or, more accurately, H 3

O +

(oxonium ion), as protons react with H 2

O to form

it

What ion causes a solution

to be alkaline?

-

OH (hydroxide ion)

Write an equation for the

ionisation of water (2)

2H 2

O (l) ⇌ H 3

O + (aq) + -

OH (aq)

OR H 2

O (l) ⇌ H + (aq) + -

OH (aq)

Derive Kw using the

equation for ionisation of

water

What is the value of K w at

298K?

1.0 x 10 -14

What physical factors affect

the value of K w ? How do

they affect it?

Temperature only - if temperature is increased,

the equilibrium moves to the right so K w

increases and the pH of pure water decreases

Why is pure water still

neutral, even if pH does not

equal 7?

[H + ] = [ - OH]

Give an expression for pH in

terms of H +

pH = -log 10 [H + ]

What is the relationship

between pH and

concentration of H + ?

Lower pH = higher concentration of H +

If two solutions have a pH

difference of 1, what is the

difference in [H + ]?

A factor of 10

How do you find [H + ] from

pH?

[H + ] = 10 -pH

How do you find [OH -

] from

pH? (at 298K)

Find [H + ], use K w

(equal to 1 x 10 -14 at 298K) to

calculate [ -

OH]

What is different when

finding [H + ] from the

concentration of diprotic and

triprotic acids?

Need to multiply the concentration of the acid by

the number of protons to find [H + ]

How do you calculate the

pH of a strong alkaline

solution?

Use Kw to calculate [H + ] from [OH -

]

Use pH=-log[H + ]

Define the term strong acid

One which fully dissociates in water ( HX → H + +

X -

)

Define the term strong base.

One which fully dissociates in water (XOH → X +

+ -

OH)

What is the difference

between concentrated and

strong?

Concentrated means many mol per dm 3

, strong

refers to amount of dissociation

What is a weak acid and a

weak base?

Weak acids and bases do not fully dissociate in

water. They only partially dissociate into their

ions.

Give some examples of

strong acids

HCl, H 2

SO 4

, H 3

PO

Give some examples of

strong bases

NaOH, CaCO 3

, Na 2

CO

Give some examples of

weak acids

CH 3

COOH (ethanoic), any organic acid

Give some examples of

weak bases

NH 3

What is K a ? (expression)

How would you work out the

pH of a weak acid?

Use the equation for K a

, subbing in values for [A -

]

and [HA].

Use pH=-log[H + ] equation to find pH

What is a titration?

The addition of an acid/base of know titration to a

base/acid of unknown titration to determine the

concentration. An indicator is used to show that

neutralization has occurred, as is a pH meter.

Draw a diagram of the

equipment that could be

used for a titration.

Draw the titration curve for a

strong acid with a strong

base added

Draw the titration curve for a

strong acid with a weak

base added

Draw the titration curve for a

weak acid with a weak base

added

Draw the titration curve for a

weak acid with a strong

base added

Define the term equivalence

point.

The point at which the exact volume of base has

been added to just neutralise the acid, or

vice-versa.

What generally happens to

the pH of the solution

around the equivalence

point?

There is a large and rapid change in pH, except

in the weak-weak titration.

How would you calculate the

concentration of a reactant if

you know the volume and

conc of the other reactant and

the volume of that reactant

added

Calculate mols of one reactant

use balanced equation to work out mols of the

other

Use conc = mol/vol to calculate concentration

What is the end point?

The volume of acid or alkali added when the

indicator just changes colour. If the right indicator

is chosen, equivalence point = end point.

What are the properties of a

good indicator for a

reaction? (3)

Sharp colour change (not gradual) - no more than one drop of

acid/alkali needed for colour change

End point must be the same as the equivalence point, or

titration gives wrong answer.

Distinct colour change so it is obvious when the end point has

been reached.

What indicator would you

use for a strong acid-strong

base titration?

Phenolphthalein or methyl orange, but

phenolphthalein is usually used as clearer colour

change.

What indicator would you

use for a strong acid-weak

base titration?

Methyl orange

What indicator would you

use for a strong base-weak

acid titration?

Phenolphthalein

What indicator would you

use from a weak acid-weak

base titration?

Neither methyl orange or phenolphthalein is

suitable, as neither give a sharp change at the

end point.

What colour is methyl

orange in acid? In alkali? At

what pH does it change?

Red in acid; yellow in alkali. Changes at about

pH = 4-5. Approx same as pK a value

What colour is

phenolphthalein in acid? In

alkali? At what pH does it

change?

Colourless in acid; red in alkali. Changes at

about pH = 9-10. Approx same as pK a value

What is the

half-neutralisation point?

When volume = half the volume that has been

added at the equivalence point

Define a buffer solution

A solution that resist changes in pH when small

amount of acid/alkali are added.

What do acidic buffer

solutions contain in general

terms?

A weak acid and a soluble salt of that acid that

fully dissociates

Write a reaction for an

acidic buffer with added acid

A - + H + → HA, opposes addition of H +

Write a reaction for an

acidic buffer with added

alkali.

HA + OH - → H 2

O + A -

How else can you achieve

an acidic buffer solution

other than just mixing the

constituents?

Neutralise half of a weak acid (meaning the acid

must be in excess) with an alkali - this forms a

weak acid / soluble salt mixture.

What do basic buffer

solutions contain in general

terms?

Weak base and soluble salt of that weak base

How can you calculate the

pH of buffer solutions?

Use the K a of the weak acid, sub in [A - and [HA],

calculate [H + ] → pH

How can you calculate the

new pH of a buffer solution

when acid or base is

added?

Calculate number of moles of H + and A - and HA

before acid or base is added. Use equations to

work out new moles of A - and HA → find [H + ] →

pH

Which buffer system

maintains blood pH at 7.4?

What happens when

acid/alkali is added?

H + + HCO 3

-- ⇌ CO 2 + H 2

O

Add OH - → reacts with H + to form H 2

O, then

shifts equilibrium left to restore H + lost

Add H + → equilibrium shifts to the right, removing

excess H +

What products are buffers

found in?

Shampoos, detergents → important to keep pH

right to avoid damage to skin, hair, fabrics