Acids, Bases and buffer 5.1.3

What are acids

Proton donors

What are bases

Proton acceptors

what are monobasic acid and example

Release 1 proton

HNO₃

What are dibasic acids and example 

Release 2 protons 

H₂SO₄

What are tribasic acids and example

Release 3 protons

H₃PO₄

what is a strong acid

Proton donor that fully dissociates

What is a Weak acid

A proton donor that partially dissociates

Strong base definition

A proton acceptor that fully dissociate ( High OH⁻ conc)

Weak base definition 

A proton acceptor that Partially dissociate ( low OH⁻ conc) 

equation to calculate pH

pH = -log [H⁺]

Equation to calculate concentration of [H⁺]

[H⁺] = 10^-ph

How to work out [H⁺] of di basic

X by the amount of protons released

X by 2

what does a larger Ka value mean

Stronger acid and Greater Dissociation

what are the Ka equations

Weak acids → Ka = ( [H⁺] ² / [HA] )

Buffers → Ka = ( [H⁺] [A⁻] / [HA] )

How to work out pKa

pKa = -log (Ka)

How to work out Ka from pka

Ka = 10^-pka

pKa and ka relationship

Lower value of pKa → High value of Ka → more dissociation, stronger acid

General Dissociation of a acid

HA ⇌ H⁺ + A⁻

How to work out pH for a weak acid / monobasic

What is the assumption when calculating weak acid

[ H⁺] = [ A⁻] → [ H⁺]²

what are conjugated acid and base pairs ( Image)

Anything that loses H+ = Acid

anything that gains H+ = Base

what are conjugated acid and base pairs

Two species that are different from each other by a Proton

Units of Ka

mol dm⁻³

What reaction happens in all aqueous solutions contain

H₂O (l) ⇌ OH⁻(aq) + H⁺(aq)

What factor affects Kw

Temperature only

equation to find pH of pure water

Kw = [ H⁺]²

what is the Kw equation

Kw = [ H⁺]² → ionic water ( aq)

Kw = [ OH^-][ H⁺] → Bases

Why is Kw constant

Water dissociates in small amounts

Water concentration is so large effectively constant

What is Kw value at 25⁰C

1.00 × 10¹⁴ mol²dm⁻⁶

What does changing temperature do to kw value

Increases → Kw value increases

decreases → Kw value decreases

What type of reaction is H₂O dissociation

Endothermic

effect of increasing and decreasing temp on dissociation of water

Increase temp → equilibria shifts to right ( products) more dissociation

Decrease temp → equilibria shifts to left ( Reactants)

How to work out concentration of Strong bases

work out [ H⁺] = 10^-pH

Kw = [ H⁺][ OH⁻]

[ OH⁻] = Kw / [ H⁺]

equation to use for bases

Kw = [ H⁺][ OH⁻]

Buffer definition

Maintains pH when small amounts of acid / alkali added

Function of acidic and Basic buffers

Acidic → Maintain pH below 7

Basic → Maintain pH above 7

What are the two ways to make acidic buffer

weak acid + its salt

Partially Neutralise weak acid

Acidic buffer ( Image)

Acidic buffers why is the Ion from the salt

Salt fully dissociates

Larger mols of Ion

How to make a Basic buffer

Weak base + its salt

Acidic buffer adding strong base to weak acid (Image)

Partially Neutralising weak acid products

Salt + H₂O

Effects of increasing [ H⁺] on Buffer

increase [ H⁺] → Equilibria moves to the left

[ H⁺] reacts with [ A⁻]

Effects of increasing [ OH⁻] on Buffer

increase [ OH⁻] → Equilibria moves to Right ( products)

Reacts with Acid

Example of Buffers

Blood

What is the pH blood is maintained at

pH 7.35 - 7.45

Equation for blood buffer

H₂CO₃(aq) ⇌ HCO₃⁻ (aq)+ H⁺(aq)

Effects of increasing [ H⁺] on Blood Buffer

Equilibria shifts to the Left → more H₂CO₃ made

prevents blood pH from decreasing

Effects of decreasing [ H⁺] on Blood Buffer

More H₂CO₃ made → dissociates into H⁺

equilibria shifts to right

stops pH from rising ( too alkaline)

ka equation for weak monobasic acids ( only)

General buffer equation

How to work out pH from buffer solution

work out mols of reactants

limiting mol = [ A⁻] mol

ICE table → limiting reactants gets used up

[ HA] mol = initial - change

use mol value in [ H⁺] = (Ka [ HA] / [ A⁻] )

-log [ H⁺] = pH

how to work out concentration in dilution

( Mol / new volume (dm³)

What are indicators made from

Weak inorganic acids

How to calibrate a pH probe

place bulb of pH meter into distilled water

reading should be 7

How to improve pH experiment

Rinse pH probe in distilled water after each reading

is there a suitable indicator for weak acid/ weak base titrations and why?

No suitable indicator

there’s no rapid pH change

Example of weak alkali

NH₃ ( aq)

Measuring pH experiment how to know what was in the flask and burette on graph

If pH starts above 7 → alkali in flask / acid in burette

if pH starts below 7 → acid in flask/ alkali in burette

What is the equivalence point ?

shows when the substances are neutralised

How to know choose a suitable indicator

endpoint is close to equivalence point and is within the rapid pH change

Strong acid - strong Base pH curve graph( Image)

Strong acid - Weak base pH curve graph ( Image)

pH isn’t 13 / 14 then its a weak base

Why can pH at the equivalence point be below 7 in a equation with Ammonium salt as product ( SA-WB)

NH₄X ⇌ NH₄⁺ + X⁻

NH₄⁺ ⇌ NH₃ + H⁺ ( in a solution)

H⁺ decreases pH

How to work out volume of base needed for neutralisation on pH curve graph

use the rapid pH change line

Weak acid - strong base pH curve ( Image)

Why can pH at the equivalence point be above 7 in a equation with a Salt as product ( WA-SB)

Salt dissociate to form

HA ⇌ A⁻ + H⁺ ( in water)

A⁻ accepts H⁺ so acts as base

Weak acid - Weak base pH curve graph (Image)

Weak acid- Weak base pH curve why is point of equivalence pH 7

Salt dissociates into weak acid and weak base

Which constant to use acids and bases/ water

bases/ water → KW

Acids → Ka

acid dissociation constant equation

General equation for all weak acids

HA ⇌ H⁺ + A⁻

How to prove that a Acid is weak using a pH meter

if SA → full dissociates so [ HA ] = [ H⁺]

pH = - Log [ H⁺] ( stated)

if pH is greater then its a weak acid

How to do write a conjugate acid-base pairs equation between a SA and WA

SA acts as a acid and donates proton

WA acts as base and accepts proton ( cation)