chapter 20- acids, bases and pH

what is a bronsted-lowry acid

a proton donor

what is a bronsted-lowry base

a proton acceptor

what is a conjugate acid-base pair

contains two species that can be interconverted by transfer of a proton

what is a hydronium ion

H₃O+

monobasic acid

gives off one proton per molecule

dibasic acid

gives off two proton per molecule

tribasic acid

gives off three proton per molecule

monobasic acid example

hydrochloric acid

dibasic acid example

sulfuric acid

tribasic acid example

phosphoric acid 

what are carbonates

bases that neutralise acids to form a salt, water, carbon dioxide

acid + base

salt and water

acid + alkali

salt + water

what type of pH has a low H+ value

alkali high value

what type of pH has a high H+ value

acidic low value

what is the equation for finding pH from H+ conc

pH= -log[H+ (aq)]

what is the equation for finding the H+ conc from pH

10^-pH

what type of scale is the pH scale

a logarithmic scale 

• a change in one pH = 10x difference in H+ conc

what is a strong acid

one which completely dissociates in aqueous solutions

what is a weak acid

one which partially dissociates in aqueous solutions

what can affect Ka

temperature

for a strong acid Ka is

high

for a weak acid Ka is

low

what are the units for Ka

moldm-3

what is pKa

the negtaive logarithim of Ka

how to find pKa from Ka

-logKa

how to find Ka from pKa

10^-pKa

for strong acid pKa is

low

for weak acids pKa is

high

how to calculate pH change in a solution

find the total end volume

total end volume/ start volume = amount of dilution

concentration/ amount of dilution

what is Kw

the ionic product of water

what is the equation for Kw

Kw= [H+][OH-]

what is the equilbrium constant for Kw at 25 degrees

1×10^-14

what is the equation without assumptions for the Ka of a weak acid

Ka= [H+][A-]/[HA]

what are the two assumptions you can make when finding the Ka of a weak base

• that [H+] is approximately equal to [A-]

• that [HA] at equilbrium is roughly equal to [HA] initially 

what is the equation for Ka of weak acids

Ka= [H+}² / [HA]

pH of a strong base

Kw= [H+][OH-]

what is a buffer solution

is one where the pH does not change significantly if small amounts of acid or alkali are added to it.

how to make an acidic buffer

• weak acid

• salt of the weak acid

• eg. ethanoic acid and sodium ethnaote

how to make a basic buffer

• weak base

• salt of the weak base

• eg ammonia and ammonia chloride

how do buffers work when acid is added

• shifts left to remove H+ ions added

how do buffers work when alkali added

• the OH- react with the H+ to form water

• equilibrium shifts right to produce more H+

how do you find the pH of a buffer solution

[H+]= Ka [HA]/[A-]

what is the buffer in blood

H₂CO₃ / HCO₃^-

what happens to the buffer in blood when alkali is added

• reacts with the H+ to make water 

• so blood equilibrium shifts right to form more H+ amd HCO₃^-

what happens to the buffer in blood when acid is added

• blood equilbrium shifts left

method for construction of a pH curve

• Transfer 25cm3 of acid to a conical flask with a volumetric pipette

• Measure initial pH of the acid with a pH meter

• Add alkali in small amounts (2cm3 ) noting the volume added

• Stir mixture to equalise the pH

• Measure and record the pH to 1 d.p.

• Repeat steps 3-5 but when approaching endpoint add in smaller volumes of alkali

• Add until alkali in excess

where is the equivalence point 

lies at the mid point of the extrapolated vertical portion of the curve.

phenolpthalein colour change

acid colourless

base pink

methyl orange

acid red

alkali yellow

assumptions for the Ka equation

[H+]=[A-]

[H+] from water is negligible

why is water neutral at all temps

in pure water [H+] always equals [OH-]