Acids and Bases

Acid

substances which can act as a proton (hydrogen ion) donor and are monoprotic or polyprotic depending on the number of protons available for donation

Electrolytes

form ions in solution

strong electrolytes

Fully ionise or dissociate (single arrow

• strong acids fully ionise (very little acid remains in solution)

• soluble ionic compounds e.g. strong bases fully dissociate

weak electrolytes

partially ionise (double arrows)

• weak acids

• weak bases

Bronsted Lowry theory of Acids and bases

acids are proton donors- usually resulting in H3O+ water

• HCL + H2O → H3O+ +Cl

bases are proton acceptors - usually resulting in OH- water

• NH3 + H2O →← OH- + NH4+

conjugate pairs

the result of swapping protons, in the case of reversible reactions a product which has accepted a proton can re-donate it and a product which has given a proton can re-accept it.

• the weaker the acid the stronger it’s conjugate base

• the weaker the base the stronger the conjugate acid

Amphoretic substances

can act as an acid or a base e.g. water

successive ionisation

the degree of ionisation of an acid decrease and the strength of the conjugate base increases

Autoionisation of water

since water is an amphoteric substance it is able to ionise itself (autoionization)

• 2H2O →← H3O(aq) + OH-(aq)

• H2O (l) → ← H+ + OH-

Kw

Kw = (H3O+) (OH-) = 1.0 × 10^-14

because water is a very weak electrolyte the equilibrium constant is very low

• as temperature increases, the Kw value increases and vice versa

Strong acids

• good conductors of electricity

• HCL, H2SO4 and HNO3

weak acids

• not good conductors of electricity

• CH3COOH

• H2CO3

• H2SO3

• H3PO4

• Oxalic acid: HO2C2O2H

Acid Strength

determined by the concentration of H+ they produce in solution

Base strength

determined by the concentration of OH- they produce in solution

Strong bases

• any soluble hydroxide

weak base

Ammonia: NH3

Hydrolysis/hydrolysis equations

• when salts dissolve in water they may react with water to produce H+ or OH- theis is caled hydrolysis

• a chemical breakdown of a compound due to a reaction with water

• Hydrolysis equations show what happens when an acid or weak base dissolves in water

• acidic species will donate a proton to water

• base will accept a proton from water to form OH

Ka values

the equilibrium constant for ionisation of an acid, shows the extent to which the reaction occurs

• a Large Ka value ( larger than 1) indicates that the acid will fully ionise in solution, strong acid

• weak acids have low Ka value

Ka values of polyprotic acids

• polyprotic acids have the potential to undergo multiple hydrolysis reactions, however the tendency decreases with each successive ionisation (Ka value decreases)

• successive ionisations will not happen to completion, therefore the expected mols of H3O+ will not be produced.

Salts

• produced when a acid and a bases are reacted together

neutral salts

• salts which have been produced by a strong acid and a strong base, therefore neither component of the salt will undergo hydrolysis with water

Basic salt

• produced when a weak acid and a strong base are reacted together

• basic salt completely dissociate in water

Acidic salts-

• strong acids and weak bases react

Arrhenius theory of acids and bases

• an acid will ionise to form hydrogen ions in solution, and will increase the conc of H+

• A base will form hydroxide in solution; it will increase the concentration of OH- in solution

Why do hydrogen ions form hydronium ions in solution

Protons are not stable by themselves in water

limitations of Arrhenius

• limited to interactions in aqueous solution

• e.g. Hcl, HNO3, and H2SO4 are not acids under arrhenius definition as they do not dissolve in water

• it doesn’t explain why some salts are acidic and basic

Davy Theory

acids contain hydrogen ions which could be replaced metals and bases are substances which react with acids to form salts

polyprotic acids

• acids that can donate two or more protons

• become less acidic as they lose protons/with every successive ionisation the degree of ionisation of the acid decreases

why are only some substances that contain hydrogen acidic under normal conditions

• hydrogen atoms will only be acidic if they already have a slight positive charge, generally only happens if they are bonded to a highly electronegative atom such as oxygen or chlorine.

what does an acid and a base produce

salt + water

what are the formulas involved in pH calcullations

Kw=(H+)(OH)=1.0 x 10^-14 for any aqueous solution at 25C

pOH= -log(OH)

pH= -log(H+)

pH= 14 - pOH

(H+)= 10^-pH