C1 Acid-base chemistry 1 Flashcards
physicochemical modes of action for dyspepsia
antacids
raft-forming agents
anti-foaming agents
pharmacological modes of action for dyspepsia
H2 receptor antagonists
proton pump inhibitors
main chemical bases of treatment
- reduced acidity in the stomach (physicochemical and pharmacological modes of action)
- prevention of acid reflux (physicochemical mode of action)
simply describe the mode of action of antacids
consume acid by use of a base
describe magnesium hydroxide as an antacid giving its balanced equation with hydrochloric acid
Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
- consume stomach acid
- products formed are not acidic or basic
- neutralisation
Arrhenius definition of an acid
a substance that increases the concentration of H+ when dissolved in water
arrhenius definition of a base
a substance that increases the concentration of OH- when dissolved in water
Bronsted-Lowry definition of an acid
a proton donor
Bronsted-Lowry definition of an base
a proton acceptor
describe Bronsted-Lowry definitions in general
- more general
- should always use these definitions in aqueous solutions
what occurs in an acid-base reaction?
a proton is transferred from acid to base
how do protons exist in water?
- not independently
- attach strongly to water to form hydronium ions
describe and explain the deionisation of hydrochloric acid in water. give the chemical equation
- essentially 100% of the HCl dissolved in water ionises to the hydronium and chloride ions (one-headed arrow)
- I.e. HCl transfers all of its protons and the reaction goes to completion
- HCl is a strong acid
describe and explain the deionisation of acetic acid in water. give the chemical equation
- only a proportion of its protons are transferred to water
- I.e. the reaction is considered an equilibrium (equilibrium arrow required)
- acetic acid is a weak acid
describe strong acids
- fully ionise in aqueous solutions
- transfer all their protons to water in aqueous solutions
- reactions go to completion
describe weak acids
- partially ionise in aqueous solutions
- don’t transfer all their protons to water in aqueous solutions
- reactions are equilibria
what do equilibrium constant values (Ka) provide?
quantitative indication of whether an acid is strong or weak
define conjugate acid
the acid (proton donor) on the product side of the reaction
define conjugate base
the base (proton acceptor) on the product side of the reaction
what do conjugate acid / base pairs differ by?
just a proton
what are all of these?
conjugate acid / base pairs
does water behave as an acid or a base?
- both
- a small amount of water molecules are ionised in water
- one water molecule can act as an acid if another acts as a base (this produces one hydroxide ion and one hydronium ion)
if 2 water molecules become ionised to produce a hydroxide ion and a hydronium ion, which is the conjugate acid and which is the conjugate base?
- hydronium ion is the conjugate acid
- hydroxide ion is the conjugate base
equation for K (equilibrium constant) of this general acid dissociation equation
what can the general acid dissociation equation for K (shown in the image) be simplified to and what does this turn the equation into?
- since [H2O] can be considered constant in aqueous solutions, it can be taken out
- this makes K = Ka (acid dissociation constant)
what does the magnitude of Ka indicate?
a quantitative measure of acid strength
compare HCl (Ka = 10^7) and ethanoic acid (Ka = 1.76 x 10^-5) in terms of their Ka and equilibriums
HCl
- equilibrium lies far to the right
- A- is favoured over HA
- strong acid
CH3COOH
- equilibrium lies to the left
- HA is favoured over A-
- weak acid
why is pKa often used over Ka?
- pKa helps to avoid very small and very large numbers being used
- gets rid of lots of decimal places
describe the Ka and pKa values of strong acids
large Ka
small pKa
describe the Ka and pKa values of weak acids
small Ka
large pKa
general formula for K for base dissociation and Kb
how is the strength of a base described?
- in terms of the pKa of its conjugate acid
describe Ka and pKa values of strong bases
- small Ka
- large pKa
- the larger the pKa of the conjugate acid of the base, the stronger the base
- BH+ favoured over B
describe Ka and pKa values of weak bases
- large Ka
- small pKa
- the smaller the pKa of the conjugate acid of the base, the weaker the base
- B favoured over BH+
what is Kw? describe how this is derived from Ka and Kb formulae
- ionisation constant for water
- Ka x Kb
what does the equation of Kw confirm about the strength of bases and conjugate acids?
- concentration of hydronium and hydroxide ions multiplied is the ionisation constant for water (Kw)
- this is seen after cancelling out common components in fractions
- this confirms that a STRONG BASE WILL HAVE A WEAK CONJUGATE ACID
what is the Kw of pure water at 25 degrees C?
10^-14
what is pKw?
14
pH of pure water at 25 degrees C
7
when does the pH of neutrality vary?
with temperature
how does the degree of ionisation of water change with temperature?
- degree of ionisation of water increases with temperature
- hence, the pH of neutrality decreases
pH of neutrality at 37 degrees C
6.8
