acids and bases Flashcards
what is the bronsted-lowery definition for an acid
an acid is a substance which can behave as a proton donor - H+
hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion
HCl (aq) → H+ (aq) + Cl- (aq)
what is the bronsted-lowery definition for a base
a base is a substance which can behave as a proton acceptor - has a lone pair
hydroxide (OH-) ion is a Brønsted base as it can accept a proton to form water
OH- (aq) + H+ (aq) → H2O (l)
what are conjugate acid-base pairs
most acid-base reactions are reversible
the acid which gives up a proton can accept a proton and thus behave as a base
the species formed when a base accepts a proton can give up a proton and behave ad an acid
how are acids related to bases in an equation
acid = proton + conjugate base
how are bases related to acids in eq
bases + proton = conjugate acid
examples of conjugate pairs
Identify the acid-base conjugate pairs in the following reactions:
HCO3- (aq) + H2O (l) ⇌CO32- (aq) + H3O+ (aq)
HCO3- (aq) + H3O+(aq) ⇌ CO2 (g) + H2O (l) + H2O (l)
H2SO4 (aq) + HNO3 (aq) ⇌ HSO4- (aq) + NO2+ (aq) + H2O (l)
HSO4- (aq) + OH- (aq) ⇌ SO42- (aq) + H2O (l)
Answers
The pairs in the order acid/base are:
HCO3- and CO32- ; H3O+ and H2O
H3O+ and H2O ; (CO2 + H2O) and HCO3-
H2SO4 and HSO4- ; (NO2+ + H2O) and HNO3
HSO4- and SO42- ; H2O and OH-
give an example of acid-base eq
For example, ethanoic acid (CH3COOH) is a weak acid that partially dissociates in solution
When equilibrium is established there are CH3COOH, H2O, CH3COO- and H3O+ ions present in the solution
The species that can donate a proton are acids and the species that can accept a proton are bases
CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)
acid base conjugate base conjugate acid The reactant CH3COOH is linked to the product CH3COO- by the transfer of a proton from the acid (CH3COOH) to the base (CH3COO-) Similarly, the H2O molecule is linked to H3O+ ion by the transfer of a proton These pairs are therefore called conjugate acid-base pairs
what is a conjugate pair
two species that are different from each other by a H+ ion
how to work out which is which in the equation
the B-L base is the reactant that has gained a hydrogen on the product side - thus the conjugate acid - now has a positive charge
the B-L acid is the one that loses the hydrogen on the product side - thus the conjugate base
what does the strength of an acid refer to
pH not conc
what is a strong acid
low pH 1-3
high conc of H+ ions - which fully dissociate into ions
what is a weak acid
high pH 4-7
low conc of H+ ions - does not fully dissociate
what is the definition of pH
pH = -log[H+]
where [H+] is the concentration of hydrogen ions in mol dm–3
how do you find the conc of H+ ions
concentration of H+ of a solution can be calculated if the pH is known by rearranging the above equation to:
[H+] = 10-pH
what does log 10 mean
pH scale is a logarithmic scale with base 10
This means that each value is 10 times the value below it. For example, pH 5 is 10 times more acidic than pH 6.
to how many values is pH usually read
2.d.p
worked example of a pH calc
Question 1: Find the pH when the hydrogen concentration is 1.60 x 10-4 mol dm-3
Answer 1:
The pH of the solution is:
pH = -log[H+]
= -log 1.6 x 10-4 = 3.80
Question 2: Find the hydrogen concentration when the pH is 3.10
Answer 2:
The hydrogen concentration can be calculated by rearranging the equation for pH
pH = -log[H+]
[H+] = 10-pH
= 10-3.10 = 7.94 x 10-4 mol dm-3
how are strong acids ionised in solution
Strong acids are completely ionised in solution
HA (aq) → H+ (aq) + A- (aq)
Therefore, the concentration of hydrogen ions, H+, is equal to the concentration of acid, HA
The number of hydrogen ions formed from the ionisation of water is very small relative to the [H+] due to ionisation of the strong acid and can therefore be neglected
The total [H+] is therefore the same as the [HA]
what are dibasic/diprotic acids
two replaceable protons and will react in a 1:2 ratio with bases
give an example of a dibasic acid
Sulfuric acid is an example
H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (l)
You might think that being a strong acid it is fully ionised so the concentration of the hydrogen is double the concentration of the acid
This would mean that 0.1 mol dm-3 would be 0.2 mol dm-3 in [H+] and have a pH of 0.69
However, measurements of the pH of 0.1 mol dm-3 sulfuric acid show that it is actually about pH 0.98, which indicates it is not fully ionised
The ionisation of sulfuric acid occurs in two steps
H2SO4 → HSO4- + H+
HSO4- ⇌ SO42- + H+
Although the first step is thought to be fully ionised, the second step is suppressed by the abundance of hydrogen ions from the first step creating an equilibrium
The result is that the hydrogen ion concentration is less than double the acid concentration
how to find log on calculator
Make sure you know how to use the antilog (base 10) feature on your calculator. On most calculators it is the 10x button, but on other models it could be LOG-1, ALOG or even a two-button sequence such as INV + LOG
how can water act as a base and acid
Water molecules can function as both acids and bases. One water molecule (acting as a base) can accept a hydrogen ion from a second one (acting as an acid).
how is equilibrium set up in water
However, the hydroxonium ion is a very strong acid, and the hydroxide ion is a very strong base. As fast as they are formed, they react to produce water again.
The net effect is that an equilibrium is set up.
2H2O = H3O+ + OH-
At any one time, there are incredibly small numbers of hydroxonium ions and hydroxide ions present. Further down this page, we shall calculate the concentration of hydroxonium ions present in pure water. It turns out to be 1.00 x 10-7 mol dm-3 at room temperature.
You may well find this equilibrium written in a simplified form:
H2O = H+ + OH-
This is OK provided you remember that H+(aq) actually refers to a hydroxonium ion.
what is Kw
Kw is essentially just an equilibrium constant for the reactions shown.
it can be written as
Kw = [H+][OH-]
what is the value of kw at 298k
1.0x10-14 mol2dm-6
what is the ph of water at 298k
7
That means that you can replace the [OH-] term in the Kw expression by another [H+].
[H+]2 = 1.00 x 10-14
Taking the square root of each side gives:
[H+] = 1.00 x 10-7 mol dm-3
Converting that into pH:
pH = - log10 [H+]
pH = 7
That’s where the familiar value of 7 comes from.
how does kw change with temp
it varies
how does ph of pure water change in eq
The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process. Using the simpler version of the equilibrium:
The forward reaction absorbs heat.
According to Le Chatelier’s Principle, if you make a change to the conditions of a reaction in dynamic equilibrium, the position of equilibrium moves to counter the change you have made.
According to Le Chatelier, if you increase the temperature of the water, the equilibrium will move to lower the temperature again. It will do that by absorbing the extra heat.
That means that the forward reaction will be favoured, and more hydrogen ions and hydroxide ions will be formed. The effect of that is to increase the value of Kw as temperature increases.
- Kw increases so H+ also increase when temp increases so shift right opppose change in conc / increase in temp
how to work out the ph or pOH of mixed solution
calculate the moles of H+
calculate moles of OH-
calculate moles excess of H+ and OH-
calculate excess [H+] or [OH-] depending on which is larger
calculate ph = -log10 (H+)
or 14 - (-log10(h+))