5.1.3 Acids and Bases (completed) Flashcards
What is a weak acid?
It partially dissociates in an aqueous solution
What is a base?
A base is a hydrogen ion or proton acceptor
What is an alkali?
A base that dissolves in water
What is the equation for the neutralisation of acids with metal oxides/hydroxides?
Metal oxide/hydroxide + Acid –> Salt + Water
What is the equation for the neutralisation of acids with alkalis?
Acid + Alkali –> Salt + Water
What is the equation for the neutralisation of acids with carbonates?
Metal Carbonate + Acid –> Salt + Water + Carbon Dioxide
What is a titration?
A technique used to accurately measure the volume of one solution that reacts exactly with another solution in order to find it’s concentration
What are concordant results?
Results within 0.10 of each other
How do you make a standard solution?
- Weigh the solid using a balance
- Transfer solid from weighing boat to beaker wash any left behind with distilled water
- Dissolve solid fully with distilled water
- Transfer solution to volumetric flask then rinse beaker and funnel into flask to ensure all solid is transferred
- Fill with distilled water to graduation mark
- Add a stopper and invert to mix
Which base treats acid indigestion?
Magnesium hydroxide
What is a buffer solution?
A solution that minimises pH changes when small amounts of an acid or a base are added.
What two components does a buffer solution contain?
A weak acid HA and its conjugate base A-
In a buffer solution, what occurs to increase pH when acid is added?
[H+] increases
H+ ions added react with conjugate base A- of buffer removing most H+ ions
Equilibrium shifts to the left
In a buffer solution, what occurs to decrease pH when alkali is added?
[OH-] increases
H+ ions react with OH-
Equilibirum shifts to the right so more HA dissociates to restore H+ ions
How can the pH of a buffer solution be calculated?
Ka=[H+][A-] / [HA]
pH = -log[H+]
What is the dissociation constant?
Ka- It represents the strength of an acid and can sometimes be shown as a pKa value
What is the pH of blood between?
7.35 and 7.45
What is the equilibrium for the buffer system in blood?
H2CO3(aq) ⇌ H+(aq) + HCO3-(aq)
When acid is added to blood, what happens?
[H+] increases
H+ ions react with HCO3- produing carbonic acid
Equilibrium position shifts to the left removing most of the H+ ions
Excess H2CO3 is converted to CO2 which is removed in the blood
When alkali is added to blood, what happens?
[OH-] increases
H+ ions react with OH- ions to produce water
Equilibrium position shifts to the right
H2CO3 dissociates to restore H+ ions
What are the significant points on a pH titration curve?
At first, excess of acid, pH increases slowly
Vertical section, pH increases rapidly
Equivalance point, centre of the vertical section, each are neutralised
At the end, excess of base, pH increases slowly
What is the equivalence point?
The volume of one solution that exactly neutralises the other solution
What is an an acid base indicator?
A weak acid that has a distinctive colour change from it’s conjugate base
When is an indicator suitable for a titration?
The pH indicator range is in the vertical section of the pH titration curve
What is a Bronsted-Lowry acid?
A proton donor
What are conjugate systems?
When the acid and base are related
Acid ⇌ Proton + Conjugate Base
Base + Proton ⇌ Conjugate Acid
What is a monobasic acid?
It has one H+ which it can replace and donate e.g. HCl
Dibasic has two e.g. H2SO4
How do you work out pH of strong acids?
pH = -log [H+]
As they are strong the dissociate into a 1:1 ratio so it’s just the concentration of the acid is the concentration of H+
How do you work out pH of weak acids?
[H+] = √[HA]Ka
pH = -log[H+]
You can work this out as yje ions of [H+] and [A-] are formed in equal amounts so they can be written as [H+]²
What is the ionic product of water?
Kw = 1.0 x 10^-14 mol²dm^-6
How do you work out the pH of a strong base?
Kw = [H+][OH-] pH = -log[H+]
What is the endpoint?
The point when the titration changes colour
How do you use a pH meter?
- Before use rinse probe with distilled water
- Place probe in buffer of pH 4
- Calibrate probe and then place in buffer of pH7 and then pH 10 to complete calibration
What is enthalpy of neutralisation?
The energy change when a sufficient amount of acid or alkali is neutralised to produce one mole of water
-57kJ mol-1
Use q=mcΔT
How do you convert from g/dm³ to mol/dm³
Divide by Mr
What could be added to a weak acid to turn it into a buffer?
e.g. HCOOH
HCOONa / NaOH in order to produce HCOO- which is the conjugate base
What factors determine pH of buffer solution?
- Ka of acid
- Temperature of solution
- Ratio/concentrations of weak acid and conjugate base
How do you calculate the percentage dissociation of an acid?
[H+]
—— x 100
[HA]
If the volume of a buffer solution is greater than expected how does the pH change?
Stays the same
The ratio of [HA] : [A] is the same
