Reactivity 3.1 Flashcards
Svante Arrheniu’s theory of acids
acidic substances dissociated in aqueous solutions to produce hydrogen ions
Basic substances dissociated in aqueous solutions to produce hydroxide ions
Bronsted-Lowry acid
hydrogen ion (proton) donor
Bronsted-Lowry base
hydrogen ion (proton) acceptor
What is a hydrogen ion?
Hydrogen atom that has lots its electron
Why is the hydrogen ion basically a proton?
Very small and large charge density
What happens when Bronsted-Lowry acid molecules mix with water?
They ionise
HX -> H+ + X-
What is the hydrogen ion attracted to?
Water molecules, forming the hydronium ion
What are the chemical properties of acids in aqueous solution due to?
Presence of hydronium ions
Conjugate-acid base pairs
Species that differ by one proton (acid written first)
Amphiprotic substances
substances that are capable of both accepting and donating a proton depending on reaction conditions
When does water act as a base
when reacting with a stronger acid
When does water act as an acid
when reacting with a stronger base
Amphoteric substances
Used to describe substances that can react as acid or basics
Example of amphoteric substances
ZnO
ZnO + 2H+ -> Zn2+ + H2O
Zn + H2O + OH- -> Zn(OH)4)2-
pH scale
measured scale that extends from 0-14 that show relative acidity, using a logarithmic scale
what is the midpoint of the pH scale
at 25 degrees, 7
Formula for pH
-log[H3O+]
Formula for [H3O+] from pH
[H3O+] = 10-pH
For strong monoprotic acids [HA] =
[H3O+]
Significant figures in the [H3O+] become
DP in pH
Self-ionisation of water
As it is an amphiprotic susbtance, it can act as an acid or a base depending on the cirumstances. Therefore, it can react with itself
H2O + H2O -> H3O+ + OH-
As water is a poor conductor of electricity, we can assert that there are very few of these ions formed
Ion product constant (Kw)
[H3O+] x [OH-] = 10^-14 mol2 dm-6 at 25 degrees
What is the ion product constant true for?
All aqueous solutions
Strong acids
Virtually completely ionise in aqueous solution
Weak acids
Partially ionise in solution(<100%)
Strong acids example
HCl
HBr
HI
HNO3
H2SO4
Weak acids
Acetic/ethanoic acid: CH3COOH
H2CO3
H3PO4
What is used to express the ionisation of weak acids?
A double arrow, as they exist in equilibrium. The resulting solution is a mixture of both reactants and products, and the concentration of hydronium ions is equal to the concentration of the conjugate bate (ignoring self-ionisation of water)
Strong base
readily accepts a hydrogen ion
Strong base examples
OH-
O2-
LiOH
NaOH
KOH
weak bases
partially ionise in water to produce hydroxide ionswea
weak base examples
ammonia: NH3
ethylamine: CH3CH2NH2
carbonate ion: CO32-
C2H5NH2
The strength of a conjugate base is inversely..
proportionate to that of its acid
e.g. HCl (strong base) has a very weak conjugate base (Cl-)
Strong acids (100% deionised in aqueous solution) have a…
weak base (negligible tendency to be protonated in aqueous solution)
Weak acids (exist in a solution in a mixture of HA, A- and H3O+) make
Weak bases (moderate tendency to be protonated in aqueous solution
Very weak acids (neglible tendency to dissociate) make
strong bases (100% protonated in aqueous solution)
Concentrated
Large amount of solue dissolved in a solvent
Dilute
Small amount of solute dissolved in a solvent
Distinguishing between strong and weak acids/bases relies on the fact that:
Strong acids and bases have a greater extent of ionisation in solution, so their resulting solution with have a greater concentration of ions.
This property can be used to distinguish between them, if solutions of the same concentration are compared at the same temperature
Tests that can be done to compare strong/weak acids/bases
Electrical conductivity: depends on concentration of ions. Stronger acids/bases have a higher conductivity than weak acids/bases
Rate of reaction: reactions of acids with bases depend on the conentration of H+ ions. A faster rate of reaction will occur with a higher concentration, so strong acids will have a faster reaction
pH: measure of H3O+ concentration. A strong acid will have a lower pH than a weak acid of the same molar concentration
Monoprotic
Have one ionisable hydrogen atom per molecule
Diprotic
Have two ionisable hydrogen atoms per molecules
Triprotic
Has two ionisable hydrogen atoms per molecules; thus, ionises and reacts with water in three stages
What happens when an acid is added to a metal (hydr)oxide base
the properties of both the acid and the base are “lost”
i.e. the acid is neutralised by the base
Parent acids and bases
The original acids and bases that a salt (produced in a neutralisation) reaction came from
How to identify parent acids and bases
Split salt into cation and anion. Add hydroxide ions to the cation (making parent base) and protons to the anion (parent acid)
How can the unknown concentration of an acid/base be determined?
By an acid/base titration using a standard solution
Equivalence point
Point at which the acid and base are in equal proportions and have neutralised each other i.e. neither reactant is in excess / reactants are present according to mole ratio
Describe pH titration curve of a strong acid being added to a strong base
- Starts at high pH (i.e. 13)
As more volume of acid is added, the pH slowly descreases.
At point of inflection, the pH plummets, passing the equivalence point
Will eventually reach a pH of ~1
End point
Moment of colour change (where both colours of indicator are present)
Burette
Used to deliver volumes accurately
Titre
volume delivered from the burette
volumetric pipette
used for accurate transfer of fixed volumes
Aliquot
Known volume delivered from the pipette
Titration
Process of mixing the reactants in a volumetric analysis until they are present in the exact mole ratio represented by the balanced equation. This is a procedure for accurately determining the concentration of an unknown solution
Standard solution
Solution of accurately known concentration
Traits of primary standard
Readily available in pure form
Have a known formula
Be easy to store without deteriorating or reacting with the atmosphere
E.g. Sodium carbonate
What should a volumetric flask be rinsed with, just prior to use?
Deionised wter
How is the primary standard transferred to the volumetric flask?
using a funnel
what error will occur if not all the primary standard is transferred into the volumetric flask?
Concentration of standard solution will be lower than the calculated concentration, as less mass (moles) would have been transferred than calculated
What volume would’ve been dispensed if the burette is emptied completely?
an unknown volume, greater than 50,00cm3
Uncertainty for burette reading
0.05H
how many decimal places must be recorded for a burette reading?
2
Formula for titre volume
V(titre) = V(final) - V(initial)
What should burette be rinsed with just prior to us?
The solution with which it is to be filled
how many dp are pipettes accurate to?
2
what are pipettes rinsed with just prior to use?
the solution with which they are to be filled
Why is the hand holding the pipette close to the end?
support pipette and reduce chance of it breaking
Where to never pipette from?
volumetric flask
why is the standard solution poured into a beaker and the aliquot taken from the beaker?
to avoid contamination of the stock of standard solution
what should the beaker be rinsed with jut prior to pipetting?
the standard solution
what error would occur if the beaker was rinsed with water just prior to pipetting?
the solution in pipette would be diluted -> less titre volume added
Where does the aliquot get transferred to?
Conical flask
What quantities are known about the solution in the conical flask?
concentration and volume
What should conical flask be rinsed with just prior to use?
deionised water
where is indicator placed in titration?
conical flask
when do you stop adding solution from the burette?
when the indicator changes colour
why does the indicator change colour?>
addition of acid from burette changes pH of solution; and colour of indicator depends on pH
What can the tip of the burette and the sides of the conical flask be rinsed down with?
Water, as it won’t affect the moles
Concordant meaning
Within uncertainty
Note about end point v.s. equivalence point
as the indicator is an acid/base itself, a slight excess of the solution needs to be added past the equivalence points to get to the end point. Hence, the end point will always be an approximation of equivalence point
Titration question to find concentration of the aliquot
Balanced chemical equation
Averafge titre volume
Average amount of moles used (in titre)
Moles of aliquot solution used to neutralise titre (according to balanced chemical equation)
Concentration of aliquot, using c = n/V
How to make a primary solution?
Weigh solids + weight bottle
Add weighed solid crystals to the flask through a funnel
Reweigh the empty weighing bottle + lid (i.e. weigh by difference)
Add deionised water until the flask is 1/3 full. Swirl the flask to dissolve the solute. Add water until 1cm below the etched mark.
Add hte final amount of water using a dropping pipette, until the bottom of the meniscus sits at the edged line. Add lid and invert flask to ensure even distribution of solute
Alkalis
bases that provide OH- as the only anion inn aqueous soluton
alkalis examples
NaOH
KOH
Strong alkalis
Alkalis that are soluble and give rise to a high concentration of OH- ions.
Weak alkalis
Less soluble alkalis that only give rise to a few hydroxide ions in solution
Do all alkalis contain hydroxide ions?
No, but they all give rise to hydroxide ions in aqueous solutions
e.g. NH3 + H20 -> NH4+ + OH- (therefore alkali)
