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Theories of acids and bases
Brønsted-Lowry Acid-Base theory
This theory gives a new definition for acids and bases. The fundamental idea is that when acids and bases react the acid forms its conjugate base and the base forms its conjugate acid via the exchange of protons.
Brønsted–Lowry acid: A proton donor
Brønsted–Lowry base: A proton acceptor
Deduce the acid and the base in a reaction
- All acids contain hydrogen
- All bases have a lone pair of electrons to bond to hydrogen
Acid+base examples
These are not given in the data booklet so they will need to be memorized.
Name of acid |
Formula |
Hydrochloric acid |
HCl |
Nitric acid |
HNO3 |
Sulfuric acid |
H2SO4 |
Ethanoic acid |
CH3COOH |
Carbonic acid |
H2CO3 |
Phosphoric acid |
H3PO4 |
Benzoic acid |
C6H5COOH |
Conjugate acid-base pairs
These are pairs of acid and bases which differ by one hydrogen ion (proton).
E.g the conjugate base of is NH3
Amphiprotic and amphoteric species
Amphiprotic species: is a compound which is able to gain a hydrogen ion to form a conjugate acid or lose a hydrogen ion to form a conjugate base. Thus these species act as both a Brønsted-Lowry acid and base.
Amphoteric species: Can act as either a base or an acid. All amphiprotic species are also amphoteric.
Common amphiprotic species
Deduce if a species is amphiprotic
Very often in multiple choice you will be asked to identify the amphiprotic species from a list of compounds. To identify it check each option one by one to see if it fits both the acid and the base criteria. If it does, then congratulations you've identified the amphiprotic species.
Amphiprotic must fit the following properties:
- Contains hydrogen
- Has a lone pair of electrons to accept a hydrogen ion.
Properties of acids and bases
Acid reactions
Acids undergo reaction with different elements such as metals, metal hydroxides and metal/hydrogen carbonates. Each of these have different products. These need to be memorized.
Type of reaction |
Reaction word equation |
Pseudo-equation |
Example |
Acid-metal |
Acid + metal = Salt + hydrogen |
(a)H + (b) = (a)(b) + H2 |
|
Acid-metal hydroxide |
Acid + metal hydroxide = Salt + water |
(a)H + (b)OH = (a)(b) + H2O |
|
Acid-metal/hydrogen carbonate |
Acid + metal/hydrogen carbonate = Salt + water + carbon dioxide |
(a)H + (b)CO3 = (a)(b) + H2O + CO2 |
|
Acid-Base neutralization reactions
In neutralization reactions an acid and a base react together to form a salt and water.
- This neutralization always occurs when the pH of the solution is 7.
- The solution forms a salt, dependent on the acid and base.
- The reaction is always exothermic and thus the enthalpy change (ΔH) is negative
Ionic formula and spectator ions
The reason it neutralizes is due to the double-displacement reaction which occurs. We can show this by writing the ionic formula
For example:
Determining formula of salt produced
- Remove a hydrogen from the acid
- Remove the hydroxide/ammonia part of the base
- Combine together and balance
Non-hydroxide neutralization examples
The base can be a hydroxide but it is also good to know what happens with ammonia or an oxide.
Acid base titrations
An acid-base titration is used to find the concentration of an unknown acid/base, if the concentraton of one of them is known.
Procedure
The procedure is to add the acid/base solution with the known concentration (titrant) to the solution with the unknown concentration (Anylate) slowly until the reaction reaches neutralization. This point can be determined using an indicator which changes colour dependent on the pH of the solution. You record the volume of titrant required to neutralize the anylate then calculate the moles and determine the concentration of the anylate.
Titrant: The solution with the known concentration in buret, can be an acid or a base
Analyte: The solution with the unknown concentration in conical flask, can be an acid or a base.
Buret: a graduated glass tube with a tap at one end, for delivering known volumes of a liquid in titrations.
Titration curve
Equivalence point: is the point in a titration where the amount of titrant is enough to completely neutralize the analyte. Thus at this point the moles of titrant is equal to the moles of the solution with unknown concentration. The straight vertical line in the titration curve.
Indicators
A good indicator is bright, colorful and has a distinct change depending on whether acidic or basic. Indicators must also have their changing point within the range of the neutralization. These are themselves either weak acids or weak bases which react in a reversible reaction which depends on the acidity of the surroundings.
Phenolphthalein: A ph indicator which is colourless in the presence of an acid and pink in the presence of a base. It helps determine the point of neutralisation
Buffer region
The region where the acids and bases don't have the same concentration, so there are still products or reactants to accept or donate hydrogen ions
Thermometric titrations
Heat is released in the neutralisation reaction. However, once the reaction has been neutralized it will stop releasing heat. This means you can determine the equivalence point without an indicator.
Enthalpy of neutralisation: Energy released when 1 mol of water is produced through the neutralisation of an acid and a base.
The pH scale
What is pH?
pH is a measure of the concentration of H+ ions within a solution. The lower the pH, the lower the concentration; the higher the pH, the greater the concentration. The idea of pH is to map the wide range of concentrations onto a value ranging from 0 to 14. This is done through the use of a logarithm. The formula for pH is given below
Concentration of hydrogen ions = [H+]
Concentration of hydroxide ions = [OH-]
Example hydrogen ion conc. to pH question
Using pH to distinguish between acid, basic and neutral
the pH of a solution tells us whether that solution is acid, basic or neutral.
- pH<7 → acidic
- pH = 0 → neutral
- pH>7 → basic
Comparing pH
As the pH scale is logarithmic a one unit change in pH represents a 10-fold change in [H+]
So going from pH 4 to pH 0, is a 10,000 fold increase in [H+] !
The ionic product of water \(K_w\)
This is a value which allows us to determine the relationship between [H+] and [OH-] for aqueous solutions.
When an acid and a base reacts, the hydrogen and hydroxides disassociate and then combine together in a double displacement reaction. We can split this up into two partial reactions, one with the salt and one with water. The water one looks like this.
The equilibrium value is given by
As the reaction lies so far to the left the water concentration has a constant value and we can ignore it. This gives us an expression for what's called the ionic product constant of water
\(K_w\)= \( [H^+][OH^-] \)
The value of \(K_w\) depends on the temperature, however at 298K this value is known to be \(1.0 \times 10^{-14}\)
This means if we know the concentration of either hydrogen or hydroxides we can calculate the concentration of the unknown one.
Strong and weak acids and bases
Definition of strong and weak
Strong acid/base: Ions fully dissociate. pH either very low or very high
Weak acid/base: Ions partially dissociate, in a reversible reaction. pH on either side of 7, but not too extreme.
Some must-know strong and weak
Strong acids: HCl, H2SO4, HNO3
Strong bases: Alkali hydroxides (LiOH, NaOH, KOH, RbOH, CsOH), Ba(OH)2
Weak acids: CH3COOH, H2SO3, HNO2, H3PO4
Weak bases: Mg(OH)2, NH3
Properties of strong vs weak
|
Strong |
Weak |
pH |
At ends of spectrum |
Towards middle of spectrum |
Electrical conductivity |
High |
Low |
Rate reaction |
High |
Low |
Buffers
A solution which resists pH changes upon the addition of small amounts of strong acid or base. It can either accept hydrogen ions or donate them depending on the change in acidity.
Acid deposition
Any precipitation of acid rain
Why is rain acidic
Acid rain contains dissolved CO2 which lowers its pH to around 5.6.
CO2 (g) + H2O (l) ⇌ H2CO3 (aq)
Forms hydrogen carbonate. This process above is NATURAL, this is NOT considered "acid rain".
Acid rain comes from oxides of 2 main elements: Sulfur and Nitrogen.
Sulfuric and sulfurous acid
Sulfur can be turned into gaseous oxides from burning of fossil fuels. The sulfur present in amino acids (ex. cysteine) from decayed dinosaurs and plants is combusted. This then reacts with water to form sulfuric acid (strong acid) or sulfurous acid (weak acid).
S (s) + 3O2 (g) → 2SO3 (g)
SO3 (g) + H2O (l) → H2SO4 (aq)
H2SO4 (aq) → H+(aq) + HSO4-(aq)
S (s) + O2 (g) → SO2 (g)
SO2 (g) + H2O (l) → H2SO3 (aq)
H2SO3 (aq) ⇌ H+(aq) + HSO2-(aq)
Nitric and nitrous acid
Nitrogen can be turned into gaseous oxides in the heat of combustion engines. Usually nitrogen gas and oxygen gas will not react, but at high temperatures (ex. in an engine), they have enough energy to overcome the Activation Energy. This then reacts with water to form nitric acid (strong) and nitrous acid (weak).
Forming nitric acid and nitrous acid:
N2 (g) + 2O2 (g) → 2NO2 (g)
2NO2 (g) + H2O (l) → HNO3 (aq) + HNO2 (aq)
HNO3 (aq) → H+(aq) + NO3-(aq)
HNO2 (aq) ⇌ H+(aq) + NO2-(aq)
Ways to stop/prevent acid rain
- Wash coal to remove sulphur
- Catalytic converter on cars, prevent release of sulphur dioxide.
- Alternative cleaner fuels
- Instal "scrubbers" in power plant to clean emissions of sulphur dioxide before it leaves the plant
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