Acids and Bases Flashcards
An acid
A substance which dissolves in water to release aqueous protons , H+ (aq)
- Acidic solutions have a PH between 1-6
Examples:
Hydrochloric acid HCl
Nitric acid HNO3
Sulfuric acid H2SO4
pH
Potenz hydrogen ~a measure of how many aqueous protons are present in the solution.
Strong acids
- Fully dissociate in solution
- high concentration of protons
Examples:
Hydrochloric acid ~ HCl
Nitric acid ~ HNO3
Sulfuric acid ~ H2SO4
Phosphoric acid ~ H3PO4
Weak acids
- Partially dissociate in solution
- LOW concentration of protons
- The forward reaction is incomplete and the weak acid quickly accepts the proton back after releasing it.
Examples:
Ethanoic acid ~ CH3COOH
Methanoic acid ~HCOOH
Citric acid ~ C6H8O7
Making an acid
A polar covalent molecule is placed in water and ionises to release an aqueous proton.
Hydrochloric acid:
HCl (g) + aq ——- H+(aq) + Cl- (aq)
Nitric acid:
HNO3(l) + aq ——— H+(aq) + NO3-(aq)
Sulfuric acid:
H2SO4(l) + aq ——- 2H+ (aq) + SO4 2- (aq)
Ethanoic acid :
CH3COOH (l) + aq ——– CH3COO- (aq) + H+ (aq)
A base
- a proton ,H+ , acceptor
- neutralise acids
- metal oxides
- metal hydroxides
- metal carbonates
- alkalis
Alkalis
A base which dissolves in water to form aqueous hydroxide ion , OH- (aq).
- sodium hydroxide NaOH
-Potassium hydroxide KOH - Calcium hydroxide Ca(OH)2
- Ammonia NH3
Ammonia
- when ammonia gas is bubbled into water , a SMALL AMOUNT reacts with water to form AQ HYDROXIDE IONS.
- the reaction is REVERSIBLE
- The ammonia ACCEPTS A PROTON in the forward reaction so its behaving as a base.
- The concentration of hydroxide ions is LOW so the solution is WEAKLY ALKALINE.
NH3 + H2O ——— NH4+ +OH-
Corrosive
Alkalis are MORE corrosive than acids.
Alkalis ~ corrosive at 0.5 mol dm -3
Acids ~ corrosive at a concentration greater than 6.5 mol dm -3
Neutralisation reactions
An ACID reacts with an ALKALI to produce WATER.
- An acid is an aqueous proton
- An alkali is an aqueous hydroxide
Ionic equation :
H+ (aq) + OH- (aq) ——– H2O (l)
A salt
A salt is formed when the H+ ion in an acid is replaced by a METAL ION or NH4+.
Acid reactions
Metal + acid —— salt + hydrogen
Metal oxide + acid —— salt + water
Metal hydroxide + acid —– salt + water
Metal carbonate + acid —– salt + water + carbon dioxide
Metal hydrogen carbonate + acid —- salt + water + carbon dioxide
Ammonia + acid —— ammonium salt
Anhydrous
A salt which does not contain any water molecules.
Has NO water of crystallisation.
Hydrated
A crystalline salt compound which contains water.
has water of crystallisation
Water of crystallisation
This is the water needed for the crystals to form.
Shown by a DOT in the formula.
Working out the formula of a HYDRATED SALT
Calculate MASS of WATER in the hydrated salt:
Mass of hydrated salt - Mass of anhydrous salt.
Calculate MOLES of ANHYDROUS salt:
n =m/M
Calculate MOLES of water:
n=m/M
Calculate the MOLAR RATIO of the anhydrous salt to water:
Divide by the smallest number of moles to find formula.
Water of crystallisation METHOD
- Take an empty CRUCIBLE and weigh it using a BALANCE.
- Add several SPATULAS of your HYDRATED salt into the crucible and weigh again.
- Calculate the MASS of hydrated salt added by subtracting the two values given by the balance.
- HEAT the crucible using a BUNSEN BURNER , driving of the water of crystallisation as STEAM.
5.Weigh the crucible every couple of minutes until the mass STOPS DECREASING.
At this point, we have driven off all of the water and are left with the ANHYDROUS salt which is WHITE.
6.Calculate the MASS of the anhydrous salt by subtracting the FINAL MASS from the mass of the EMPTY CRUCIBLE.
Empirical formula to dot formula
Divide the number of hydrogen atoms by 2.
Example:
MgCl2H10O5 (empirical formula)
MgCl2 . 5H2O (Dot formula)
Ways to IMPROVE the method for water of crystallisation
- Use a balance to a higher resolution
- INCREASE the mass of the hydrated salt to reduce % uncertainty.
- Use a lid to avoid spitting
- Controlled heating ~ use an oven set below the decomposition temperature to prevent the decomposition of the hydrated salt.
Diprotic acid
An acid which has TWO H+ ions that can be replaced by a different positive ion.
