Friday, 1 October 2010

Acid and Bases

Acid and bases

Daily used acids
  1. Hydrochloric acid (HCl): digesting, cleansing agent
  2. Sulphuric acid (H2SO4): car batteries fertilizers (NH4)SO4, detergents and paints.
  3. Nitric acid (HNO3): etch copper, fertilizers
  4. Ethanoic acid (CH3COOH): vinegar and storage of pickled food
  5. Carbonic acid (H2CO3): soft drinks
  6. Lactic acid: makes milk sour; citric acid found in fruits
  7. Formic acid (HCOOH) in ants and tartaric acid (C4H6O4) in grapes
Properties of (dilute) acids:
  1. Sour and conducts electricity (H2 produced)
  2. Turns blue litmus paper red
  3. Dilute acid + metal H2 + salt (Note CH3COOH→CH3COOH-+H+) Also note that the metals that are low in the reactivity series won’t have reaction such as Cu. (colour gass bubbles of H2 evolved, heat releasing)
  4. Very dilute nitric acids reacts to give H2, dilute acid gives NO while concentrated nitric acid gives NO2.
  5. Dilute acid + CO32-/HCO3- salt + CO2 + H2O
  6. Neutralization: metal oxide/hydroxide salt + H2O. (OH- gives more water.) Note that neutralization is very heat releasing.
Acid is the substances that produces H+ as the only cation when dissolved. Basicity is the max # of H+ produced by one molecule of an acid. Note that all H+ of the mineral acid can be ionized. For organic acid, ethanoic acid is monobasic, phosphorus acid (H3PO3) is dibasic and phosphoric acid (H3PO4) is tribasic.

Water must be present for acid to perform the typical acidic properties since water must be present for acid to give H+ which is responsible for the typical acidic properties.

HCl dissolves in water but not in the organic solvent such as dry methylbenzene so that HCl in water behaves as an acid. In methylbenzene HCl doesn’t ionize so that it can’t conduct electricity too.

Note that we add conc. acid into water instead of adding water into conc. acid. Note that heat is released during ionization.

The fizzy drink/tablet/power contains solid acid and NaHCO3 where H+ is released when dissolved in water. Then it reacts with hydrogencarbonate and gives carbon dioxide since H++HCO3-H2O+CO2, and effervescence occurred. Baking powders also contain NaHCO3 or a mixture of that with acid. NaHCO3Na2CO3+H2O+CO2.

Concentrated acid gives acid mist and a strong smell when we open the bottle. Concentrated nitric acid must be kept in a brown bottles since it decomposes under light: 4HNO32H2O+O2+4NO2 (brown fumes).

(dilute/conc.) HNO3 are strong oxidizing agent since NO3- under the presence of H+ behave as an electron acceptor. HNO3 attacks most metal (except Au,Pt).
  1. Reduction to NO2 (reddish brown fumes): NO3-+2H++e-NO2+H2O
  2. Reaction with Mg/Cu to gives NO2: X+4HNO3X(NO3)2+2NO2+2H2O. Note that the oxidation number is reduced from +5 to +4 so that it is an oxidizing agent. Also note that the conc. nitric acid has no reaction with Al or Fe, maybe due to the protective oxide layer.
  3. Dilute /moderately conc. HNO3 is reduced as NO: NO3-+4H++3e-NO+2H2O.
  4. Reaction with Mg/Cu to gives NO: 3X+8HNO33X(NO3)2+2NO+4H2O. Colourless NO is further react with O2 to become NO2. Therefore reddish brown fumes Is found at the mouth of the tube.
  5. Cu does not react with non-oxidizing acids but can be oxidized to copper(II) ions.
  1. Substances that reacts with acids (H+) to form salt and water only. (Neutralization)
  2. Metal oxides/hydroxides are bases. Carbonates are not since they gives CO2 too.
  3. Bases can be regarded as chemical opposites of acids.
  4. Bases that soluble in water is called alkalis, like NaOH, KOH, Ca(OH)2 and NH3(aq).
  5. Alkali is the compound that gives OH- as the only anions when dissolved in water.
Note that for the first three compounds OH- already exist in the solid alkalis but for NH3 there’s a change in bonding: NH3+H2O↔NH4++OH-.

Alkalis taste bitter and slippery for dilute alkalis. Conc. alkalis are corrosive and able to burn skins so that they are also called “caustic” alkalis. It turns red litmus paper blue and are electrolytes since there exists mobile ions in solution form.

Ammonium compound + alkali salt + water + ammonia: it gives pungent choking smell (from NH3) and turns wet red litmus paper blue.

Metal hydroxides are insoluble in water except NaOH and KOH. When they are added to other metal’s ionic compound, they will form participate in the solution (metal hydroxides):
  1. Cu2+ + 2OH- → Cu(OH)2 pale blue ppt.
  2. Fe2+ + OH- → Fe(OH)2 dirty green ppt.
  3. Fe3+ + OH- → Fe(OH)3 reddish brown ppt.
  4. Mg2+/Al2+/Zn2+/Pb2+: white ppt.
When excess OH- or NH3 is added, some of them form complex salts and dissolves in water:
  1. Al(OH)3 + OH- → [Al(OH)4]- (colourless aluminate ion)
  2. Pb(OH)2 + 2OH- → [Pb(OH)4]2- (colourless plumbate ion)
  3. Zn(OH)2 + 2OH- → [Zn(OH)4]2- (colourless zincate ion)
  4. Zn(OH)2 + 4NH3 → [Zn(NH3)4]2+ + 2OH- (colourless tetraamminezinc ion)
  5. Cu(OH)2 + 4NH3 → [Cu(NH3)4]2+ + 2OH- (deep blue tetraamminecopper(II) ion)
Conc. alkalis are corrosive and attacks skin readily. It separates oily masses and water: turning chicken foot from pale pink into translucent.

Drying agent: hygroscopic substances absorb water to become jelly-like substances while deliquescent substances absorbs more water to become solution.

Solution as a drying agent: H2SO4, insert the air tube into the bottle containing drying agent solution (the tube is inserted into the solution) and the dried air is collected at another side.

Solid drying agent: (neutral): silica gel, CaCl2 and Na2SO4, (alkali) CaO

The molarity of a solution is the # of moles of solute dissolved in 1 dm3 of the solution. Mathematically the molarity of a solution (mol dm-3, or M)= # of moles of solute (mol) / volume of solution (dm3).

The pH of a solution = -log[H+] where [H+] is the concentration of H+ in that solution. (Note that it is base 10). Inversely [H+]=10-pH.

A solution is a acidic if its pH<7 and is alkali if pH>7. It’s neutral when it’s pH=7. The pH of H2O is equal to 7 (ONLY at 25 degree celcius).

In alkali solution, litmus solution gives blue colour, methyl orange change from orange to yellow and phenolphthalein changes from colourless to red.

In acidic solution litmus solution and methyl orange gives red colour and phenolphthalein is still colourless.

Universal indicator is a mixture of several indicator and gives different colour to different pH. pH paper can also be used. pH meter, pH sensor can be also used.

Strong acids/alkalis almost completely ionize to give H+ or OH- while weak acids/alkalis only partially ionizes.. Also the ionization of weak acids/alkalis reacts in both directions.

Stronger acids/alkalis conducts electricity better, and react with metals faster.

The rate of reaction decreases as [H+] decreases. However, when there’s a dilute strong acid and conc. weak acid with the same pH value , when diluted/react with Mg, the pH of conc. weak acid raises slower since when the hydrogen ions are used up, some of the un-ionized acid will be ionized to gives more hydrogen ion. We can say # of moles of acid molecules in weak acid># of moles of acid molecules in strong acids.

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