Chemistry : Acid and Bases (late)

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 + CO₃^2-/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. NaHCO₃→Na₂CO₃+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: 4HNO₃→2H₂O+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+4HNO₃→X(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+8HNO₃→3X(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.

Bases:

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⁺ + 2OH^- → Fe(OH)₂ dirty green ppt.

3)       Fe³⁺ + 3OH^- → 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.

Neutralization: reaction between acid and base where salt and water are the only product. Water comes from combining hydrogen and hydroxide ions: H⁺(aq)+OH^-(aq)→H₂O(l).

1)       Acid and alkali, e.g. HCl + KOH → KCl + H₂O

2)       Acid and base (insoluble metal hydroxide), e.g., 3HNO₃ + Fe(OH)₃ → Fe(NO₃)₃ + 3H₂O

3)       Acid and base (insoluble metal oxide), e.g., H₂SO₄ + CuO → CuSO₄ + H₂O

4)       Exceptions: NH₃ + HCl → NH₄Cl (ammonia dissociate as NH₃ + H₂O → NH₄⁺ + OH^-)

Acid salts: salts containing H⁺, like NaHSO₄, they can still react with base (dissociate to give H⁺). They can be named by hydrogen(anion). e.g., NaHSO₄ is sodium hydrogensulphate.

Soluble and insoluble salts

1)       Nitrates are all soluble, e.g., Pb(NO₃)₂.

2)       Halides are soluble except silver/lead(II) halides (AgX or PbX₂)

3)       Sulphates are soluble except CaSO₄, PbSO₄, BaSO₄ (Barium sulphate)

4)       Hydrogensulphates are all soluble, like NaHSO₄.

5)       Carbonates are all insoluble except group I-carbonate (Li₂/Na₂/K₂CO₃) and ammonium carbonate (NH₄)CO₃.

Preparing soluble salt

1)       Acid on metal/insoluble base/insoluble carbonate, e.g., CuO + H₂SO₄ → CuSO₄ + H₂O

Since H₂, CO₂ and H₂O are the only side products, the gas escapes while H₂O can be evaporated, we obtain the salt by the following step:

-          Put excess metal/base/carbonate in dilute acid.

-          Filter the remained metal/base/carbonate.

-          Apply steam bath to crystallize the salt, cool down and filter the salt.

-          Wash and dry the salt.

2)       Neutralization by titration

-          Put a known volume of dilute NaOH in the beaker, by titration determine the amount of acid needed to neutralize the alkalis with the help of indicators.

-          Without indicators, neutralize the alkalis exactly so that the solution is neutral. Apply similar method to obtain the soluble salt.

Preparing insoluble salt by precipitation

We mix two soluble solution containing the cations and anions we want. When we mix them together, they form ppt which is the salt we want. We filter the salt, wash and dry to obtain the salt.

For example, mixing Pb(NO₃)₂ and HCl to obtain insoluble PbCl₂.

Use of neutralization

-          Treatment of industrial waste

-          Producing fertilizers (NH₄NO₃ which is nitrogen-rich)

-          Weak bases to treat indigestion

-          Scrubber to neutralize SO₄ by CaO.