REACTIONS OF ACIDS. J:\Science\Chemistry\Stage 1 Notes\Acids & Bases\Reactionsofacids.doc

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1 REACTIONS OF ACIDS 1. Acids taste sour We do not attempt to taste strong acids as they are too dangerous. They do taste sour, but then they proceed to destroy cells on your tongue and mouth. If you vomit, the burning feeling in your throat and mouth comes from the hydrochloric acid in your stomach s digestive juice. Weak acids in foods give them their sour taste. e.g. citric acid lemons, oranges, grapefruit. tartaric acid grapes. oxalic acid rhubarb, sour sobs. 2. Acids react with Active Metals Acids will react with most metals, except Au, Ag, Cd, Cu and Pb very slow. Active Metal + Acid Hydrogen Gas + Ionic Salt Zn (s) + 2HCl (aq) H 2(g) + ZnCl 2(aq) In the above reaction, the ionic salt formed is zinc chloride. The first part of the ionic salt comes from the metal (i.e. zinc) and the second part comes from the acid (i.e. chloride). 3. Acids react with Carbonates & Bicarbonates Carbonate + Acid Carbon Dioxide + Water + Salt CaCO 3(s) + 2HNO 3(aq) CO 2(g) + H 2 O (l) + Ca(NO 3 ) 2(aq) NaHCO 3(s) + HCl (aq) CO 2(g) + H 2 O (l) + NaCl (aq) sodium bicarbonate (bicarb. soda)

2 4. Acids react with Metal Oxides Metal Oxide + Acid Water + Ionic Salt Fe 2 O 3(s or aq) + 3H 2 SO 4(aq) 3H 2 O (l) + Fe 2 (SO 4 ) 3(aq) iron (iii) sulfate 5. Acids react with Bases (Metal Hydroxides) Common bases: NaOH sodium hydroxide (caustic soda) KOH potassium hydroxide (potash) NH 3 ammonia NH 4 OH ammonium hydroxide (ammonia sol n) Ca(OH) 2 calcium hydroxide (limewater) CaO calcium oxide (lime) Base + Acid Water + Ionic Salt NaOH (aq) + HNO 3(aq) H 2 O (l) + NaNO 3(aq) 6. Acids react with Indicators An indicator is a chemical compound that changes colour depending on the nature (i.e. acidic, basic or neutral) of the chemical it is combined with. There are many different indicators that exist, but the following four are the most common ones used in the laboratory (and the ones that you need to know the various colours of!). Indicator Acid (H + ) Neutral Base (OH - ) Phenolphthalein Colourless Colourless Crimson Methyl Orange Red Orange Yellow Methyl Red Pink Red Yellow Litmus Red Purple Blue(ish)

3 7. The ph Scale The ph scale describes the strength of an acidic or basic solution. Neutral Weak 7 Weak Acid 6 8 Base Strong 3 11 Strong Acid 2 12 Base The further away from neutral (ph = 7) a substance is, the more dangerous it is. The ph scale is a logarithmic scale, so each number is actually a power of 10 different to the previous number. For example, an acid of ph 3 is 10 stronger than an acid of ph 4, and 100 stronger than an acid of ph 5, etc. The ph of a solution can be calculated using the following formula: ph = -log 10 [H 3 O + ] where [H 3 O + ] is the concentration of the hydronium ions present in the solution. You do not need to be able to use this formula until next year. In the laboratory, we use ph paper, universal indicator or electronic meters to measure the ph of a solution.

4 8. Amphiprotic Substances An amphiprotic substance is a substance that can act as an acid or a base, depending on what you react it with. They are usually the hydrogen salts of diprotic or triprotic acids. For example, HS -, HCO 3 -, HSO 3 -, HSO 4 -, H 2 PO 4 -, HPO 4 2-, H 2 O. With a strong base they act as acids: HS - + OH - S 2- + H 2 O Acid Base With a strong acid they act as bases: HS - + HCl H 2 S + Cl - Base Acid Water is also amphiprotic - it can donate or accept protons, but it is a poor example to give equations for. 9. Anion & Cation Hydrolysis Anions are negatively charged ions. When they react with water, they act as bases, thus forming hydroxide ions (OH - ) in solution (i.e. ph > 7). CH 3 COO - + H 2 O CH 3 COOH + OH - 2- CO 3 + H 2 O - HCO 3 + OH - Base Acid ph > 7 Cations are positively charged ions. When they react with water, they act as acids, thus forming hydronium ions (H 3 O + ) in solution (i.e. ph < 7). NH H 2 O NH 3 + H 3 O + Acid Base ph < 7

5 Some of the polycharged metal ions actually have water molecules firmly attached to the vacant spots in their outer shell of electrons. For example: Al(H 2 O) H 2 O Al(H 2 O) 5 OH 2+ + H 3 O + Acid Base ph < 7 The Al has 6 H 2 O molecules attached to it. One of the protons (H + ) jumps off one of the water molecules attached to the Al onto a free water molecule. The same thing happens with Fe(H 2 O) 6 3+ and Pb(H 2 O) FORMULAE YOU MUST KNOW Acids: Sulfuric acid H 2 SO 4 Sulfurous acid H 2 SO 3 Hydrochloric acid HCl Nitric acid HNO 3 Carbonic acid H 2 CO 3 Phosphoric acid H 3 PO 4 Acetic acid CH 3 COOH Bases: Those listed in Acids react with Bases. Citric Acid Formic Acid