Chemistry Acids and Bases An indicator tells you if a substance is acidic or basic because the indicator is one colour in an acid and another in a base. Indicator Litmus Phenopthalein Methyl Orange Screened Methyl Orange Bromothymol Blue Acid Red Colourless Red Red Yellow Base Blue Pink Yellow Green Blue The pH Scale The strength of an acid or base is indicated by its pH. The pH tells you the amount of H + ion (acid) or OH- ions (base) in solution. The pH scale is a number line which runs from 0 to 14. Therefore, the strongest acid has a pH- 0. The strongest base has a pH- 14. And, neutral is pH- 7. Acids An acid is defined as a substance which produces “free hydrogen ions” H+ when dissolved in water. Example: HCl(g) ---------w-a-t-e-r-------- > H+(aq) + Cl-(aq) Some common acids: -Hydrochloric acid: HCl -Nitric acid: HNO3 -Sulphuric acid: H2SO4 -Ethanoic acid: CH3COOH The strength of an acid: -Strong acids are completely ionized when dissolved in water. Hence, many H+ ions are available for reaction. HCl(aq) ----------------- > H+(aq) + Cl-(aq) (Reaction goes to completion). List of Strong Acids: Strong Acids Symbol Hydrochloric Hydrobromic Hydroiodic Nitric Sulfuric Chloric Perchloric HCl HBr HI HNO3 H2SO4 HClO3 HClO4 -Weak acids are partially ionized. Few H+ ions are not available for reaction. CH3COOH(aq) CH3COO-(aq) + H+(aq) (Reaction is reversible). The Basicity (protocity)- the number of H+ ions liberated per molecule of acid dissolved. Example: Monobasic Acids- 1 H+ ion HCl, HNO3, CH3COOH, etc. Dibasic Acids- 2 H+ ions H2SO4 Tribasic Acids- 3 H+ ions H3PO4 Some important acid reactions - Acid + Base ----- > Salt + Water - Acid + Metal ----- > Salt + Hydrogen - Acid + Carbonate ------ > Salt + Water + Carbon Dioxide - Acid + Hydrogencarbonate -------- > Salt + Water + Carbon Dioxide Salt formed as product Salts are substances formed when the hydrogen of an acid is replaced by a metal ion. When naming a salt, the metal (ion) in the reaction forms the first part of the name, while that of the acid forms the rest of the name. Name of Acid Hydrochloric acid Sulphuric acid Nitric acid Ethanoic acid Name in SALT Formed Chloride Sulphate Nitrate Ethanoate Examples: 1) Zinc + Hydrochloric acid ----- > Zinc chloride + Hydrogen Zn(s) + 2HCl(aq) ------ > ZnCl2(aq) + H2(g) 2) Calcium carbonate + Nitric Acid ------- > Calcium nitrate + Carbon dioxide + Water CaCO3(s) + 2HNO3(aq) ------- > Ca(NO3)2(aq) + CO2(g) + H 2O Bases Bases are substances which react with H+ ions to produce salts and water only. Bases (OH-) include metal oxides, hydroxides and ammonia gas. Some common bases: -Sodium hydroxide: NaOH -Calcium hydroxide: Ca(OH)2 -Potassium hydroxide: KOH -Aqueous ammonia- NH4OH The strength of a base: -Strong bases are completely ionized when dissolved in water. Hence, many OH- ions are available for reaction. NaOH(aq) ----------------- > Na+(aq) + OH-(aq) (Reaction goes to completion). List of Strong Bases: Strong Bases Symbol Lithium Hydroxide Sodium Hydroxide Potassium Hydroxide Calcium Hydroxide Barium Hydroxide Rybidium Hydroxide Caesium Hydroxide Strontium Hydroxide LiOH NaOH KOH Ca(OH)2 Ba(OH)2 RbOH CsOH Sr(OH)2 -Weak bases are partially ionized. Few OH- ions are not available for reaction. NH3(g) + H2O(l) NH+(aq) + 0H-(aq) (Reaction is reversible). Some important base reactions -Acid + Base ------------- > Salt + Water (Example shown above for acid reaction with base). -Alkali + Ammonium compound --------------- > Salt + Water + Ammonia NaOH(aq) + NH4Cl(aq) ------------------ > NaCl(aq) + H2O(l) + NH3(g) Determining Neutralisation Point When an acid or base reacts, the point at which the acid/base is just used up has a pH of 7. This is the neutralization point of the reaction. Neutralisation point can be determined in 2 ways: -Thermometrically: by determining the highest temperature of the reaction. -Titration: several drops of indicator are placed in a known volume of the alkali. The acid is run carefully from a burette until the exact point when the indicator changes colour. This is the neutralization point of the reaction. Acid/ Base Properties of Oxides Oxides contain oxygen and another element. Oxides show acid/base properties. Acid Oxides: The oxides of most non-metals when dissolved in water form acidic solutions. These oxides are also referred to as acid anhydrides. SO2(g) + H2O(l) ------------ > H2SO3(aq) SO3(g) + H2O(l) ------------ > H2SO4(aq) CO2(g) + H2O(l) ------------ > H2CO3(aq) Neutral Oxides: The few non-metals produce oxides that are not acidic or basic are CO, NO and N2O. Basic Oxides: The oxides of the most metals are basic. They react with acids to give a salt and water only. MgO(s) + 2HNO3(aq) ------------- > Mg(NO3)2(aq) + H2O Soluable basic oxides react with water to produce alkalis such as Na2O, K2O and CaO. K2O(s) + H2O(l) --------------- > 2KOH(aq) Amphoteric Oxides: A few metals react with both acids and bases. The most common ones are PbO, Al 2O3 and ZnO. PbO(s) + 2HNO3(aq) --------------- > 2KOH(aq) ZnO(s) + 2NaOH(aq) --------------- > ZnCl2(aq) + H2O(l)