Acids and Alkalis Year 11

CONTENTS Acidity and alkalinity Indicators pH Acids General methods for making salts Making salts from metal oxides Making salts from metal carbonates Making salts from metals Making salts from alkalis – by titration Making insoluble salts – by precipitation Questions

Acids Sour Turn litmus red pH less than 7 Dissolve carbonate rocks Corrode metals Conduct electricity

Acids Acids are substances that give H + ions when added to water. Hydrogen ion is equal to a proton. So acid is a proton donor

Common Acids are: Hydrochloric acid HCl Sulphuric acid H 2 SO 4 Nitric acid HNO 3 Phosphoric acid H 3 PO 4 Ethanoic acid CH 3 COOH Carbonic acid H 2 CO 3

Bases A substance which neutralises an acid is called a Base If a base dissolves in water, it is also called an alkali So alkalis are bases that dissolve in water. Sodium hydroxide NaOH base and alkali Potassium hydroxide KOH base and alkali Ammonia solution NH4OH base and alkali Copper(II) oxide CuO is a base Magensium oxide MgO is a base Metal oxides and hydroxides are generally bases Ammonium hydroxide is a base (alkali)

Alkalis Bitter Turn litmus blue pH greater than 7 Slippery caustic

Alkalis An alkali is a substance that gives OH - ions when added to water All alkalis dissolve in water.

Neutralisation reactions Acids and alkalis neutralise each other to form a salt and water. Example: Hydrochloric acid + Sodium hydroxide  Sodium chloride + Water HCl + NaOH  NaCl + H 2 O Reactions of acids and alkalis are called neutralisation reactions Neutralisation reaction is expressed by: H+ + OH-  H2O

Neutralization examples Indigestion is caused by too much amounts of hydrochloric acid in the stomach. It is cured by indigestion tablets. Indigestion tablets contain magnesium oxide that neutralises the excess HCl.

Neutralization examples Fields with acidic soil can be improved (by neutralisation) by adding lime. The lime contains calcium hydroxide Ca(OH) 2 which is an alkali

Neutralization examples Lakes affected by acid rain can be neutralized by adding lime.

Strong vs. weak Strong = complete dissociation Example, when HCl is dissolved in water, all of it becomes H + + Cl - HCl, H2SO4, HNO3 are strong acids HCl  H+ + Cl- H2SO4  2H+ + SO4 2- Weak = incomplete dissociation Acetic acid (vinegar) also called Ethanoic acid is a weak acid. CH3COOH CH3COO- + H+ CH 3 COOH, small amounts dissociate into H + and CH 3 COO - CH3-COOH, H2CO3, HF are weak acids (Ethanoic acid, Carbonic acid, Hydroiodic acid More dissociation  stronger acid

Strong and weak bases Strong bases ionises completely NaOH  Na+ + OH- NaOH, KOH, Ba(OH) 2, Ca(OH) 2 Weak bases ionises partially Mg(OH) 2, CuO, Al 2 O 3, NH 4 OH NH4OH NH4+ + OH- Strong acids have a small pH(1-2) strong bases have high pH(13-14) Neutral solutions have pH 7 Smaller the pH, stronger the acid Larger the pH, stronger the base

Indicators Indicators are substances that change colour when placed in an acid or an alkali. There are four important indicators: Methyl orange Phenolphthalein Litmus paper Universal indicator. Universal indicator (pH indicator) is the best as it shows the pH of the solution so that we can know how strong is the acid or base

Colour Methyl orangePhenolphthaleinLitmus paperUniversal indicator HCl RedColourlessRed NaOH Orange/yellowPinkBluePurple/violet

strongly weakly neutral weakly strongly acidic acidic alkaline alkaline HCl CH 3 COOH H 2 O NH 3 NaOH HYDROCHLORIC ETHANOIC AMMONIA SODIUM ACID ACID HYDROXIDE Acidic solutions (acidic)pH < 7 Alkaline solutions (basic) pH > 7 Neutral solutions pH = 7

Measuring pH IndicatorAdd a few drops of universal (full-range) indicator to the solution and compare the colour against a chart. The colour depends on how strong is the acid or base pH metersElectrical/electronic devices which measure the conductivity of the solution. You place the probe in any solution and read off the pH value on a scale or digital read-out

REACTIONS OF ACIDS Neutralisation Acid reacts with a base to form salt and water. Base can be a metal oxide, metal carbonate or metal hydroxide. Ammonium hydroxide is also a base. Calcium oxide + Nitric acid  Calcium nitrate + water CaO + HNO3  Ca(NO3)2 + H2O Copper(II) oxide + Hydrochloric acid  Copper(II)chloride + water CuO + HCl  CuCl2 + H2O Sodium hydroxide + Sulphuric acid  Sodium sulphate + water 2NaOH + H2SO4  Na2SO4 + 2H2O Ammonium hydroxide + hydrochloric acid  Ammonium chloride + water NH4OH + HCl  NH4Cl + H2O

Acid reacts with metals to form salt and hydrogen gas Most metals react with dilute acids to form salt and hydrogen Copper, Silver, Gold etc do not react with acids because these metals are not reactive. Magnesium + sulfuric acid  Magnesium sulfate + Hydrogen Mg + H2SO4  MgSO4 + H2 Zinc + Hydrochloric acid  Zinc chloride + Hydrogen Zn + HCl  ZnCl2 + H2 Acids react with carbonate to form salt, carbon dioxide and water Sodium carbonate + Hydrochloric acid  Sodium chloride + Carbon dioxide + water Na2CO3 + 2HCl  NaCl + CO2 + H2O Copper(II) carbonate + sulphuric acid  Copper(II) sulphate + Carbon dioxide + water CuCO3 + H2SO4  CuSO4 + CO2 + H2O

Salts Salts formed from an acid and a base. First part of a salt is from the base (+). First part of a salt i s normally a metal ion or ammonium ion (NH4+) Second part from the acid (-) For example, Sodium chloride is a salt. Sodium part comes from a base like sodium hydroxide. Chloride part comes from an acid (hydrochloric acid) Second part of the salt depends on the acid. Hydrochloric acid HCl - chloride Cl (-) Sulphuric acid H2SO4 - sulphate SO4 (2-) Nitric acid HNO3 - nitrate NO3 (-) Carbonic acid H2CO3 - carbonate CO3 (2-) Phosphoric acd H3PO4 - phosphate PO4 (3-)

How to make salts? Acid react with metal Example: Making Zinc sulphate. Take sulphuric acid in a beaker. (bec. the salt is sulphate) Add excess zinc powder and warm the mixture. Wait until no more bubbles come out. Cool the mixture and filter to get rid of excess Zinc. Take the filtrate in an evaporating dish, heat and evaporate until crystallisation point. Cool it in room temperature; filter to get the crystals of Zinc sulphate. Dry the crystals using filter paper or in an oven.

From an acid and an alkali (both soluble) Example: When hydrochloric acid and potassium hydroxide solution are mixed, we can make Potassium chloride salt. KOH (aq) + HCl (aq)  KCl (aq) + H2O (l) First conduct a titration to know what volume of acid and base exactly required for neutralisation. It needs an indicator. Then mix acid and base in larger volume in the same ratio you found in the titration for complete neutralisation. This time no indicator must be used so that salt will be pure. Heat and evaporate the resulting solution until crystallisation point. Cool it and filter to get the crystals of potassium chloride. Dry the crystals using filter papers or in an oven.

Double decomposition or Precipitation. ( make an insoluble salt) This method is useful to make a salt which is insoluble in water. For example to make calcium carbonate salt which is insoluble. Mix solutions of sodium carbonate and calcium chloride; both soluble and the calcium carbonate precipitates. Na2CO3 (aq) + CaCl2 (aq)  CaCO3 (s) + 2NaCl (aq) When two soluble salt solutions when mixed, the insoluble salt is precipitated out. The mixture is then filtered and the solid salt is left for drying.

Soluble and insoluble Following statements help you to identify if a chemical can dissolve in water or not. All compounds of SODIUM, POTASSIUM and AMMONIUM are soluble in water. All NITRATE compounds are soluble. All CHLORIDES are soluble except Silver chloride and Lead(II) chloride. All SULPHATES are soluble except Barium sulphate and Lead(II) sulphate. All CARBONATES are insoluble except sodium carbonate, potassium carbonate and ammonium carbonate. All metal oxides and metal hydroxides are insoluble in water except sodium, potassium and calcium oxide/hydroxide.

Soluble or insoluble??? Sodium carbonate Na2CO3 Soluble Copper(II) sulphate CuSO4 Soluble Lead(II) chloride PbCl2 Insoluble Silver sulphate Ag2SO4 Soluble Barium chloride BaCl2 Soluble Ammonium sulphate (NH4)2SO4 Soluble Iron(III) oxide Fe2O3 Insoluble

Copper(II) hydroxide Cu(OH)2 Insoluble Silver chloride AgCl Insoluble Barium sulphate BaSO4 Insoluble Lead(II) nitrate Pb(NO3)2 Soluble Sulphuric acid H2SO4 Soluble Potassium chloride KCl Soluble Calcium sulphate CaSO4 Soluble Magnesium carbonate MgCO3 Insoluble

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