Acids

  • An acid is a compound which when dissolved in water produces hydrogen ions (H+ / proton)
  • acids in daily life includes ethanoic acid found in vinegar & tomato juice, citric acids found in citrus foods, lactic acid found in sour milk & in muscle respiration….
  • common lab acids include hydrchloric acid, sulphuric acid and nitric acid
  • Strong acids completely ionize in water producing lots of H+ ions
  • Weak acids partially ionize in water producing few H+ ions

 

Properties of Acids

An acid is a substance that produces hydrogen ions (H+) when dissolved in water. Acids are proton donors.

Indicators

  • Have pH between 1 (strong) and 6 (weak)
  • Turns blue litmus red
  • Turns methyl orange indicator red

Strong acids completely ionize in water producing lots of H+ ions

Weak acids partially ionize in water producing few H+ ions

Chemical properties

  • Acid + metal → salt + hydrogen gas
  • Acid + base → salt + water
  • Acid + metal carbonate → salt + carbon dioxide + water

 

Properties of Bases

Bases are substances which neutralize acids to form a salt and water only. They are proton acceptors (form OH ions) and are mainly water insoluble.

  • Have pH 8 to 14.
  • alkalis have a slippery feel
  • Alkalis are hazardous, dilute alkalis are irritants & concentrated alkalis are corrosive and brn skin
  • Alkalis turn red litmus indicator paper (or solution) to blue.
  • Base + acid → salt + water (+ CO2 when base is a metal carbonate)
  • Base + ammonium salt → salt + ammonia gas + water
  • Strong alkalis completely ionize in water producing lots of OH- ions
  • Weak alkalis partially ionize in water producing OH- ions

 

Indicators

  • Have pH between 8 (weak) and 14 (strong)
  • Turns red litmus blue
  • Turns methyl orange indicator yellow

 

Strong alkalis completely ionize in water producing lots of OH ions

Weak alkalis partially ionize in water producing OH ions

 

Neutral

  • Neutral substances are pH 7.
  • Acidity in soil:

Optimal plant growth requires a soil pH between 5 and 8. Soil pH imbalance to be too acidic reduces plant growth yield. Soil acidity is neutralized by adding lime or powdered limestone.

 

pH scale

pH is the concentration of H+ ions per dm3 of solution

Universal indicator solution is used to determine the pH of a substance by matching the color change to the pH color chart.

 

Color changes of indicators
Indicator Colour in acids Color in alkalis
Phenolphalein colourless yellow
Methyl orange red yellow
bromothylmol blue yellow blue
litmus red blue

Types of Oxides

  • Metal oxides are basic in nature e.g. iron oxide and magnesium oxide
  • Non-metal oxides are acidic in nature e.g. sulphur oxide and carbon dioxide
  • Aluminum, zinc and lead form amphoteric oxides e.g. zinc oxide
  • Oxides that react with neither acids nor bases are neutral e.g. nitric oxide and carbon monoxide

 

Preparation of Salts

  • A salt is a compound formed when all the hydrogen atoms of an acid are replaced by a metal.
  • Naming salts involves 2 parts; the name of the metal and the acid ending
  • calcium + hydrochloric acid = calcium chloride
    
    
Type of Salt Acid used
Sulphate Sulphuric acid
Nitrate Nitric acid
Chloride Hydrochloric acid
Ethanoate Ethanoic acid

 

Salts can either be soluble or insoluble

 

Soluble Salts Insoluble Salts
All sodium, potassium and ammonium salts
All nitrates
Chlorides Except silver and lead
Sulphates Except barium, lead and calcium
Potassium, sodium and ammonium carbonates All other carbonates

Preparation of Soluble Salts

Method A: Neutralization

  • Put a certain amount alkali in a flask
  • Add phenolphthalein
  • Add acid from a burette, stirring, until it goes colorless
  • Find out how much acid you used
  • Repeat, to be more accurate
  • Evaporate water from neutral solution

 

Method B: Titration

  • Phenolphthalein is added to an alkali (soluble base)
  • Add acid to solution using burette; note volume of acid required for solution to change color
  • Repeat without indictor using noted acid volume
  • Heat in evaporating dish to form soluble salt crystals

 

Preparation of Insoluble Salts

Method C: Precipitation

  • 2 soluble salts added to water and mixed Note: one soluble salt should always be a potassium or sodium solution (eg. potassium sulfate)
  • Filter out and clean precipitate with distilled water
  • Dry insoluble salt precipitate in oven

 

Test for Aqueous Cations

 

Cation with aqueous NaOH with aqueous Ammonia
Aluminum (Al3+) White soluble precipitate, turns colorless in excess White precipitate, insoluble in excess
Ammonium (NH4+) Pungent ammonium gas produced turns damp red litmus blue
Calcium (Ca2+) White precipitate, insoluble in excess Faint or no precipitate
Copper (Cu2+) Blue precipitate, insoluble in excess Blue precipitate, soluble in excess to give a dark blue solution
Iron(II) (Fe2+) Dirty green precipitate, insoluble in excess Dirty green precipitate, soluble in excess
Iron(III) (Fe3+) Reddish-brown precipitate, insoluble in excess Reddish-brown precipitate, insoluble in excess
Zinc (Zn2+) White precipitate, soluble and turns colorless in excess White precipitate, soluble and turns colorless in excess
Chromium (Cr3+) Grey green precipitate, soluble to give dark green solution in excess Grey green precipitate, insoluble in excess

 

Test for Anions

Sulfate ions (SO42-):

Add dilute nitric acid, then add aq. barium nitrate. White precipitate formed\

 

Sulphite ions (SO32-):

Add acidified potassium permanganate and heat. Color changes from pink to colorless

 

Halide ions:

Add nitric acid, then aqueous silver nitrate

Chloride (Cl) White precipitate
Bromide (Br) Cream precipitate
Iodide (I) Yellow precipitate

 

Nitrate ions (NO3):

Add aqueous sodium hydroxide then add warm aluminum foil. Pungent gas produced, turns damp red litmus blue

 

Carbonate ions (CO32-):

Add dilute hydrochloric acid. If bubbles/ gas produced turn limewater cloudy, carbonate ion present

Test for Gases

Gas Test and test result
Ammonia (NH3) Damp red litmus paper turns blue
Carbon dioxide (CO2) Bubble gas through–from colorless to cloudy
Chlorine (Cl2) Bleaches red/blue litmus paper
Hydrogen (H2) Place lighted splint, squeaky pop
Oxygen (O2) Place glowing splint, splint relights

 

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