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.
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 |