Hydrochloric acid (Acid):
- Litmus - RED
- Methyl Orange - RED
- Phenolphthalein - CLEAR
- Universal Indicator - RED
- Litmus - N/A
- Methyl Orange - ORANGE
- Phenolphthalein - CLEAR
- Universal Indictaor - GREEN
Sodium Hydroxide (Alkali):
- Litmus - BLUE
- Methyl Orange - YELLOW
- Phenolphthalein - PINK
- Universal Indicator - BLUE
Reactions of Acids
Metal + Acid -> Salt + Hydrogen
Magnesium is the most reactive.
e.g. zinc + sulphuric acid -> zinc sulpahte + hydrogen
Carobnate + Acid -> Salt + Water + Carbon dioxide
copper(II) carbonate + sulphuric acid -> copper sulphate + water + carbon dioxide
Alkali + Acid -> Salt + Water
Alkalis (hydroxide) are soluble bases
e.g. sodium hydroxide + hydrochoric acid -> sodium chloride + water
Base + Acid -> Salt + Water
A base is a metal oxide
copper(II) oxide + sulphuric acid -> copper sulphate + water
Properties of Acids
1. They are liquids.
2. They are solutions of compounds in water.
3. If concentrated they can be corrosive.
4. Acids taste sour (for example, vinegar).
5. Turn blue litmus paper red - this is an easy test for an acid!
6. Usually react with metals to form salts.
7. Acids contain hydrogen ions.
8. Turn Universal Indicator from green to red, and have a pH less than 7.
Common Acids used in Laboratories
1. Hydrochloric acid, HCl(aq)
2. Nitric acid, HNO3(aq)
3. Sulphuric acid, H2SO4(aq)
Properties of Alkalis
1. They feel soapy to touch.
2. They are soluble bases.
3. Like acids, they can burn the skin.
4. They turn red litmus blue - this is how you test for an alkali!
5. Alkalis contain hydroxide ions (OH-).
6. They taste bitter.
7. Turns Universal Indicator from green to blue or purple.
Common Alkalis used in Laboratories
1. Sodium hydroxide, NaOH(aq)
2. Ammonia, NH3NH4OH(aq)
3. Calcium hydroxide, Ca(OH)2(aq)
Common Examples of Neutral substances
2. Sodium chloride solution, NaCl(aq)(common salt)
3. Sugar solution C6H12O6(aq)
Acid and Alkali Molecules
Bases and alkalis both neutralise acids - to form salt and water ONLY.
The alkali has neutralised the acid by removing its H+ ions, and turning them into water.
Neutralisation reactions involve a temperature rise as heat energy is released. These types of reactions are examples of exothermic reactions.
Acidic industrial waste is often neutralised using calcium oxide. This is sometimes called lime.
Soluble salts can be produced by adding an insoluble metal, metal oxide or metal carbonate to an acid
1. Measure out 25cm³ of acid using a measuring cylinder
2. Add excess solid to the acid and stir
3. Filter off the excess solid
4. Pour the filtrate into an evaporating basin
5. Heat to partially evaporate the solution
6. Leave the solution to cool and crystallise
7. Decant the excess solution and blot the crystals dry on a paper towel
Choosing a Method for Soluble Salt Preparation
METAL + ACID
The metal must be of medium reactivity for this method to work
BASE + ACID
The base must be INSOLUBLE for this method to work
CARBONATE + ACID
The carbonate must be INSOLUBLE for this method to work
ALKALI + ACID
An alkali (SOLUBLE base) must be available
Are used to:
1) to find the concentration of an unknown substance - in analysis
2) to find the exact reacting volumes of an acid and an alkali when used to make salt crystals
1) Fill the burette with dilute hydrochloric acid (using a funnel). Remember to fill the dead space below the tap and make sure there are no air bubbles.
2) Use a pipette and safety filler to measure 10cm³ of sodium hydroxide solution into at least 3 conical flasks.
3) Add 3 drops of a suitable indicator (e.g. phenolphthalein) to each (NOT U.I.)
4) Record the initial burette reading and add the acid into the sodium hydroxide solution until the phenolphthalein turns clear
5) Record the final burette reading and calculate the volume used
When two soluble ionic salt solutions are mixed, the ions involved may re-combine to produce an insoluble salt
Sodium, Potassium & Ammonium salts: ALL soluble
Nitrates: ALL soluble
Chlorides: soluble except for silver, lead and chloride
Sulphates: soluble except for barium and calcium sulphate
Carbonates: insoluble except for sodium, potassium & ammonium carbonates
e.g. lead nitrate + sodium chloride -> lead chloride + sodium nitrate
Calculating Energy Changes
Temperature change in a reaction can be converted into an enthalpy change in Joules using the following equation:
Energy change = mass of solution being heated x specific heat capacity of solution x temp. change
Q = m c ΔT
ΔH for Exothermic and Endothermic Reactions
- More energy is released when bonds are made
- Less energy is taken in when bonds break
- ΔH = negative
ΔH = bonds broken - bonds formed
Enthalpy Changes ΔH
Enthalpy (H) is the internal chemical energy stored within a compound.
Exothermic reactions: ΔH = negative because make>break
- Products are more stable than the reactants - they have stronger bonds
- Energy is released as heat when bonds are made
Endothermic reactions: ΔH = positive because break>make
- Products are less stable than the reactants - weaker bonds
- Energy is taken in from the surroundings when bonds are made