acids and bases Flashcards
what is an acid?
An acid is a substance that dissociates or ionises to form H+ ions when dissolved in water.
strong acids:
hydrochloric acid HCl
sulfuric acid H2SO4
nitric acid HNO3
weak acids:
ethanoic acid CH3COOH
carbonic acid H2CO3
phosphoric acid H3PO4
citric acid C6H8O7
properties of acids
- Acids have a sour taste.
- Acids turn blue litmus paper red.
- Acids react with reactive metals to form a salt and hydrogen.
acid + metal → salt + hydrogen - Acids react with carbonates to form a salt, water and carbon dioxide.
acid + carbonate → salt + water + carbon dioxide - Acids react with metal oxides and hydroxides to form a salt and water only.
acid + metal oxide → salt + water
acid + metal hydroxide → salt + water
- Strong acids are strong electrolytes. They dissolve in water to form solutions that can conduct electricity due to the presence of mobile ions that act as charge carriers.
what is a base?
A base is a substance that dissociates or ionises to form OH- ions when dissolved in water.
A base is a substance that reacts with an acid to produce salt and water only.
All metal oxides and hydroxides can act as bases.
properties of bases
- Alkalis have a bitter taste and soapy feel.
- Alkalis turn red litmus paper blue.
- Alkalis react with acid to form a salt and water only.
alkali + acid → salt + water - Heating an alkali with an ammonium salt produces ammonia gas.
alkali + ammonium salt → ammonia + water + salt - Strong alkalis are strong electrolytes. They can dissolve in water to form solutions that conduct electricity due to presence of mobile ions that act as charge carriers.
reactions of acids
- metal + acid → salt + hydrogen
Testing for the presence of hydrogen gas:
Place a lighted splint at the mouth of the test tube when effervescence is observed. Hydrogen gas will extinguish the lighted splint with a pop sound.
However, not all metals will react with acids to give a salt and hydrogen gas.
When unreactive metals such as copper, silver or gold are added to dilute acids, there is no reaction. - acid + carbonate → salt + water + carbon dioxide
acid + hydrogen carbonate → salt + water + carbon dioxide
- A hydrogen carbonate refers to any compound that contains the hydrogen carbonate ion, HCO3
Testing for the presence of carbon dioxide gas:
Bubble the gas through limewater (aqueous calcium hydroxide).
Carbon dioxide forms a white precipitate with limewater - acid + base → salt + water
- neutralisation - acid + sodium sulfite → sodium salt + sulfur dioxide + water
- Testing for the presence of sulfur dioxide gas:
Place a strip of filter paper soaked in acidified potassium manganate(VII) solution in the gas sample. Sulfur dioxide gas turns purple acidified potassium manganate(VII) colourless. Sulfur dioxide also turns orange acidified potassium dichromate(VI) green.
reactions of alkali
- alkali + ammonium salt → salt + water + ammonia
Testing for presence of ammonia gas:
Place a moist red litmus paper in the gas sample. Ammonia gas turns moist red litmus pape blue. Ammonia gas is colourless and pungent - alkali + acid → salt + water
strength of acids and bases
- The strength of an acid or base refers to the extent to which an acid
or base dissociates when dissolved in water - strong acids and bases: strong electrolytes
[An electrolyte is a substance that conducts an electric current as a result of its dissociation into ions when dissolved in a solvent such as water. Freely-moving ions act as mobile charge carriers that can conduct electricity. A strong electrolyte completely dissociates in solution and contains only ions. Strong electrolytes
are good conductors of electricity.]
strong acid: an acid that dissociates (or ionises) completely in water to form
hydrogen ions, H+
strong base:will dissociate fully in water to produce OH− ions.
weak acids and bases
- A weak acid: an acid that dissociates (or ionises) partially in water to form hydrogen ions, H+
- weak electrolyte: conduct only a small electrical current as it
contains a relatively lower concentration of mobile ions. - weak base: dissociate partially in water to form hydroxide ions OH-
concentration of acids and bases
Concentration refers to the amount of substance (solute) dissolved in a fixed volume of solution
The strength (extent of dissociation in water) of an acid or base cannot be changed.
The concentration of an acid or base can be changed.
basicity of acids
Acids are classified as monobasic, dibasic or tribasic depending on the number of H atoms in a molecule that are able to form H+ ions when it dissociates in water.
Hydrochloric acid is able to produce 1 H+ on per molecule when it dissociates in water. It is classified as a monobasic acid.
Sulfuric acid is dibasic as each molecule produces 2 H+ ions when it dissociates in water.
Phosphoric acid is tribasic as it produces 3 H+ ions per molecule when it dissociates in water.
- basicity does not determine strength
reactions of oxides
Basic oxides react with acids
Example: calcium oxide + nitric acid → calcium nitrate + water
CaO(s) + HNO3(aq) → Ca(NO3)2(aq) + H2O(l)
Amphoteric oxides react with acids
Example: zinc oxide + hydrochloric acid → zinc chloride + water
ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
Amphoteric oxides react with bases
Example: zinc oxide + sodium hydroxide → sodium zincate + water
ZnO(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2O(l)
Acidic oxides react with bases
Example: sulfur dioxide + sodium hydroxide → sodium sulfite + water
SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l)
classification of oxides
metal oxides
basic oxides: Reacts with acids to form salt and water, does not react with bases
- soluble in water:
e.g. (mostly group 1)
potassium oxide (K2O)
sodium oxide (Na2O)
lithium oxide (Li2O)
calcium oxide (CaO)
- insoluble in water
e.g.
magnesium oxide (MgO)
copper(II) oxide (CuO)
iron(II) oxide (FeO)
iron(III) oxide (Fe2O3)
Amphoteric Oxides: Reacts with acids and bases to form salt and water
- insoluble in water:
e.g. zinc oxide (ZnO)
aluminium oxide (Al2O3)
lead(II) oxide (PbO)
non-metal oxides
Acidic Oxides: Reacts with bases to form salt and water, does not react with acids
soluble in water:
Examples: group V-VI
nitrogen dioxide (NO2)
phosphorous(V) oxide (P2O5)
carbon dioxide (CO2)
sulfur dioxide (SO2)
sulfur trioxide (SO3
insoluble in water;
e.g. silicon dioxide (SiO2)
Neutral Oxides:
Does not react with an acid or a base
Examples:
carbon monoxide (CO)
nitrogen monoxide (NO)
