Combination and decomposition Flashcards
What is a combination reaction
When reactions occur in which two or more substances combine to form a SINGLE product.
element A + element B -> compound AB
What does a metal and a non-metal form?
An ionic compound
What do two non-metals form?
Molecular compound
Covalent compounds
Eg equation for rusting
4Fe + 3O2 -> 2Fe(2)O(3)
Why do the atoms of different elements combine?
The atoms of different elements will combine if the new substance formed is MORE stable than the original substances. Sometimes, heat energy is required to start the reaction.
Reactive metals are not very stable so they tend to react quickly with non-metals elements.
What is an oxidation reaction
When an element reacts with oxygen.
What are rapid oxidation reactions also called? (with heat and flame released)
Combustion, or burning
How to tell that an acid and metal is reacting? (not combination reaction as two products are formed)
A gas (hydrogen is produced) is released. (Colourless, odourless) Exothermic Test for hydrogen using glowing splint.
eg Mg and oxygen
2Mg + O2 -> 2MgO
A bright light is seen
White powder.
eg Sulphur and oxygen
S + O2 -> SO2
Colour of burning sulphur is blue flame
Sulphur is dissolvable in water.
pH of sulphur is acidic ie turns UI red.
SO2 dissolves in water to form sulphurous acid. H2SO3
eg Cu and oxygen
Inside envelope = colours
Outside of envelope = black layer
2Cu + O2 -> 2CuO
The product formed is in a solid state at room temp because ionic compound so there are strong electrostatic forces holding the atoms together, and the particles are closely packed.
Eg Fe and sulphur
When heated together, melted and formed a grey solid once cooled.
Mixture goes red when heated
No magnetic properties
Fe (II) + S -> FeS
Requires heat
Sulphur: solid at room temp. Yellow. Non-magnetic. Crystalline solid
Iron: Solid at room temp. Metallic. Magnetic. Grey-silver
Product: solid at room temp. Ionically bonded. Not magnetic
eg of Colours of combination reaction
MgO is white ash solid. Relatively unreactive. Ionic solid.
CaO is white powdery solid at room temp. Relatively unreactive.
S burns in O2 (blue flame) to produce SO2 which has a strong odour and irritates the lungs and throat.
CuO is a black powder
What is a decomposition reaction?
A compound is separated into smaller chemical species. The species formed can be elements or simpler compounds.
eg XY -> X + Y
XYZ -> XY + Z
What is needed for a decomposition reaction?
Heat (thermal decomposition), catalyst (catalytic decomposition) or electrical current (electrolytic decomposition)
Equation for thermal decomposition of METAL CARBONATE
Metal carbonate ——> (heat) Metal oxide + carbon dioxide
Equation for thermal decomposition of METAL HYDROGEN CARBONATE
Metal hydrogen carbonate —–> (heat) metal carbonate + CO2 + water
The CO2 and water are colourless, odourless gases.
Eg of colours of Metal carbonate thermal decomposition
ZnCO3 (white powder) -> ZnO (white powder) and CO2
PbCO3 (white powder) -> DOES NOT DECOMPOSE
CuCO3 (green powder) -> CuO (black powder) and CO2
NaCO3 (white powder) does NOT thermally decompose because too stable. Requires VERY high temperatures. Using a Bunsen it won’t
K2CO3 (white powder) -> K2O (pale yellow powder) and CO2
How to test for CO2?
Use delivery tube. Limewater -> Milky
What is CO2?
A colourless, odourless gas
Equation for thermal decomposition of metal hydroxides
Metal hydroxide -> metal oxide + water
How to test for water?
Add piece of blue cobalt chloride paper. Will change from blue (dehydrated) to pink (hydrated)
Which metal hydroxides and carbonates decompose easily?
Metal hydroxides and carbonates of the less reactive metals (low on reactivity series). Eg CuCO3 decomposes more readily than CaCO3.
Which metal carbonates do not decompose at all?
Na2CO3 and K2CO3 because they are too stable.
Which metal HYDROGENCARBONATES decompose readily?
ALL metal hydrogencarbonates that exist will decompose readily.
Colours of carbonates, hydroxides and oxides?
Most metal carbonates, hydroxides and oxides are white. However, compounds of iron and copper can be coloured.
Iron II compounds - carbonate (green), hydroxide (green), oxide (black)
Iron III compounds - hydrogen (rust/red), oxide (rust/red)
Copper compounds - carbonate (green), hydroxide (blue-green), oxide (black)
Manufacture of lime (thermal decomposition)
Limestone is heated to manufacture lime:
CaCO3 -> CaO + CO2
Lime is used as an agricultural chemical to reduce the acidity of soils.
What causes biscuits and cakes to rise in baking?
When heated, sodium hydrogencarbonate (baking soda) releases carbon dioxide gas, which causes biscuits and cakes to rise during baking
2NaHCO3 -> (heat) Na2CO3 + CO2 + H2O
What is catalytic decomposition?
When a catalyst is used to speed up decomposition but is not used up in the reaction.
Two ways a catalyst can speed up rate of reaction
Provide a specific site for reaction collisions to occur.
Provide an ALTERNATIVE pathway for the reaction that needs a lower amount of energy to start (Ea)
Catalytic decomposition of H2O2
Without a catalyst, the decomposition of H2O2 at room temp is very slow. A catalyst speeds up the reaction so that it can be observed at room temp. Only a small amount of catalyst is needed because it does not get used up during the reaction.
H2O2 is a stable, colourless, poisonous compound often used for bleaching purposes. Use MnO2 or KI as a catalyst.
2H2O2 -> (catalyst) 2H2O + O2
Heat is given off.
Test for O2 by holding glowing splint above test-tube. The splint relights.
The rate of a catalysed reaction is very rapid at first. The rate levels off after a short time as all the reactant (H2O2) is used up.
The rate of an uncatalysed reaction is much slower than a catalysed reaction. However, the volume of oxygen produced in an uncatalysed reaction will steadily increase over time until it reaches the same volume as the catalysed reaction when all the reactant is used up.
