Combination and decomposition

Question 1

What is a combination reaction

Answer 1

When reactions occur in which two or more substances combine to form a SINGLE product.

element A + element B -> compound AB

Question 2

What does a metal and a non-metal form?

Answer 2

An ionic compound

Question 3

What do two non-metals form?

Answer 3

Molecular compound

Covalent compounds

Question 4

Eg equation for rusting

Answer 4

4Fe + 3O2 -> 2Fe(2)O(3)

Question 5

Why do the atoms of different elements combine?

Answer 5

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.

Question 6

What is an oxidation reaction

Answer 6

When an element reacts with oxygen.

Question 7

What are rapid oxidation reactions also called? (with heat and flame released)

Answer 7

Combustion, or burning

Question 8

How to tell that an acid and metal is reacting? (not combination reaction as two products are formed)

Answer 8

A gas (hydrogen is produced) is released. (Colourless, odourless)
Exothermic 
Test for hydrogen using glowing splint.

Question 9

eg Mg and oxygen

Answer 9

2Mg + O2 -> 2MgO
A bright light is seen
White powder.

Question 10

eg Sulphur and oxygen

Answer 10

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

Question 11

eg Cu and oxygen

Answer 11

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.

Question 12

Eg Fe and sulphur

Answer 12

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

Question 13

eg of Colours of combination reaction

Answer 13

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

Question 14

What is a decomposition reaction?

Answer 14

A compound is separated into smaller chemical species. The species formed can be elements or simpler compounds.

eg XY -> X + Y
XYZ -> XY + Z

Question 15

What is needed for a decomposition reaction?

Answer 15

Heat (thermal decomposition), catalyst (catalytic decomposition) or electrical current (electrolytic decomposition)

Question 16

Equation for thermal decomposition of METAL CARBONATE

Answer 16

Metal carbonate ——> (heat) Metal oxide + carbon dioxide

Question 17

Equation for thermal decomposition of METAL HYDROGEN CARBONATE

Answer 17

Metal hydrogen carbonate —–> (heat) metal carbonate + CO2 + water

The CO2 and water are colourless, odourless gases.

Question 18

Eg of colours of Metal carbonate thermal decomposition

Answer 18

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

Question 19

How to test for CO2?

Answer 19

Use delivery tube. Limewater -> Milky

Question 20

What is CO2?

Answer 20

A colourless, odourless gas

Question 21

Equation for thermal decomposition of metal hydroxides

Answer 21

Metal hydroxide -> metal oxide + water

Question 22

How to test for water?

Answer 22

Add piece of blue cobalt chloride paper. Will change from blue (dehydrated) to pink (hydrated)

Question 23

Which metal hydroxides and carbonates decompose easily?

Answer 23

Metal hydroxides and carbonates of the less reactive metals (low on reactivity series). Eg CuCO3 decomposes more readily than CaCO3.

Question 24

Which metal carbonates do not decompose at all?

Answer 24

Na2CO3 and K2CO3 because they are too stable.

Question 25

Which metal HYDROGENCARBONATES decompose readily?

Answer 25

ALL metal hydrogencarbonates that exist will decompose readily.

Question 26

Colours of carbonates, hydroxides and oxides?

Answer 26

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)

Question 27

Manufacture of lime (thermal decomposition)

Answer 27

Limestone is heated to manufacture lime:
CaCO3 -> CaO + CO2

Lime is used as an agricultural chemical to reduce the acidity of soils.

Question 28

What causes biscuits and cakes to rise in baking?

Answer 28

When heated, sodium hydrogencarbonate (baking soda) releases carbon dioxide gas, which causes biscuits and cakes to rise during baking

2NaHCO3 -> (heat) Na2CO3 + CO2 + H2O

Question 29

What is catalytic decomposition?

Answer 29

When a catalyst is used to speed up decomposition but is not used up in the reaction.

Question 30

Two ways a catalyst can speed up rate of reaction

Answer 30

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)

Question 31

Catalytic decomposition of H2O2

Answer 31

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.