Unit2Topic3 - Different Types Of Chemical Reactions Flashcards
Signs that a chemical reaction is taking place
(i) change in appearance (eg colour)
(ii) energy change (eg heat, light, sound, electricity)
(iii) formation of precipitate
(iv) production of gas
What is an exothermic reaction?
Gives out more energy than it takes in
What is an endothermic reaction?
Takes in more energy than it gives out
Types of exothermic reactions
Combustion of fuels
Neutralisation
Displacement
Combustion of fuels
Combustion is the reaction of a fuel with oxygen that produces at least one oxide and releases energy.
Burning is an example of combustion.
fuel + oxygen -> carbon dioxide + water + heat
Neutralisation
When an acid and an alkali react, the H+ and OH- ions form water. Water is neutral.
H+ (aq) + OH- (aq) -> H2O (l)
Displacement
A more reactive element will displace a less reactive element from its compound.
Types of endothermic reactions
Thermal decomposition
Cracking
Photosynthesis
Thermal decomposition
Thermal decomposition is the breaking down of a compound using heat.
Eg. CaCO3 (s) -> CaO (s) + CO2 (g)
Cracking
The reaction used to break large hydrocarbon molecules into smaller molecules.
Photosynthesis
Energy + carbon dioxide + water -> glucose + oxygen
Light + 6CO2 + 6H2O -> C6H12O6 + 6O2
Consider…
CH4 (g) + 2O2 (g) ->CO2 (g) + 2H2O (g)
fuel(methane) + oxygen -> carbon dioxide + water + energy
Burning methane is an exothermic reaction because the energy required to break the bonds of the reactants (methane and oxygen) is less than the energy released on forming the bonds of the products (carbon dioxide and water).
All chemical reactions involve bond breaking and bond forming processes:
Breaking bonds is an endothermic process.
Bond making is an exothermic process.
Metal carbonates thermally decompose to give …
A metal oxide and carbon dioxide
Eg. CuCO3 (s) -> CuO (s) + CO2 (g)
(CuCO3 is green, CuO is black, heat over arrow)
Thermal decomposition of limestone
When limestone (calcium carbonate) is heated strongly it breaks down to calcium oxide (quick lime) and carbon dioxide.
This can take place in a lime kiln.
Thermal decomposition of calcium carbonate equations
Calcium carbonate -> calcium oxide + carbon dioxide
(Two calcium things are white, oxide is basic)
CaCO3 (s) -> CaO (s) + CO2 (g)
Uses of limestone
Blast furnace
Neutralising acidic soil
Blast furnace
Limestone is added to blast furnace during extraction of iron from its ore.
The calcium carbonate thermally decomposes to form calcium oxide.
The calcium oxide combines with the acidic impurities, silicon dioxide (sand), thus removing them.
Calcium oxide + silicon dioxide -> calcium silicate (slag)
CaO + SiO2 -> CaSiO3
Uses of calcium oxide
Mixed with water to produce limewater
CaO + H2O -> Ca(OH)2
To make plaster
Advantages of limestone quarrying
- provides jobs
- boosts economy
- provides raw material for use in blast furnace and neutralising soil
Disadvantages of limestone quarrying
- eye sore
- ruins animals’ habitats
- noise/dust pollution
- increased traffic congestion
What has increased the concentration of CO2 and other greenhouse gases in our atmosphere?
Human activity such as burning fossil fuels
Global warming
Many suggest these greenhouse gases form a blanket around the earth, trapping heat and causing the average temperature at the earth’s surface to rise.
The effect of high concentrations of CO2 in the atmosphere
Current temperature of atmosphere = 14°C.
2.7 billion years ago temp = 70°C due to intense volcanic activity, releasing large quantities of CO2.
Over time CO2 levels decreased as bacteria and plants slowly turned CO2 into carbonates (CO32-), organic material and oxygen via photosynthesis.
During last 100 years the average temp has risen by 1°C.
This has been enough to cause flooding, drought and destruction of habitats.
Wegener’s Theory
In 1912 Alfred Wegener proposed the theory of continental drift.
Originally, all our present continents were joined together in a single land mass. About 200 million years ago the land mass started to break up and form continents. These continents could move and drift apart.
Wegener couldn’t explain how or why they could move apart.
Current theory of continental drift
Plate tectonics
Continents move due to convectional currents in the mantle.
Where do earthquakes and volcanoes occur?
Around plate boundaries
What causes an earth quake?
Caused by the stresses and strains when 2 continental plates slide past each other or one slides under another. As the plates slide past they might even bend. Forces are suddenly released which causes the ground to shake violently and breaks and cracks develop in the ground.
What causes a volcano?
Caused by plates moving apart. This causes the earth’s crust to be stretched. In areas of weakness, the crust may crack and molten magma and gases can escape through the cracks, resulting in a volcano.
What is the chemical formula for rust?
Iron (III) oxide
Fe2O3.xH2O
How is rust formed?
Iron reacting with both oxygen and moisture at room temperature.
How is rusting sped up?
By salt
Methods used to prevent rusting
Painting Oiling Plastic coating Suitable metal coating/plating (including galvanising) Sacrificial protection
What is galvanising?
Galvanising is a process that bonds a layer of zinc metal onto the surface of steel.
How does galvanising work?
Zinc reacts with air to form protective zinc oxide layer.
What is the advantage of galvanising?
Even if the zinc coating gets scratched or damaged, the exposed steel still won’t rust as the zinc will react instead of the iron.
What is sacrificial protection?
A more reactive metal, in good contact with a less reactive metal, will always react first, therefore protecting less reactive metal from corrosion.
Example of sacrificial protection
Iron boats often have a lump of magnesium bolted to their hulls - the Mg turns to MgO rather than iron turning to rust.
What is a redox reaction?
A reaction where oxidation and reduction take place together.
Oxidation is…
(i) the addition of oxygen
(ii) the removal of hydrogen
(iii) the removal of electrons
…to/from a substance.
Types of oxidation reactions
Combustion of fuels
Rusting
Examples of oxidation reactions
Mg is a grey shiny metal. Burns with bright white light. Forms a white ash.
Sulfur is a yellow brittle solid. Burns with blue flame. Forms pungent gas sulfur dioxide.
Reduction is…
(i) the removal of oxygen
(ii) gain of hydrogen
(iii) gain of electrons
Copper oxide reduction example
Starts CuO is black, changes to pink/copper-coloured solid.
Condensation inside tube.
CuO (s) + H2 (g) -> Cu (s) + H2O (g)
H2 acts as a reducing agent.
Why is the excess hydrogen burned?
To prevent farmable H2 gas building up in the lab.
What is a reducing agent?
A substance that will remove oxygen from a compound such as an oxide. It is oxidised.
What is an oxidising agent?
A substance that will provide oxygen to a substance being oxidised. It is reduced.
What happens to the magnesium atom as it forms the compound? 2Mg + O2 -> 2MgO
The magnesium atom loses to electrons and becomes a magnesium ion. Loss of electrons is oxidation.
Mg - 2e- -> Mg2+
What happens to copper (II) ion as it forms the compound?
CuO + H2 -> Cu + H2O
The copper ion gains two electrons and becomes a copper atom. Gain of electrons is reduction.
Cu2+ + 2e- -> Cu
Important oxidation and reduction reactions in industrial processes
(i) during the extraction of iron in the blast furnace
(ii) during extraction of aluminium by electrolysis
(iii) during production of ammonia in the Haber process
Redox reactions: during the extraction of iron in the blast furnace
Iron(III) oxide + carbon monoxide -> iron + carbon dioxide
Fe2O3 (s) + 3CO (g) -> 2Fe (l) + 3CO2 (g)
The iron (III) ion has gained 3 electrons to become an iron atom. Gain of electrons is reduction. Fe3+ + 3e- -> Fe
Redox reactions: during the extraction of aluminium by electrolysis
Aluminium oxide (bauxite) -> aluminium + oxygen 2Al2O3 (s) -> 4Al (l) + 3O2 (g)
The aluminium ion has gained 3 electrons to become an aluminium atom. Gain of electrons is reduction. Al3+ + 3e- -> Al
How is the oxide ion oxidised?
Two oxide ions gain 2 electrons each to become an oxygen molecule. The loss of electrons is oxidation.
Redox reactions: during production of ammonia in Haber process
Nitrogen + hydrogen -> ammonia
N2 (g) + 3H2 (g) -> 2NH3 (g)
Reduction is gain of hydrogen.
