Chemistry C2

Rust is hydrated iron (III) oxide Fe2O3

It occurs when Iron is exposed to oxygen and moisture (water in the air)

Preventing Rusting: using barrier methods in order to create a barrier between the iron and oxygen and water

• Paint, Oil or Grease- Cars and Machinery
•  Plastic coating- Bicycles
• Plating with another metal-- Tin – cans--Chromium – handlebars--Thin layers of metals are deposited on Iron using electrolysis-- Coating with Zinc which is GALVANISING
•  Sacrificial protection involves coating Iron with a more reactive metal which will react with oxygen and water before the Iron so Iron will be left intact. e.g. Magnesium coating for ships

REDOX reactions involve Reduction and Oxidation reactions occurring at the same time (simultaneously).

Oxidation is--Gain of Oxygen--Loss of Hydrogen--Loss of electrons

Reduction is Loss of Oxygen-- Gain of Hydrogen--Gain of electrons

Common Oxidation reactions:

• CombustionC + O2 →CO2
• Rusting4Fe + 3O2 + 2xH2O → 2Fe2O3.xH2O
• Burning elements  2Mg +O2 → 2MgO      S + O2 → SO2

Common Reduction reactions:

Reduction of copper(II) oxide--CuO + H2 → Cu + H2O

All reactions are REDOX reactions because if one compound loses oxygen, another must gain oxygen. But we classify the reaction according to what is the main purpose of the reaction.

• Soft water Lathers easily with soap-Contains Na+/ and/or K+ ions
• Hard water does not produce a lather readily with soap- Contains Ca2+ and/or Mg2+ ions Reacts with soap to form scum (Calcium Stearate or Magnesium Stearate). Will lather with detergent
• Temporary Hardness can be removed by boiling-Contains Ca2+ /Mg2+ and HCO3- Forms limescale (CaCO3). Formed when CO2 dissolves in rainwater forming carbonic acid which falls on limestone:  CO2 + H2O→ H2CO3: H2CO3 +CaCO3 →Ca(HCO3)2. Softening during boiling: Ca(HCO3)2 → CaCO3 +CO2 +H2O
• Permanent Hardness cannot be removed by boiling- Contains Ca2+/Mg2+ and SO42- Softening by adding washing soda: Na2CO3 +Ca(HCO3)2 → 2NaHCO3 +CaCO3. This is a precipitation reaction. A precipitation reaction involves ions in two solutions reacting together to make an insoluble substance. E.g. CO32-(aq) +Ca2+(aq) →CaCO3(s)
• Softening using an Ion-Exchanger: To replace the Ca2+ or Mg2+ ions with either Na+ or K+ The hard water is passed through a column containing Na+ ions on resin beads. The hard water ion (Ca2+) replaces the soft water ion on the resin bead. The soft water ion travels out of the column with the water.

Testing for Hardness

• Add a known volume of soap solution to the water sample
• Shake well
• Record observations
• if lather then SOFT water
•  if scum forms or no immediate lather then it is HARD water

To distinguish between Temporary Harness and Permanent Hardness:

• Boil sample of known hard water
• Add known volume of soap solution
• Shake
• Record observations
• if lather forms then Temporary hard water, 
• if no lather forms then Permanent hard water

Advantages and Disadvantages of Hard Water

Advantages

• Hard water tastes better
• It is better for brewing beer
• It is good for tanning leather
• It provides Ca2+ for healthy bones and teeth

Disadvantages

• Produces scum which wastes soap
• Produces ‘fur’ or limescale in kettles and hot water pipes so appliances are less efficient and need replaced more often
• Dishwasher salt needed to soften water, adding to the cost.

• Relative Atomic Mass – RAM is the mass of an atom compared with that of the Carbon-12 isotope which has a mass of exactly 12. It is a weighted average. It is the mass of one mole of that element.

• Relative Formula Mass –RFM is the sum of the RAMS of all the atoms present. It is the mass of one mole of that substance. Units are gmol-1

• Mole of a substance contains 6x1023 particles which is known as Avogadro’s Number

• Moles = Mass  for SOLIDS

                  RFM

• Moles – Volume x Concentration for LIQUIDS

The rate of a reaction can be measured by the rate at which a reactant is used up, or the rate at which a product is formed. The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts, are all factors which affect the rate of a reaction. Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

Measuring rates

Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, the chemical weathering of rocks is a very slow reaction: it has a low rate of reaction. Explosions are very fast reactions: they have a high rate of reaction.

Reactants and products

There are two ways to measure the rate of a reaction: 

• Measure the rate at which a reactant is used up
• Measure the rate at which a product is formed.

Things to measure

The measurement itself depends on the nature of the reactant or product:

• The mass of a substance - solid, liquid or gas - is measured with a balance
• The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette

It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical axis, and the time goes on the horizontal axis.

For example, if 24 cm³ of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12 cm³ hydrogen / min.

Changing concentration or pressure

• If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:
• There are more reactant particles in the same volume
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing particle size

• If a solid reactant is broken into small pieces or ground into a powder:
• Its surface area is increased
• More particles are exposed to the other reactant
• There is a greater chance of the particles colliding
• The rate of reaction increases

Changing the temperature

• If the temperature is increased: The reactant particles move more quickly
• More particles have the activation energy or greater
• The particles collide more often, and more of the collisions result in a reaction
• The rate of reaction increases

Using a catalyst

• Catalysts increase the rate of reaction without being used up.
• They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases.
• Different reactions need different catalysts.
• Catalysts are important in industry because they reduce costs.

Tests

• Hydrogen – lighted splint goes POP - 2H2 + O2 → 2H2O
• Oxygen – relights a glowing splint
• Carbon dioxide – limewater (calcium hydroxide) turns from colourless to milky.
• Water – anhydrous copper(II) sulfate turns from white to blue

Hydrogen – H₂ – Diatomic element

• Colourless odourless gas
• Collected from the reaction of a metal and acid
• Is a reducing agent
•  Used in meteorological balloons as it is light
• Rocket engines

Carbon and Carbon dioxide

• Carbon dioxide is a colourless odourless gas
• Carbon dioxide reacts with water to form carbonic acid

CO2 + H2O → H2CO3

• Carbon dioxide reacts with calcium hydroxide (limewater)
• CO2 +Ca(OH)2 → CaCO3 + H2O
•  Cloudy limewater changes to colourless again with excess carbon dioxide
• CaCO3 + CO2 + H2O → Ca(HCO3)2
• Burning fossil fuels releases carbon dioxide which contributes to global warming to due the ‘Greenhouse effect’ where carbon dioxide absorbs heat energy, preventing it escaping from the earth, which warms the planet, melts ice-caps and causes sea levels to rise.

Uses of Carbon dioxide:

• Fire extinguishers-As it is denser than air, so covers the burning fuel, preventing the fuel from getting oxygen which is needed for burning.
• Making carbonated drinks 
• As it has a low solubility in water. It will also give the drink an acidic taste.

Nitrogen

• A colourless odourless gas (N2)
• Unreactive gas as it contains a TRIPLE covalent bond which requires a LOT of energy to break before the atoms can react.

Uses

• Liquid Nitrogen is used as coolant
• In food packaging to keep food fresh
• In the manufacture of ammonia (Haber process)
• N2 + 3H2 → 2NH3     450°C/ 200 atm/Iron catalyst
• This is a Reduction reaction as N is gaining H. Uses of Ammonia: Manufacture of fertilizers/Production of nitric acid/Manufacture of nylon

Oxygen

• A colourless odourless gas (O2)
• Reaction 
• With Hydrogen2H2 + O2 →2H2O Blue flame
• With CarbonC + O2 → CO2Orange sooty flame
• With SulfurS + O2 → SO2 Blue flame via a Red liquid
• With Magnesium 2Mg + O2 → 2MgO Bright white light
• With Zinc2Zn + O2 → 2ZnOYellow solid when hot, white when cold
• With Iron3Fe + 2O2 → Fe3O4
• With Copper 2Cu + O2 → 2CuOBlack powder

Uses

• Breathing apparatus/Steel Making/Welding/Rocket engines
• The atmosphere contains about 80% Nitrogen and 20% Oxygen
• And small amounts of other gases such as Carbon dioxide, Water vapour,Noble gases

Sulfur 

• Brittle yellow solid
• Combustion when S impurities in fossil fuels are burnt
• S + O2 → SO2
• SO2 reacts with water to form ACID RAIN
• H2O + SO2 → H2SO3 (sulfurous acid)

Effects of ACID RAIN:

• Corrosion of Limestone
• Death of fish in rivers and lakes
• Defoliating trees (Deforestation)

Prevention of Acid Rain:

• Burn less fossil fuel/Remove sulfur from fuels before burning
• Reaction with Iron:
• Fe + S → FeS
• Grey          Yellow Black

This is the chemistry of compounds of carbon. These are often Fossil Fuels: e.g. Natural gas, L.P.G (liquid petroleum gas), petrol, diesel, paraffin, peat, lignite, coal, coke, candle wax

CH4 +   O2 →CO2+ H2O

Crude oil is a mixture of liquids with different b.pts. These liquids are separated in an oil refinery by fractional distillation. 

Cracking: Each fraction from the crude oil is composed of many different chemical compounds and must be processed further to make them useful. Some of the larger compounds from the kerosene fraction can be broken down into smaller compounds by a process called thermal cracking. This involves breaking large hydrocarbon chains into smaller ones - you usually need heat and a catalyst to do this. There will be an example of cracking later.

All the compounds found in crude oil are composed of hydrogen and carbon, they are called HYDROCARBONS. 

A Hydrocarbon is a substance which is composed of carbon and hydrogen only

Hydrocarbons are classified into groups of similar compounds known as a HOMOLOGOUS SERIES we will look at 2 such families: Alkanes & Alkenes.

Homologous-means that they have the same general formula (see later), similar chemical properties (reactions) and a gradual change in their physical properties (melting/boiling points or are they solid liquid or gas )

Other stuff 

• Carbon always forms 4 bonds
• Hydrogen always forms 1 bond
• Oxygen always forms 2 bonds
• Meth- you put this at the start of the name for a molecule with 1 carbon
• Eth – you put this at the start of the name for a molecule with 2 carbons
• Prop – you put this at the start of the name for a molecule with 3 carbons
• But- you put this at the start of the name for a molecule with 4 carbons

Alkanes (CnH2n+2)

• Alkanes do not have a functional group, which makes them less reactive organic molecules. 
• They are saturated as they contain single C-C bonds (no C=C double bonds).
• They contain C-H bonds which are very strong and difficult to break.
• CH4 - Methane
• C2H6 - Ethane
• C3H8 - Propane
• C4H10 - Butane
• C5H12 - Pentane

Combustion Due to the strong C-H bonds in alkanes, they release a lot of heat energy when burnt and so are a good source of fuel. They burn with a BLUE flame. Complete Combustion: In a plentiful supply of oxygen0--Alkane + Oxygen → Carbon dioxide + Water or CH4 + 2O2 → CO2 + 2H2O

Incomplete Combustion: In a limited supply of oxygen produces TOXIC C0--Alkane + Oxygen → Carbon monoxide + Water--2CH4 + 3O2 → 2CO + 4H2O

Alkenes (CnH2n) Alkenes have the functional group C=C (double bond).

• C2H4 - Ethene/C3H6 - Propene/C4H8- Butene/C5H10 - Pentene
• Alkenes are made from alkanes by cracking at HIGH temperatures (>800°C) using a Zeolite catalyst (containing Al, Si, O) Alkane → Alkene + AlkanesC=C is very reactive and is easily broken to form single bonds. This is an example of an ADDITION reaction because H2 adds across the double bond. C2H4 + H2 → C2H5
• Test for unsaturation – i.e. - to determine the presence of C=C
• Alkanes are stable and ‘saturated’ so will not react with the Bromine water. The orange-brown colour will remain.
• Alkenes can undergo an addition reaction using up the bromine in the Bromine water. Hence, the orange-brown colour will disappear, leaving a colourless solution. C2H4 + Br2 → C2H4Br2
• Combustion Alkenes burn with a yellow flame, in the same way as alkanes. However, they are not usually used as fuels.
• Complete Combustion: In a plentiful supply of oxygen Alkene + Oxygen → Carbon dioxide + Water- C2H4 + 3O2 → 2CO2 + 2H2O
• Incomplete Combustion: In a limited supply of oxygen produces TOXIC CO
• Alkene + Oxygen → Carbon monoxide + Water- C2H4 + 2O2 → 2CO + 2H2O
• Alkenes can also add on to each other to form a polymer in a polymerisation reaction. 

Alcohols (CnH2n+1OH)  Alcohols have the functional group, OH which makes them a good starting material to make a variety of compounds. CH3OH - Methanol/C2H5OH - Ethanol/C3H7OH - Propanol/C4H9OH - Butanol/C5H11OH - Pentanol

Uses: They are also used as solvents because they mix well with water due to their OH group.

Combustion-Due to the strong C-H bonds in alcohols, they release a lot of heat energy when burnt and so are a good source of fuel. They also burn with a BLUE flame.

Complete Combustion: In a plentiful supply of oxygen. Alcohol + Oxygen → Carbon dioxide + Water. C2H5OH + 3O2 → 2CO2 + 3H2O

Incomplete Combustion:  is not as likely to happen with an alcohol because it contains an oxygen which helps to promote complete combustion. It therefore creates less pollution than petrol and is known as a ‘clean’ fuel

Industrial Preparation Ethanol can be prepared from ethene and steam via an addition reaction at high temperature and pressure.

Social impact and harmful effects of ethanol in alcoholic drinks

• Health problems include heart disease, cirrhosis of the liver, damage to the nervous system, brain damage and alcoholism. 
• Social impact of abusing alcohol include relationship and work problems. In addition over 30% of road accidents involve people who have had alcohol. The legal limit for driving a car is 80mg of alcohol per 100cm3 of blood.

Carboxylic acids (CnH2n+1COOH) 

• Contains the functional group COOH- HCOOH – Methanoic acid/CH3COOH – Ethanoic acid/C2H5 COOH – Propanoic acid/C3H7 COOH – Butanoic acid/C4H9 COOH – Pentanoic acid
• Ethanoic acid is a weak acid which is diluted to form vinegar. 
• A weak acid is one which only partially dissociates into ions: i.e. CH3COOH → CH3COO- +H+ compared with inorganic acids which are strong acids and completely dissociate: HCl → H+ + Cl-
• Reactions of ethanoic acid:  with sodium carbonate: 2CH3COOH + Na2CO3 → CH3COONa + H2O + CO2/sodium hydroxide: CH3COOH + NaOH → CH3COONa + H2O/and magnesium :2CH3COOH + Mg → (CH3COO)2Mg + H2

Chemical reactions can be classed as

Exothermic – Give OUT heat energy

Bond making is Exothermic – Energy is released when bonds are formed/Neutralisation/ Combustion/Displacement Reactions/Dissolving/Hydration/Rusting

Endothermic – Take IN heat energy

Bond breaking is Endothermic – Energy is required to break a bond/Thermal Decomposition/Photosynthesis/Dissolving/Electrolysis

Overall Energy Change: is a balance of the energy taken IN when bonds BREAK and the energy RELEASED when bonds FORM

Example; CH4   +   2O2 → CO2  +  2H2O (Exothermic)

Energy is required to break the bonds in methane and oxygen, this is endothermic. Energy is released when bonds are made in carbon dioxide and water, this is exothermic. More energy is given out than taken in, so the overall reaction is exothermic.

Thermal Decomposition is breaking a substance down using HEAT and is therefore ENDOTHERMIC.

Thermal Decomposition of Calcium carbonate

• Calcium carbonate – CaCO3 is a white solid
• Compound found in LIMESTONE and MARBLE.
• It is insoluble in water
• CaCO3 →CaO + CO2
• White →White  but during heating it turns orange.
• Calcium oxide is known as ‘Quick Lime’
• There is a loss in mass due to the loss of CO2.
• The process occurs in a lime kiln.
• Uses of calcium carbonate are to neutralize soil acidity and in the Blast Furnace to remove acidic impurities in the production of iron.
• Limestone has to be quarried
• Advantages of Quarrying: Provides employment, Provides landfill sites, Puts money into the local economy, Provides better transport links
• Disadvantages: An eyesore Noise pollution, Dust pollution, Destroys habitats

A good way to remember the order of a reactivity series of metals is to use the first letter of each one to make up a silly sentence. For example: People Say Little Children Make A Zebra Ill Constantly Sniffing Giraffes.

Observations of the way that these elements react with water, acids and steam enable us to put them into this series. The tables show how the elements react with water and dilute acids:

Element       Reaction with water

• Potassium        Violently
• Sodium        Very quickly
• Lithium        Quickly
• Calcium        More slowly

Element         Reaction with dilute acids

• Calcium        Very quick
• Magnesium   Quickly
• Zinc             More slowly
• Iron              More slowly than zinc
• Copper         Very slowly
• SilverBarely reacts
• GoldDoes not react