The early periodic table
In the early 1800s only atomic mass was known and elements were put in order of size.
In 1864 John Newlands created his 'law of octaves' - he arranged the atoms elements in order of mass and said that every eighth element had similar properties. He then listed the then known elements in columns of 7 and rows of 8 this was called 'Newland's table of Octaves' but the pattern broke down on the third row. This meant his idea was largely unaccepted
- He didn't leave gaps for unknown elements
- His groups contained elements that didn't have similar properties
- He mixed up metals and non-metals
In 1869 Dimitri Mendeleev used the 50 known elements he had to create a new table putting the elements in order of mass but he left gaps in order to keep elements with similar properties in the same vertical columns (groups). He then used the gaps to predict the properties of these missing elements which when discovered matched the predictions confirming the validity of the table
This was the beginning of the modern periodical table
The modern periodic table
Mendeleev's table was good, but as it was arranged in order of atomic mass, there were slight problems. At the beginning of the 20th century scientist disscovered protons, neutrons and electrons and slight ammendments were made to the table to arrange the elements in order of atomic number (proton number).
- Elements in the same group react in similar ways - they have the same number of electrons in their highest energy level
- The group number tells us the number of electrons in the elements outer shell
Group 1 + 2 - reactive metals
Middle - transition metals
Group 0 - noble gases
Group 3-7 - mix of other metals and non-metals
Group 1 (ALKALI METALS)
As you go down group 1 the elements become MORE reactive and have LOWER melting and boiling points. The alkali metals have a low density, the first three elements in the group are less dense than water. they become more reactive as:
- the single elctron is further from the positive nucles so is less strongly attracted to it
- the inner electrons shield the attraction
- they are extremely reactive
- they are soft, can be cut with a knife
- have a silvery shiny surface when first cut but react with oxygen so fast that they soon turn grey
The alkali metals always form ionic compounds with non-metals that carry a 1+ charge. they react with water to produce hydrogen gas. When the alkali metals bond with the halides they all form a white powder that dissolve easily in water.
Group 7 (THE HALOGENS)
As you go down the group the halogens become LESS reactive with HIGHER meltng and boiling points. they become less reactive as it becomes harder to gain an extra electron because the outer shell is further from the positively charged nucleus meaning there is less attraction and also more shielding .
- The halogens are all non-metals with coloured vapours. They all exist as molecules made up of two atoms with covalent bonds.
- They form ionic bonds with metals and become 1- ions called halides
- They form covalent bonds with other non-metals
- A more reactive halogen can displace a less reactive one from solutions of its salts
The transition elements all have similar properties, this is because they of their metallic bonding and giant structures
- Good conductors of heat and electricity
- Hard and strong
- High densities
- High melting points (apart from mercury)
- less reactive than group one elements - corode a lot slower
When they form compounds they are often very colourful, the colour depends on which transition element is present
Transition metals and their compounds are all good catalysts.
Hard water prevents you getting a good lather when washing with soap and forms scum. The scum floats on the water and sticks to the bath. Scum is not formed when washing clothes because of soapless detergents.
Hard water is caused by Calcium and magnesium ions in the water, these ions come from rock that the water must travel over for example limestone, the limestone reacts with acid rain to form the hardness
CaCO3(S) + H2O(l) + CO2 -------> Ca2+(aq) + 2HCO3(aq)
hardnes negaticve charge
Hard water is bad as it wastes money - much more soap has to be used to create a lather as first all the scum must be made removing the calcium and magnesium ions. Not only this but scale can also be formed. Scale is an insoluble solid that forms when hard water is heated. It is commonly formed in pipes, boiler and kettles. It can build up and create blockages and also reduces the efficiency of the kettle. Advantages - calcium ions help the development of strong bones and teeth, it has been suggested that hard water can help reduce heart disease
Removing Hard Water
There are two types of hard water, temporary and permanent.
Temporary hardness forms scale and can be removed boiling, it is caused by the hydrogencarbonate ion HCO3^- in Ca(HCO3). When heated, the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble - this is the scale.
- calcium hydrogencarbonate ----> calcium carbonate + water + carbon dioxide
- Ca(HCO3)2 ------> CaCO3 (scale) + H2O + CO2
Permanent hardness is caused by calcium and magnesium from some salts, such as sulfates, these cannot be removed through heating for example calcium sulfate. There are two ways to remove this hardness.
using waching soda - adding sodium carbonate to the water causes a reaction where the carbonate ions react with the calcium and magnesium ions forming an insoluble precipitate much like scum removing the hardness
using an ion exchange column- The columns contain a resin with either sodium or hrdrogen ions that exchange with the calcium and magnesium ions when water is passed through it. To save replacing the resin, it can be washed with salt (sodium chloride) to refil the sodium ions.
- It passes through a mesh screen to remove leaves and twigs
- Aluminium sulfate and lime are added to clump small pieces of dirt together which then sinks to the bottom and is colleced and removed
- The water is then passed through a filter of fine sand to remove the remaining particles
- Chlorine is added to kill bacteria
- The PH is then checked and then the water is ready to be sent out
Some people want to take this filtering further and buy a filter to fit to their taps. They usualy contain activated carbon, ion exchange resin and silver
- The carbon reduces the amount of chlorine and other impurities in the water
- The resin removes the hardness
- The silver discourages the growth of bacteria within the filter
Distilled water is totally pure water but this process is too expensive for tap water
Flouridation of water has advantages and disadvantages - helps dental hygine, help some forms of heart disease, only tiny amounts - Flourosis, ethics can't control intake
Energy and fuels
Calorimetry is used to calculate fuel energy, it involves burning a fuel and measuring the energy change
- Put 50g of water into a metal can and record the temp
- weigh the spirit purner and lid
- put the lid on the can and spirit burner underneath then light the wick
- stir the water continuously until it reaches 50*C then put out the flame
- weigh the spirit burner and lid again
Energy released = mass of water x specific heat capacity of water x rise in temperature
Mass before = 68.75 Mass after = 67.85 Mass of fuel burned = 0.9
Temp before = 21.5 Temp after = 52.5 Tem change = 31*C
50g x 4.2 x 31 = 6510Joules
0.9g released 6510Joules so 1g will release 6510/0.9 = 7233Joules
Energy transfer in solutions
Energy transfer in solutions can also be measured. You can measure the amount of energy released by a chemical reaction by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature at the end of the reaction. This is true of solids with water reactions and neutralisation reactions.
The biggest problem with the calorimeter in this case is energy being lost to the surroundings, this can be helped by:
- Using a lid
- Using cotton wool to insulate the cup
The same calculation is made as with fuels Q=mc T
Energy Level Diagrams
These show us the relative amounts of energy contained in the reactants and products
In exothermic reactions:
- The products are always lower than the reactants
- The energy change is negative
- The difference in the hieght of the bars represents the energy given out
- The initial rise shows the activation energy needed to break the old bonds to make the new bonds
In endothermic reactions:
- The products are always higher than the reactants
- The energy change is positive
- The difference in height of the bars represents the energy taken in
- The increse in energy needed is to break the old bonds
Catalysts lower the activation energy - this is represented by a lower peak on the curve
Exothermic = making bonds Endothermic = breaking bonds
The energy needed to break the bond between two atoms is called the bond energy. Bond energies are measured in kJ/mol
In exothermic reactions, the energy released making the product is larger than the energy taken in breaking the reactants bonds
In endothermic reactions the energy needed to break the initial bonds is greater than the energy released making new bonds
Hydrogen + Oxygen = Water
The reaction is exothermic and could be used to power engines. If used in a normal combustion engine the only product would be water and lots of energy would be produced. BUT Hydrogen is very hard to store safely and energy from another source would be needed.
A fuel cell uses the energy from the reaction between hydrogen and oxygen to generate electricity to power the car - the fuel cell doesn't run down or need recharging from the mains like a battery it just needs to be supplied with fuel (hydrogen)
Test for positive ions
A FLAME TEST can be used to distuingish certain metal ion as they produce characteristic flames. To carry out a flame test:
- Dip the wire loop in concentrated Hydrochloric acid to clean it
- Put a small amount of the compound to be tested in the nichrome wire loop
- Hold the loop in th eroaring blue flame of a bunsen burner
- Use the colour of the flame to identify the metal ion
Metal ion Flame colour
Lithium (Li+) Crimson
Sodium (Na+) Yellow
Potassium (K+) Lilac
Calcium (Ca2+) Red
Barium (Ba2+) Green
Test for positive ions (Sodium Hydroxide)
Many metal hydroxides are insoluble and precipitate out of solution when formed, some of these have noticable coulors. Therefore, taking taking the mystery compound and adding some Sodium Hydroxide can help identify the metal ion in the compound
Calcium WHITE (red in flame test)
Aluminium(Al3+) WHITE (but in an excess of sodium hydroxide redissolves)
Test for Negative ions
Adding acid to a compound with a carbonate will result in Carbon dioxide gas being formed which can be collected and bubbled through limewater to test.
- Acid + Carbonate ion ------> Carbon Dioxide + Water + Salt
- 2H+(aq) + CO3(aq) ------> CO2(g) + H20(l)
Halides - To test for Chloride, Bromide or Iodide ions, add dilute nitric acid followed by Silver Nitrate solution
- Chloride WHITE
- Bromide CREAM
- Iodide YELLOW
Add dilute Hydrochloric acid followed by Barium Chloride solution. If a WHITE precipitate is formed there was origionally a Sulfate ion.
- Meaure a known volume of alkali into a conical flask using a pipette
- Add an indicator to the solution in the flask
- Pour the acid into the burette and record how much acid there is
- Open the tap slightly to allow the acid into the flask, swirl the flaskwhile doing so
- Continue doing this until the indicator changes and the reaction has met its end point
- Record the new reading of acid and calculate how much acid was used
- Repeat this three time to find an average (discard anomalous results)
- Then if needed repeat without an indicator to avoid contamination
phenolphthalein - colourless in acid alkali, pink alkali
MOLES = CONCENTRAION X VOLUME
MASS = Mr X MOLES
If a reversable reaction takes place in a closed system then a state of equilibrium will always be reached. The reactions still take place in both dirrections but the overall effect is nil, the reactions take place at exactly the same rate in both directions.The position of the equilibrium can be altered by temperature and pressure.
- Raising the pressure favours the reaction that produces less molecules
- Lowering the pressure favours the reaction producing more molecules
- Raising the temperature favours the endothermic reaction, (more heat to take in)
- Lowering the temperature favours the exothermic reaction, (more needs to be given out)
Adding a catalyst does not affect the equilibrium but speeds up the reaction both ways equally.
The Haber Process
Nitrogen and Hydrogen are needed to make Ammonia, Ammonia is a very good fertilizer for crops as it contains a double source of nitrogen needed for plant growth.
The nitrigen is obtained from the air and the hydrogen from natural gas or other source like crude oil. The reaction is reversable so it reaches an equilibrium.
In industry the conditions used are:
- 200 atmospheres
- Iron catalyst
N2(g) + 3H2(g) <========> 2NH3(g)
The ammonia is formed as a gas but liquifies in the condenser and is removed
The unused Hydrogen and Nitrogen are recycled so nothing is wasted
An alcohol takes a -H atom from an alkane and replaces it with -OH, this is their functional group. The functional group gives the compound their characteristic reactions.The first three alcohols are:
- Methanol - CH3OH
- Ethanol - CH3CH2OH
- Propanol - CH3CH2CH2OH
Alcohols are flamable, they burn in oxygen(air) to form carbon dioxide and water
2CH3OH(l) + 3O2(g) ----> 2CO2(g) + 4H2O(g)
- The first three alcohols dissolve completely in water to form neutral solutions
- They also react with Sodium to give Hydrogen and Alkoxides, not as vigorous as when added to water, just fizzing
- Alcohols are used as solvents as they can dissolve things that water can't such as Hydrocarbons and oils this makes them good for perfumes or Methylated spirits
- Alcohols can be used for fuels
- Alcohols can be also oxidised by chemical oxidising agents or microbal action turning them to acids
Carboxilic acids have the functional group -COOH with a double C=O bond and a C-O-H bond. The first three are : Methanoic acid, Ethanoic acid and Propanoic acid. They are made from the oxydisation of alcohols.
- They dissolve in water to produce acidic solutions
- They react with carbonates to produce carbon dioxide
- They react with alcohols in the presence of an acid catalyst to produce esters
- They do not ionise completely when dissolved in water and so are weak acids
- Aqueous solutions of weak acids have a higher pH value than aqueous solutions of strong acids with the same concentration.
Esters have the functional group –COO–. Esters are formed from an alcohol and a carboxilic acid, an acid catalyst is normally used
Alcohol + Carboxylic acid -----> Ester + Water
Ethanol+ Ethanoic acid ------> Ethyl Ethanoate + Water
They are volatile compounds with distinctive smells and are used as flavourings and perfumes.Many esters are flammable and they are not as soluble as alcohols and carboxilic acids.