When heated some elements produces flames with a distinctive colour e.g. Lithium produces a red flame.
Sodium produces a yellow/ orange flame
Potassium produces a lilac flame
- Each element gives a charecteristic line spectrum
- When heated the electrons in an atom get excited and release energy as light
- The wavelengths emitted can be recorded as a line spectrum
- Different elements emit different wavelengths of light
- So many elements has a different pattern of wavelengths and a different line spectrum
- This means that line spectrums can be used to identify line spectrums
Line spectrums have identified new elements e.g. Caesium and rubidium. Helium was discovered in the line spectrum of the Sun
The modern periodic table
Before in the 1800s elements were arranged in order of relative atmoic mass
Dobereiner tried to organise elements into triads in 1828. These triads were based on chemical properties. The middle element of each triad had a relative atomic mass that was the average of the other two.
Newland notices that when you arranged elements in order of relative atomic mass every eighth element had similar properties. These sets of eight were called Newlands' Octaves. Unfortunately the pattern broke down of the third row. It was because he left no gaps for future elements that his work was ignored. Some of the groups didn't have similar properties and he mixed up metals and non metals.
Mendeleev arranged 50 known elements into a table of elements with various gaps. Mendeleev put the elements in order of atomic mass. But found that he had to leave gaps to keep elements with similar properties in the same groups. The gaps were clever because they predicted the properties of undiscovered elements. When they were found and they fitted the pattern they confirmed Mendeleev's ideas
Some elements have just one electron in their outer shell and they'd rather lose an electron so that they could have a full shell.
A nearly full shell would like to gain extra electrons.
Ionic bonding allows electrons to transfer.
Ionic compunds form a regular lattice
- Solid ionic compounds like sodium chloride are made up of a giant lattice of ions. Each lattice forms a single crystal
- When ionic compounds become molten or are dissolves in water they conduct electricity because the ions are able to move
Group 1- The Alkali Metals
As you go down group one, the alkali metals become more:
- Reactive because the outer electron is more easily lost, because it's further away from the nucleus
- Have a higher density because the atoms have more mass
- Have a lower melting point
- Have a lower boiling point
The alkali metals are shiny when freshly cut, but quickly react with the oxygen in moist air and tarinish
Group 7- Halogens
The halogens form diatomic molecules which are pairs of atoms e.g. cl2
They have 7 outer electrons
As you go down group 7 the Halogens:
- Become less reactive because the outer electrons are further away from the nuclus and so additional electrons are attracted less stronly
- Have a higher melting point
- Have a higher boiling point
The halogens are all non metals with coloured vapours
- Fluorine is a very reactive, poisonous yellow gas at room temperature and pressure
- Chlorine is a fairly reactive. poisonous dense green gas at room temperature and pressure
- Bromine is a dense poisounous orange volatile liquid at room temperature and pressure and forms an orange gas
- Iodine is a dark grey cristalline solid at room temperature
Chemicals in the Atmosphere
Dry air is a mixture of many gases. Some on these gases are elements e.g. oxygen, nitrogen and argon.
Other gases are compounds e.g. Carbon Dioxide
The atmosphere is 78% Nitrogen
Molecular substances useually exist as small moleules e.g. CO2 and H2O these have weak forces of attraction however strong covalent bonds
You only need a small amount of energy to overcome the weak forces between the molecules so they have low melting and boiling point
This means they're usually gases/liquids at room temperature
Covalent bonding is the sharing of electrons
This way both atoms feel like they have a full outer shell
The atoms bond due to the electrostatic attraction between the positive nuclei and the negative electrons shared between them
Chemicals in the Hydrosphere
The Earth's Hydrosphere cantains many ionic compounds called salts- this is why the sea water is salty
Solid Ionic Compounds Form Crystals
- Ionic compounds are made of charged particles called ions
- Ions with opposite charges are strongly attracted to one another
- There are very strong ionic bonds between all the ions
- Ionic compounds have high melting and boiling points
- The forces of attraction between the ions are verystrong
- It takes a lot of energy to overcome these forces and melt the compound, and even more energy to boil it.
- So ionic compounds have high melting and boiling points which makes them solid at room temperature
- When an ionic compound dissolves the ions seperate and are all free to move
- This means they're able to carry an electric current
- Similiarly, when an ionic compound melts, the ions are again free to move. So they'll carry an electronic current
Chemicals in the Lithosphere
The Earth's lithosphere is the Earth's rigid outer layer- the crust and part of the mantle. It's made up of a mixture of minerals, often containing silicon, oxygen and aluminium. Carbon forms giant Covalent structures
- The carbon atoms in Diamond each form four covalent bonds in a very rigid giant covalent structure
- This structure makes diamond the harderst natural substance
- All those strong covalent bonds give diamond a very high melting point
- It doesn't conduct electricty because it has no free electrons even when molten
- It's insoluble in water Sillicon dioxide is similar to diamond
- Each carbon forms three covalent bonds creating sheets of carbon atoms which are free to slide over each other.
- The layers are held together so loosely that they can be rbeed off onto paper to leave a black mark
- Graphite has a high melting point- the covalent bond needs lots of energy to break
Electrolysis is the splitting of substances using electricity
Electrolysis is a useful way of extracting reactive metals from ore
- The main ore of aluminium is bauxite, which contains aluminum oxide AL2O3
- Molten aluminium oxide contains free ions- so it'll conduct electricity
- The positive AL3+ ions are attracted to the negative electrode where they each pick up three electrons and then turn into neutral aluminum atoms. These then sink to the bottom
- The negative ions are attracted to the positive electrode where they lose two electrons. The neutral oxygen atoms will then combine to form O2 molecules
Metals consist of a giant structure.
Metallic ions involve free electrons which produce all the properties of metals
These free electrons come from the outer shell of every metal atom in the structure.
The positively charged metal ions are held together in a crystal by a sea of free electrons that can move.
- The free electrons carry both heat alectrical current through the metal, so metals are good conductors of heat and electricity.
- Metalic bonds mean metals have high tensile strength ( they're strong and hard to break)
- Metallic bonds are very strong, so it takes a lot of energy to break them-- you have to get the metal pretty hot to melt it
C6:Acids and Alkalis
Acid + Alkali --> Salt + Water
pH meters can be used to measure the pH of a substance. This tells you the exact pH of the substance.
Litmus paper tells you whether a solution is acidic or alkaline. Red=acidic blue=alkaline
Solid acids- citric acid and tartaric acid. Liquid= Sulfuric acid and nitric acid gases= hydrogen ch
Common alkalis= sodium hydroxide, potassium hydroxide and calcium hydroxide
Energy Transfer is Reactions
An exothermic reaction is one which gives out energy to the surroundings. This is shown by a rise in temperature e.g. most reactions
An endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and shown by a fall in temperature e.g. photosynthesis
Rates of reaction
The rate of a chemical reaction is how fast the reactants are changed into product
The rate of reaction depends on four things: Temperature, Concentration, Catalyst and Surface Area
More collisions increase the rate of reaction
When the temperature is increased the particles gain energy and therefore move faster. If they're moving faster, they're going to collide more frequently
If a solution is more concentrated it means there are more particles of rectant knocking about. This makes collisions between the reactant particles more likely
If one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area. This means that the particles around it will have more area to work on so there will be collisions more frequently
A catalyst works by giving the reacting particles a surface to stick to where they can bump into each other- increasing the number of successful collisions
Measuring rates of reaction
- Observe a mark and measure how long it takes for it to dissapear. This works if the intital solution is colourless
- Disadvantage- results are subjective- different people might not agree with the exact point when the mark disappears
Change in mass
- Measuring the speed of a reaction that produces a gas can b carried out using a mass balance. As the gas is released the mas disappearing is easily measured on the balance
- The quicker the reading on the balance drops, the faster the reaction
- This quite accurate but it has the disadvantage of releasing gas straight into the room
The volume of gas given off
- Use a gas syringe to measure the volume of gas given off
- The more gas given off during a time interval the faster the reaction
- A graph of gas volume against time could be plotted to give a rate of reaction graph
Titrations are carried out to check the purity of products
How to do a titration:
- Add a known volume of alkali to a titration flask, along with two or three drops of indicator
- Fill a burette with the acid.
- Using the burette add the acid to the alkali a bit at a time- giving the conical flask a regular swirl so that the acid and alkali mix completely
- Go slow when you think the end-point is about to be reached so that no more acid is added than is necessary to neuralise the alkali
- This indicator changes colour when all the alkali has been neutralised
- Record the voume of acid used to neuralise the alkali