History of the periodic table
- Until recently, there were 2 ways to categorise elements: Their physical/chemical properties, or their relative atomic mass.
- They had no idea of atomic structure, protons, electrons or neutrons and the only thing they could measure was the RAM and so they were aranged in RAM order.
Newlands law of octaves (1864)
- He noticed that every 8th element had similar properties and so listed them in rows of 7.
- The pattern broke down on the 3rd row with the transition metals messing it up.
- He presented his ideas to the chemical society in 1865 but they were rejected because: He mixed metals and non-metals, He didn't leave any gaps for undiscovered elements, His groups contained elements that didn't have similar properties.
History of the periodic table continued...
Mendeleev- In 1869 Mendeleev in Russia made a new table of elements with various gaps left
- He put them in order of atomic mass but he knew he had to leave gaps in order to keep elements with similar properties in vertical groups, with huge gaps in the first 2 rows for the transition metals.
- The gaps were the clever bit because they predicted the properties of the undiscovered elements and when they were found they fitted the pattern nicely.
The modern periodic table
- There wasn't much evidence to prove that certain elements went together when the table was first released so not all scientists took it seriously.
- However, Mendeleev's table helped show scientists it was actually really useful.
- In the late 19th century, protons, neutrons and electrons were discovered and the periodic table matched up well to the structure of the atoms.
The modern periodic table
- The periodic table was arranged in order of proton (atomic) number when p, n and e's were discovered.
- The periodic table can be used to work out the detailed arrangement of electrons and once you know this you can work out the chemical properties.
- The maximum number of electrons that can occupy each energy level is given by the formula 2 X n^2. Where n is the number of the energy level.
- Apart from the transition metals, all elements in the same group have the same number of electrons in their outer shell.
- The attraction of the nucleus is less when there are lots of inner electrons and this is known as sheilding.
- Increased distance and sheilding means an electron in a higher energy level can be more easily lost because of less attraction. Plus, it is less likely that the higher energy level will gain an electron.
Group 1, the alkali metals
- They are all silvery solids and their hydroxides dissolve in water to give an alkaline solution.
As you go down the group, the metals become: More reactive, Bigger atoms, Higher density, Lower melting and boiling point.
- Alkali metals are very reactive
- Lithium, Sodium, Potassium and a couple more
- They all have one outer shell electron which is why they are reactive
- They all form 1+ ions
- Always form ionic compounds
- When reacted with water, produce hydrogen gas: they react vigorously, fizzing around..
2Na +2H20 ----> 2NaOH + H2
Group 7, the halogens
- As you go down the group: They become less reactive and have higher melting and boiling points. - Halogens are all non metals with coloured vapours.
- Fluorine is a very reactive, poisonous yellow gas
- Chlorine is a fairly reactive, poisonous, dense green gas
- Bromine is a dense, poisonous, volatile red-brown liquid.
- Iodine is a dark grey crystalline solid or a purple vapour.
They all form molecules which are pairs of atoms (e.g. F2, Cl2, Br2, I2)
They form 1- ions when bonded with metals, and they form covalent bonds with non-metals.
They react with metals to form salts ... 2Al +3Cl ---> 2AlCl3
More reactive halogens displace less reactive ones... Cl2 + 2Kl ---> I2 + 2KCl
- They are good conductors of heat and electricity. They are dense, strong and shiny. Much less reactive than group 1. Very high boiling points.
- Transition metals often have more than one ion. And they normally form different coloured compounds...Fe2+ ions give green compounds and Fe3+ ions normally give red/ brown compounds (such as rust).
- Compounds are colourful as a result of the transition metal ion they contain.
- Colour of hair, in gemstones, and pottery glazes are all due to transition metals.
- Transition metals and their compounds all make good catalysts. Iron is used in the Haber Process, Nickel turns oils into fats for margarine, and Manganese(IV) oxide is good for the decomposition of hydrogen peroxide.
- The transition metals put their elements in the overlapping 3rd energy shell until its full (e.g. Iron = 2,8,14,2).
Acids, Alkalis and Titration
- Strong acids ionise almost completely in water, meaning every H+ ions turns into a hydrated proton.
- Weak acids ionise slightly, meaning only small number of H+ are formed.
- The pH of acid/alkali is measured by the concentration of H+ ions in a solution.
Titrations are used to find out concetrations
- Allow you to find out exactly how much acid is needed to neutralise an alkali (v-v)
- Put alkali in a flask with indicator (Phenolphthalein for weak acid and strong alkali, or Methyl Orange for strong acid and weak alkali). If both are strong, either.
- Adding acid a bit at a time using a burette, the indicator will change colour when all of the alkali is neutralised.
- The amount of acid used is recorded.
The water cycle:
The sun causes water from the sea to evaporate, as the vapour rises it cools and so the water condenses to form clouds. When the condensed droplets get to big, they fall as rain, the water then runs back to the sea and the cycle starts again.
Water is a solvent - It dissolves many other elements
Water dissolves most ionic compounds. Water molecules surround ions and disrupt the ionic bonding so it gradually falls apart.
Salts of sodium, potassium and ammonium dissolve.
Nitrates, chlorides and sulfates dissolve.
The solubility of a substance in a given solvent is the number of grams of the solute that dissolve in 100g of the solvent at a particular temperature
Solubility usually increases with temperature.
A saturated solution is one that cannot hold any more solid at that temperature.
A solubility curve plots the mass of solute dissolved in a saturated solution at various temperatures.
All gases are soluble to some extent
The amount of gas that dissolves is dependant on the pressure of the gas above it - the higher the pressure, the more the gas that dissolves.
Gases become less soluble as the temperature of the solvent increases (oppposite of solids).
- Hard water when mixed with soap forms a nasty scum as the 'hardness minerals' react with the soap.
- It also forms scale (mostly calcium carbonate) on the inside of pipes, boilers and kettles.
- Scale acts as a bit of a thermal insulator which is why an older/dirtier kettle is more efficient than a newer one.
- Most water is hard because it contains a lot of magnesium and calcium ions.
- Hardness can come from limestone, chalk and gypsum.
- When carbon dioxide dissolves in water you get a carbonic acid.
CO2 + H2O ---->H2CO3 This is why rainwater is slightly acidic.
- If there's calcium carbonate (CaCO3) then calcium hydrogen carbonate is formed
H2CO3 + CaCO3 -----> Ca(HCO3)2
Hard water continued...
- Ca2+ ions are good for healthy teeth and bones.
- Scale inside pipes form a protective coating which prevents poisonous ions getting into drinking water.
Remove hardness by removing dissolved Ca2+ and Mg2+ ions
1) By adding Sodium Carbonate. The carbonate ions join onto the calcium or magnesium ions and form an insoluble precipitate.
e.g. Ca2+ + CO32- ---->CaCO3
2) By 'ion exchange columns'. A water supply can be fed through an IEC to remove the hardness
e.g. Na2(resin) + Ca2+ ----> Ca(resin) + 2Na
3) Scale is mainly just calcium carbonate and can be dissolved by acid. Most de-scaling products that you buy to clean your kettle is some kind of acid.
- Micro-organisms in water can cause diseases such as Cholera and Dysentery.
Treatment of water from reservoirs:
1) Water passes through a mesh screen to remove big bits like twigs.
2) Next it's treated with ozone or chlorine to kill micro-organisms.
3) Chemicals are added to make solids and micro-organisms stick together and fall to the bottom. Bacteria are used to remove nitrates.
4) The water is filtered through gravel beds to remove all the solids. Nasty tastes and odours can be removed by passing water through 'activated carbon' filters or with 'carbon slurry'.
5) The pH is corrected if the water is too acidic or alkaline.
6) Water is chlorated to kill off any harmful micro-organisms left.
- To monitor water quality, companies take samples of water.
- Some people who still aren't satisfied buy filters that contain carbon or silver. Carbon removes the chlorine taste and silver kills bugs.
- Some people also buy 'water softeners' which contain ion exchange resins.
- Totally pure water can be produced by distillation - boiling water to make steam, and condensing the steam - but this process is too expensive to produce tap water..it is mainly for chemistry labs.
- Some purifying water processes can damage the environment.
Exothermic - Reaction gives out energy to its surroundings, usually in the form of heat.
Endothermic - Takes in energy from the surroundings, usually in the form of heat.
Measuring energy transfer:
- Can be measured by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature at the end.
- Problem is the amount of energy lost to the surroundings.
- Can be reduced by putting a lid on the cup or putting it in a beaker with cotton wool.
- This method works for reactions of solids with water and neutralisation reactions.
Energy is always required to break bonds and energy is released when bonds form.
- During a reaction, old bonds are broken and new bonds are formed.
- Energy is supplied to break existing bonds so bond breaking is endothermic.
- Energy is released when new bonds are formed, so it is exothermic.
- In an endothermic reaction, the amount of energy supplied to break the bonds is greater than the energy released when new bonds are formed.
- In exothermic, it is the other way around.
Energy and fuels, Bond energies
- To measure the energy produced when fuel is burned, you can burn the fuel and use the flame to heat up water - Calorimetry.
- This uses a metal container, usually copper, because it conducts heat so well.
- In exothermic reactions, deltaH is -ve. This means the products are at a lower energy than the reactants. The initial rise is the activation energy used to break the bonds.
- The activation energy required can be lowered by catalysts. Using a catalyst however does not affect the overall energy change.
- Known bond energies can be used to calculate overall energy change for a reaction.
Energy and food
- Energy is often measured in calories and kilocalories.
- 1 calorie = amount of energy needed to raise 1g of water by 1 degree C = 4.2 joules.
- The dietary information on food labels is in kilocalories (Kcal) (C not c)
- Foods with high proportions of fats and oils produce large amounts of energy.
- Carbohydrates produce some, but less than fats and oils.
- Protein is about the same, but we don't use the energy in our bodies.
- The energy in food is released by respiration. Excess energy is stored in the body as fat.
- If the food you eat contains less energy than your body needs, it will start to use the stored fat.
Tests for cations
Lithium -(Li+) Burns with a crimson-red flame
Sodium -(Na+) Burns with a orange-yellow flame
Potassium -(K+) Burns with a lilac flame
Calcium -(Ca2+) Burns with a brick-red flame
Barium -(Ba2+) Burns with a green flame
Some metals form a coloured precipitate with NaOH
Ammonium compound + NaOH gives off (stinky) ammonia
- You can tell if ammonia is about by the smell - cat wee.
- Or use damp red litmus paper - ammonia will turn it blue.
Tests for Anions
Testing for carbonates - Check for CO2. - Bubble it through limewater - it will go milky if CO2 is present.
- Acid + Carbonate ---> Salt + Water + CO2
Test for sulfates (SO4 2-) and Halides (Cl-, Br-, I-)
- You can test for certain ions by seeing if a precipitate is formed.
To test for sulfate ions, add dilute HCl followed by Barium chloride (BaCl2). If there is a white precipitate of barium sulfate, the original compound was a sulfate.
To test for Bromide, Iodide or Chloride ions, add dilute nitric acid (HNO3) followed by silver nitrate solution (AgNO3).
A Chloride gives a White precipitate of silver chloride
A Bromide gives a Cream precipitate of Silver bromide
An Iodide gives a yellow precipitate of Silver iodide.
Tests for organic compounds
Test for nitrates (NO3-) produces ammonia.
- Mix compound with aluminium powder, add a few drops of sodium hydroxide solution and heat, it will be reduced to ammonia.
Organic compounds burn when heated- They burn in air with an orange-yellow and/or Blue flame. The greater the proportion of carbon, the more yellowy and smokey the flame is.
- When there's plenty of air, burning a hydrocarbon produces carbon dioxide and water. If the amount of air available is reduced, then carbon monoxide and carbon (soot) can also be produced.
- Solid organic compounds will char - surfaces will get scorched with black marks of carbon.
Compounds with C=C bonds decolourise bromine water
- If the organic compound is unsaturated, it will decolourise bromine water.
- If it is saturated, the bromine water will stay brown.
Atomic absorption spectroscopy identifies metals
- Patterns of light absorbed by the metals are analysed. Each metal present in the sample produces a different pattern.
- Much faster and much more reliable than human eye.
- The steel industry uses atomic absorption spectroscopy to check the composition of the steels it produces. It takes minutes as opposed to days in the lab.
Infra-red or Ultraviolet spectrometry - Identifies which frequencies of EM radiation are absorbed. The pattern for each is unique.
Nuclear Magnetic Resonance spectrometry - Used for organic compounds, shows what the hydrogen atoms are connected to and the structure of the molecule.
Gas-Liquid chromatography - Similar to paper chrom.. but with G/L
Mass spectrometry - Can be used for elements and compounds. Tells you the mass of each molecule/particle.