- Created by: CessieJakes
- Created on: 22-05-12 10:49
History of the Periodic Table
Newlands Law of Octaves - John Newlands created this in 1864. He felt that because musical notes could be arranged in groups of 8, so could the elements. Every eighth element (when orded by atomic mass), had similar properties so he arranged them in rows of seven. However, the pattern broke down after the third row because some of the transition metals got messed up. His work was criticised for 3 main reasons:
- His groups contained elements with different properties
- He mixed up metal and non-metals
- He didn't leave gaps for undiscovered elements
Dmitri Mendeleev - A few years later, in 1869, a man called Dmitri Mendeleev came along. He drew up his own idea for the Table of Elements but he changed a few key things. He put the elements in order of atomic mass but left gaps to keep elements with similar properties together. He also swapped the odd few elements around because he found they fitted better somewhere else. This caused his work to be questioned but because he had built it around a previous idea it had more weight. Also, the gaps he left were where newly discovered elements fitted perfectly.
The Modern Periodic Table
Our modern periodic table is arranged in order of atomic number (the number of protons/electrons). This means that elements in the same group (column) have the same number of electrons in their outer shell and similar properties. The elements in the same period (row) have the same number of shells/energy levels.
Also, for metals, the more shells there are, the weaker the attraction between the outermost electron(s) and the nucleus of the atom (because the distance is bigger), and so the easier it is for the element to lose the electron(s), thus making the element more reactive (e.g. Francium). This is the opposite for non-metals and so the more shells there are, the weaker the attraction between the outermost electron(s) and the nucleus (because the distance is bigger), and so it is harder for the element to gain an electron, thus making the element less reactive (e.g. Astatine).
Group 1 - The Alkali Metals
There are a number of trends you need to learn. As you go down Group 1 the alkali metals become:
- bigger atoms
- more reactive
- high density
- lower melting and boiling points
So more/higher of everything apart from the melting and boiling points.
The alkali metals are really reactive and stored under oil. They have one outer electron and that is why they are in Group 1. They always form ionic compounds and always form 1+ ions. Reactions with water produce Hydrogen gas e.g.
2Na(s) + 2H2O(l) ----> 2NaOH(aq) + H2(g)
Group 7 - The Halogens
The trends are slightly different. As you down Group 7, the Halogens:
- Get less reactive
- Have higher melting and boiling points
They are all non metals with coloured vapours. For example, Fluorine is a poisonous yellow gas. They all form molecules which are pairs of atoms so chlorine becomes Cl2. They form both ionic bonds and covalent bonds as well as forming 1- ions. They react with metals to form salts:
2Al(s) + 3Cl2(g) ----> 2AlCl3(s)
More reactive Halogens will displace less reactive ones:
Cl2(g) + 2Kl(aq) ----> I2(aq) + 2KCl(aq)
The transition metals have a number of properties that you need to know:
- They are good conductors of heat and electricity
- They're very dense, strong and shiny
- They are fairly unreactive and don't react with water or oxygen.
- They have high melting and boiling points
- They often form more than one ion e.g. Fe2+, Fe3+
- The compounds are very colourful e.g. Copper(II) Sulphate is blue
- Transition metals and their compounds make good catalysts
- Their properties are due to the way their electron shells fill (see below)
As you get further away from the nucleus of an atom, the energy levels get closer until they overlap. This happens for the first time between energy levels 3 and 4. Transition metals do a strange thing where they fill up the 4th energy level first. Take Potassium, it fills up 2,8,8,1 just like normal. Calcium then fills up 2,8,8,2 - fine. Then, the elements go back to the 3rd energy level and fill that up. So it becomes 2,8,9,2 then 2,8,10,2 and so on.
Acids and Alkalis
Arrhenius said acids release hydrogen ions in water.
HCl(g) + H2O(l) ----> H+(aq) +Cl-(aq)
He also said that alkalis form OH- ions when in water. This worked well but it only worked for acids and bases that dissolved in water. Ammonia gas can react as a base when it isn't in water and it was reasons like this that stopped his ideas from being accepted. Also, charged subatomic particles hadn't been discovered yet so the idea of charged ions was a bit strange.
Lowry and Bronsted then came along and, working separately, came up with the a more general theory. They both said that Acids are proton donors (release H+ ions) and Bases are proton acceptors (accept H+ ions).These ideas were readily accepted because they explained things in more detail. Also, they were an adaptation of Arrehenius' ideas, not something completely new.
In acidic solutions the acid molecules dissociate, releasing lots of H+ ions. These become hydrated protons (H+(aq))
In basic solutions the water molecules dissociate into H+ and OH- ions (although they don't normall do this in pure water). Some base molecules (like NH3) can take hydrogen ions from water, causing more molecules to dissociate and leave excess OH- ions. Other bases release OH- ions straight into the solution.
Acids can be strong or weak. This is normally measured by how much the acid ionises in water. If it is strong then it fully ionises in water and if it is weak it only partially ionises in water. This basically means that a strong acid forms lots of H+ ions in water and a weak one only forms a few.
Titrations mean you can find out exactly how much acid is needed to neutralise a quantity of alkali and the same in reverse. You put a known volume of alkali in a flask (measured with a pipette) with some indicator and add put the acid in a burette. Add the acid a small amount at a time and swirl the flask to mix the two together. When the indicator changes colour when the end point (neutralisation) is reached.
There are two types of indicator that can be used (definitely not Universal Indicator!):
- Phenolphthalein for weak acid - strong alkali
- Methyl Orange for strong acid - weak alkali
To remember these I look at which one is strong (so either acid or alkali) and then I remember that an acid goes orange with UI and so use Methyl Orange for a strong acid, and an alkali goes purple so use Phenolphthalein for a strong alkali.
This can be really tricky so I'm just going to talk you through an example. You will need to practise these loads so you know you can do whatever they throw at you.
The student found that 25.0 cm3 of ammonia solution reacted completely with 32.0 cm3 of sulfuric acid of concentration 0.050 moles per cubic decimetre. Calculate the concentration of this ammonia solution in moles per cubic decimetre. Question from AQA C3 Higher Tier - June 2011. They own the copyright etc.
The first step is to work out which solution you know both facts about. For this question it is the sulfuric acid. You have the concentration and the volume so you need to work out the moles. The triangle is:
moles I remember this with the phrase "cook my venison" which is just a little phrase that I made up jokingly whilst trying to find out a decent phrase and it ended up sticking!
So you have to multiply the concentration by the volume. However, the volume is currently in cm3 and you need it in dm3 because that's what they want the answer in so you have to do 0.050 * (32.0/1000) = 0.0016moles of sulfuric acid.
Titration Calculations (Continued)
Next, write out the equation (or if they've given it to you) to work out how what the mole to mole ratio of the two reactants is. In this case it is 1 mole of sulfuric acid:2 moles of ammonia solution. Therefore, you have to take the 0.0016 and multiply it by 2 ( you have to work it out for each situation). So 0.0016 * 2 = 0.0032 moles of ammonia solution.
Finally, return to the triangle (cook my venison) and plug in the values you have. You end up with 0.0032/(25/1000) = 0.128moles/dm3 as the concentration of the ammonia solution.
After this, they may ask you to calculate the concentration in grams. This is really simple! Work out the Mr of the ammonia solution (which is 17). Then, simply do 0.128 * 17 = 2.176g/dm3!
Remember, practise makes perfect!
The Water Cycle is basically the system that recycles our water so that it never runs out. The Sun causes evaporation of water from the sea. It becomes water vapour and rises. As it rises, the vapour cools down and the water condenses to form clouds. When these condensed droplets get too big they fall as rain. The water then runs back to the sea whilst travelling over rocks in rivers etc. This means that minerals dissolve in the water. The cycle then starts again!
Water dissolves mostly ionic compounds. Water molecules surround the ions and disrupt the bonding so the structure falls apart. Water molecules are polar (hydrogen is positive, oxygen is negative) and so the opposite ions attract each other.
Sodium, Potassium and Ammonium dissolve.
All chlorides except Silver and Lead.
All Sulfates except Barium and Lead. Calcium Sulfate is only slightly soluble.
Solubility - the number of grams of the solute that dissolve in 100g of the solvent at a particular temperature.
The solubility of solutes normally increases with temperature.
Saturated solution - when no more solute will dissolve at that temperature.
Solubility curves - when you cool a saturated solution some of the solid will crystallise out. To work out what mass will crystallise out when cooled from 80C to 10C then find out how much dissolves at 10C and take that away from what dissolves at 80C. The difference is the answer.
All gases are soluble but the higher the pressure the more gas that dissolves and the lower the temperature, the more gas that dissolves. This is why your fizzy drink can bubble up so much when you take the lid off. The pressure is released and sometimes there is a change in temperature too. this means some of the gas fizzes out of the solution.
Hard water won't easily form a lather with soap, a scum forms instead (calcium stearate). It also forms limescale (calcium carbonate) on pipes and kettles. This can reduce the efficiency of heating systems due to blocking pipes and being a thermal insulator.
Hardness is caused by Ca2+ and Mg2+ ions (calcium and magnesium). Certain areas suffer from hard water due to the composite of their rocks in the area. Places with lots of limestone, chalk and gypsum are likely to suffer from this.
Hard water can be good for our teeth and bones because of the calcium in it. It can also stop poisonous lead and copper from the pipes getting into drinking water.
Removing Hard Water
Removing Hardness can either happen by adding sodium carbonate:
Ca2+(aq) + CO32-(aq) ----> CaCO3(s)
By boiling it:
Ca(HCO3)2 → CaCO3 + CO2 + H2O
Or by using an ion exchange column that contains a carbon or silver resin. This removes the calcium and magnesium ions and exchanges them for sodium and hydrogen ions. Other filters remove the chlorine taste (carbon does this) and kill microbes (silver does this).
Drinking water needs to be good quality so that it doesn't harm anyone. There are 6 main cleaning processes the water goes through:
- a mesh screen is used to remove large twigs etc.
- it's treated with ozone and chlorine to kill microbes
- chemicals are added to make solids and microbes stick together and fall to the bottom
- it is then filtered again through gravel beds and activated carbon filters to remove solids and bad tastes
- the pH is corrected
- water is chlorinated again to kill microbes
Exothermic - a reaction that gives out energy, usually shown by a rise in temperature
Endothermic - a reaction that takes in energy, usually shown by a fall in temperature
Energy transfer can be measured. Here is an example experiment (from my CGP guide!):
Place 25cm3 of dilute hydrochloric acid in a polystyrene cup and record the temperature of the acid.
Put 25cm3 of dilute sodium hydroxide in a measure cylinder and record it's temperature.
Add the alkali to the acid and stir.
Take the temperature every 30 seconds and record the highest temperature it reaches.
Endothermic is when the energy required to break old bonds is greater than the energy released when new bonds formed.
Exothermic is when the energy required to break old bonds is less than the energy released when new bonds formed.
Energy and Fuels
Fuel energy is calculated using Calorimetry. It takes 4.2 joules of energy to heat up 1g of water by 1C. The equation to work out the energy per gram of a certain fuel is: specific heat capacity * mass of water * temperature change
You can then plug this in for any experiment (they may have to work out the temperature difference and maybe the weight too but this is normally just simple subtraction!). e.g.:
4.2 * 100g * 31C = 13020 joules
They may then ask you for the energy produced by 1g of the fuel, rather than the amount used in the experiment. e.g.:
13020/0.7 = 18600 joules.
Remember though that this figure may not be accurate due to heat lose through the air and the can etc.
Fuels provide energy but have consequences. List some of the problems involved with burning fuels. (e.g. pollution, fossil fuels etc.)
In exothermic reactions the energy change is negative and an endothermic reaction is positive. This can be seen easily with the energy level diagrams because an exothermic reaction will loop downwards and an endothermic will loop upwards. The difference in height of the products and reactants is the energy taken in/released during the reaction.
The activation energy is the minimum energy needed by reacting particles for the reaction to occur and is represented by the loop from reactants to the highest point. This is lowered by catalysts meaning it is easier to start the reaction.
You can calculate the energy needed to break/make a bond. You will be given the bond energies for each particular bond. You just add up the reactants and add up the products and then take the products away from the reactants. A negative number means an exothermic reaction and a positive number is an endothermic reaction.
Energy and Food
Food energy is often measured in calories and Kilocalories. 1 calorie = the amount of energy needed to raise the temperature of 1g of water by 1C. So 1 calorie = 4.2 joules.
Food labels are very confusing because they often write kilocalories as Calories with a capital C just to confuse you! 1 Calorie is the amount needed to raise the temperature or 1kg of water by 1C. So 1 Calorie = 4200 joules.
The composition of foods determine how much energy it provides us. For instance, fats produce large amounts of energy. However, if we take in more energy than we use, the excess is stored. Too much of this too often will mean an increase in body weight which could lead to obesity.
The energy in food is used for respiration (glucose + oxygen ----> carbon dioxide + water + energy). If the food you eat does not contain contain enough energy then your body draws on it's stores. If this happens often then it will mean a decrease in body weight.
Unfortunately you need to learn all of these and I don't have any fun rhymes and sayings so you are just going to have to learn it!
Flame Tests With NaOH
Lithium - Bright Red Calcium - White Precipitate
Sodium - Yellow/Orange Copper(II) - Blue Precipitate
Potassium - Lilac Iron(II) - Green Precipitate
Calcium - Brick Red Iron(III) - Brown Precipitate (Iron 3 is the trunk of a tree!)
Barium - Green Aluminium - White, then redissolves (in excess NaOH)
Magnesium - White Precipitate
If you mix an Ammonium compound with NaOH (Sodium Hydroxide) you get Ammonia which turns damp, red litmus paper blue. It is also very smelly.
Chemical Tests (continued)
Carbonates - add a dilute acid and then bubble the gas through limewater. If the limewater goes cloudy then you have got a carbonate! Some also change colour when they decompose. Copper carbonate turns from green to black and zinc carbonate goes from white to yellow but when it cools it turns back to white.
Sulfates - Add acidified Barium Chloride (CGP say to add dilute HCl and then Barium Chloride but I have found at times that this isn't accepted in some past papers I've done). A white precipitate forms if it is a sulfate.
Halides - add dilute nitric acid and then silver nitrate solution. Chloride forms a white precipitate, bromide a cream one, and iodide a yellow one.
Nitrates - Add aluminium powder and then NaOH then heat. Ammonia is produced. Remember, this is smelly and turns damp red litmus paper blue.
Organic Compounds and Instrumental Methods
Organic compounds burn when heated with a yellow/orange flame and produce carbon dioxide and water. If there is less air available then carbon monoxide and carbon is formed.
Unsaturated compounds (C=C bonds) decolourise bromine water. If it is a saturated compound (C-C bonds) the bromine water will stay brown.
Instrumental Methods - machines can be used to analyse substances. There are 6 methods you need to know:
- Atomic Absorption Spectroscopy identifies metals
- Infrared Spectrometry identifies compounds
- Ultraviolet Spectroscopy identifies compounds
- Nuclear Magnetic Resonance Spectroscopy identifies organic compounds
- Gas-Liquid Chromatography identifies gases and liquids
- Mass spectrometry identifies elements and compounds
Again, this just needs to be practised. Here is a walk through of an example (Aqa Chemistry Higher - June 2011 paper:
When 2.1 g of an unsaturated hydrocarbon were completely burned in oxygen, 6.6 g of carbon dioxide and 2.7 g of water were the only products. Relative formula masses: CO2 = 44; H2O = 18. Calculate the empirical formula of this hydrocarbon.
First work out the mass of the carbon/hydrogen: 6.6 * (12/44) = 1.8g, 2.7 * (2/18) = 0.3g (you can check because 1.8+0.3 = 2.1g which is in the question).
The calculate the moles. Remember: mass
So do 1.8/12 = 0.15moles of Carbon, 0.3/1 = 0.3moles of Hydrogen.
Then divide both numbers by the smallest to get the ratio:
0.15/0.15 = 1, 0.3/0.15 = 2
So it must be CH2 because there are twice as many hydrogen atoms as carbon.
Empirical Formula (continued)
Sometimes they may also ask for oxygen. To work this out you just add up the carbon and hydrogen (1.8+0.3) and take it away from the weight of the compound that is given in the question (2.1). Of course, in this case the answer is 0 because there is no oxygen.
Sometimes they will give you percentages rather than masses. This is actually simpler than masses as you just assume the mass of the compound is 100g. So if you have 81.82% carbon and 18.18% hydrogen then you would assume the hydrocarbon's mass is 100g and then the carbon has a mass of 81.82g and the hydrogen 18.18g. Then you follow this through like before:
81.82/12 = 6.8183333.... = 6.818g
18.18/1 = 18.18g
Divide each one by the smallest: 6.818/6.818 = 1, 18.18/6.818 = 2.6665
Work out what you have to multiply by to give whole numbers (this question is tricky, you can't just round because it 2.6665 isn't very close to 3)
Multipling by 3 gives very nearly a whole number. 1*3 = 3, 2.6665*3 = 7.9995
So it would be C3H8.
The only way to get good at the equations etc. is to practise them like crazy! Find past papers and search on here too because I've found some good practise questions here too.
List as many advantages and disadvantages of instrumental analysis as you can (e.g. expensive, sensitive, fast etc.)