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Chemistry Unit 3

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History Of the Periodic Table

Early 1800's

- Until recently, there were two ways to categorise elements: Their physical/chemical properties or their relative atomic mass

- They had no idea of atomic structure or of protons or electrons, so there was no such thing as Atomic number and so Relative atomic mass was the only measure they could use, and so they were arranged in this order.

Newlands Law of Octaves (1864)

- He noticed every eighth element had similar properties and so listed them in rows of 7

- The pattern broke down on the third row with the transition elements like titanium and iron messing it up

- it was because he left no gaps that his work was ignored, but he was pretty close

- he presented his ideas to the chemical society in 1865 but his work was critisised because:

  • his groups contained elements that didn't have similar properties
  • he mixed up metals and non-metals
  • he didn't leave any gaps for elements that hadn't been dicovered yet.
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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 to leave gaps in order to keep elements with similar properties in vertical groups, with a huge gap in the first two rows for the transition metals

- He was prepared to leave big gaps in the first two rows before the transition metals before the thrid row

- The gaps were clever 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 late 19th century, protons, neutrons and electrons were discovered and the periodic table matched up well to the structure of the atoms

- The periodic table was arranged in order of proton number when protons, neutrons and electrons were discovered.

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The modern periodic table

- 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 shell 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 out shell.

- The attraction of the nucleus is less when where there are lots of inner electrons and this is known as shielding

- Increased distance and shielding means an electron in a higher energy level can be more easily lost because of less attracted. Plus, it is less likely that the higher the energy level will gain an electron

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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 group one, the metals become: More reactive, Bigger atoms, Higher density, Lower melting and boiling points

- Alkali metals are very reactive, they have to be stored in oil

- they are Lithium, Sodium, Potassium and a few more

- They all have one outer electron, hence group 1. This makes them very reactive

- They all form 1+ ions so they are keen to lose their outer electron to form a 1+ ion

- They always form ionic compounds, covalent bonding is out of the question

- When reacted with water, they produce hydrogen gas: they react vigorously, fizzing around 

- potassium gets hot enough to ignite and a lighted splint will indicate hydrogen with a squeaky pop

- they form a hydroxide in solution. I.e aqueous OH- ions

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Group 7, the halogens

- As you go down the group: They become less reactive and higher melting and boiling points.

- Halogens are all non-metals with coloured vapours

-Fluorine is a very reactive yellow gas

- chlorine is a dense green poisonous gas

- Bromine is a dense, poisonous volatile, red-brown liquid

- Iodine is a dark grey crystalline or a purple vapour

- They all form molecules which are pairs of atoms 

- They form 1- ions when bonded with metals, and they form covalent bonds with non-metals

-They react with metals to form salts

- More reactive halogens displace less reactive ones

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Transition Elements

- 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+ give red/brown compounds eg rust

- Compounds are colourful as a result of the transition metal they ion they contain

- Colour of hair, gemstones and pottery glazes are all due to transition metals

Transition metals and their compounds all make good catalysts. Iron is used in the harbor 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 it is full. Eg iron(2,8,14,8)

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Acids and Alkalis

- Arrhenius said acids release hydrogen ions in water

- his theory was that when mixed with water, all acids release Hydrogen ions, H+ 

- He also said that alkalis form OH- ions, Hydroxide ions when in water

- The idea worked well but it only worked for acids and bases that dissolved in water, however ammonia gas can react with a base even when it isn't dissolved in water, which is one reason the ideas weren't accepted straight away. 

- Back in the 1880's when Arrhenius first suggested that molecules ionise with water, many scientists didn't believe it was possible. Charged subatomic particles hadn't been discovered yet, so the idea of charged ions seemed strange

- Lowry and Brønsted said acids are proton donors

- Acids release H+ ions----- i.e they're proton donors

- Bases accept H+ ions----- i.e they're proton acceptors

- The idea was readily accepted because they explained the behaviour of acids and bases in solvents other than water. 

- Protons are hydrated in water, in acid they dissociate, releasing H+. Water molecules dissociate into H+ and OH-

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Acids, Alkalis and Titration

- Strong acids ionise almost completely in water. This means almost every hydrogen atom is released to become a hydrated proton 

- Weak acids ionise only very slightly, only some H+ atoms in the compound are released so only a small number of H+ ions are formed

- The pH of an acid or alkali is a measure of concentration of H+ ions in a solution, Stong acids have a typical pH of about 1 or 2, while a weak acid might be 4,5 or 6

Titrations are used to find out concentrations

- Allow you to find out exactly how much acid is needed to neutralise an alkali or vise versa

- put alkali in a flask with indicator (Phenolphthalein for weak acid, strong alkali. Methyl orange for strong acid, weak alkali)

- Adding a bit at a time using a burette, the indicator will change colour when all of the alkali is neutralised

- The amount of acid is recorded

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Acids, Alkalis and Titration

- Strong acids ionise almost completely in water. This means almost every hydrogen atom is released to become a hydrated proton 

- Weak acids ionise only very slightly, only some H+ atoms in the compound are released so only a small number of H+ ions are formed

- The pH of an acid or alkali is a measure of concentration of H+ ions in a solution, Stong acids have a typical pH of about 1 or 2, while a weak acid might be 4,5 or 6

Titrations are used to find out concentrations

- Allow you to find out exactly how much acid is needed to neutralise an alkali or vise versa

- put alkali in a flask with indicator (Phenolphthalein for weak acid, strong alkali. Methyl orange for strong acid, weak alkali)

- Adding a bit at a time using a burette, the indicator will change colour when all of the alkali is neutralised

- The amount of acid is recorded

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Titration Calculations

if they ask you for concentration in Moles Per dm^3

- Step 1: work out number of moles of known substance you have

Number of moles = concentration x volume

- Step 2: Write equation for reaction and work out moles of unknown sunstance

- Step 3: Workout concentration of unknown stuff 

Concentration = Number of moles / volume                (,r:0,s:0,i:70&tx=59&ty=33)

If they ask for concentration in Grams per dm^3

- Step 1: Work out relative atomic mass for the acid

- Step 2: Convert the concentration in moles into concentration in grams 


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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 water 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, the molecules start to surround the ions, and disrupt the ionic bonding, so the solid structure of the ionic compound gradually falls apart.

- water molecules are polar, they've got a positive hydrogen side and a negative oxygen side. The slightly positive side attracts the negative ions.

- Salts of Sodium, Potassium and Ammonium dissolve in water

- Nitrates, Chlorides and Sulfates dissolve in water

- Many covalent bonds don't in water, they don't form ions and their molecules are too big

- Water from streams, rivers, and rain dissolves a lot of substances that is comes into contact with e.g salts from rocks, fertilisers from fields, and gases in the atmosphere, such as sulfur dioxide from power stations and car exhausts .

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- The solubility of a substance in a given solvent is the number of grams that dissolve in 100g of the solvent at a particular temperature

- The solubility of solutes usually increases with temperature

- A saturated solution is one that cannot hold any more solid at that temperature- and you have to be able to see solid at the bottom to be certain that its saturated

- A solubility curve plots the mass of a 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 (opposite of solids) 

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Hard Water

 Hard water, when mixed with soap forms nasty scum as the hardness minerals react with the soap

- It also forms scale, mainly Calcium Carbonate, on the inside of pipes, kettles and boilers

- The scale acts as a thermal insulator which is why an old kettle takes longer to boil and so is less efficient.

- Hardness is cause by Magnesium and Calcium ions.

- Hardness comes from the type of rocks where your water comes from, eg limestone, chalk and gypsum

- Rain falling on some types of rocks can dissolve magnesium sulphate , and calcium sulphate

- When Carbon Dioxide dissolves in water you get a carbonic acid, this is why rainwater can be acidic

- Calcium ions in the water are good for teeth and bones and the scale inside pipes stops poisonous metal ions getting into drinking water

- Remove hardness by removing dissolved Ca2+ and Mg2+ ions, you can replace them with sodium carbonate

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Water Quality

- Water is essential for life and so it is important that it is clean and safe

- Micro-organisms in water can cause diseases such as Cholera and Dynsentry

- Most drinking water comes from reservoirs  and have to go through treatment works:

1. the water passes through a mesh screen to remove big particles i.e twigs

2. Next, its 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 chlorinated to kill off any harmful micro-organisms left. 

- To monitor water, companies take samples. Some people who are still not satisfied, buy filters that contain carbon or silver. Carbon takes away chlorine taste and silver kills bugs. Some people buy ion exchange columns which contain ion exchange resins 

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Removing Hardness

- Hardness can be removed by replacing it with sodium carbonate

- The carbonate ions join onto the calcium and magnesium ions to make an insoluble precipitate

- Ion exchange columns- sometimes a water supply is fed through an ion exchange column to remove the hardness. They have sodium ions in which exchange the magnesium and calcium ions in the water that runs through them.

- Scale is mainly just calcium carbonate, and that can be dissolved by acid

- Most descaling products that you buy to clean the kettle are some kind of acid

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- 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, this can reduced by putting a lid on cup etc

- 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 formed.

- Energy is supplied to break existing bonds s bond breaking is endothermic

- Energy is released when new bonds form, so it is exothermic

- In an endothermic reaction, the amount of energy supplied to break the bond is greater than the energy released when new bonds are formed. In exothermic, it is the other way round

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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 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

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Energy and Food

- Energy is often measured in Kilocalories

- 1 calorie = amount of energy needed to raise 1g of water by 1 degree C= 4.2 joules

- The dietary info on food labels is in kilocalories (Kcal) 

- 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 

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Test for Cations

Lithium (Li+) Burns a crimson red flame

Sodium (Na+) Burns with a orange-yellow flame

Potassium (K+) Burns with a lilac flame

Calcium (Ca2+) Burns a brick red flame

Barium (Ba2+) Burns with a green flame

Some metals form a coloured precipitate with NaOH

Ammonium compound + NaOH gives off smelly ammonia

- You can tell if ammonia is about by the smell - cat wee

- Or use damp red litmus paper - ammonia will turn it blue

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Tests for Anions

Testing for carbonates - check for CO^2 - bubble it through lime water - it will go milky if carbon dioxide is present

- Acid + Carbonate ----> Salt + water + CO^2

Test for Sulphates and Halides

- You can test for certain ions by seeing if a precipitate is formed

To test for sulphate ions, add dilute HCl followed by Barium chloride. If there is a white ppt of barium sulphate, the original compound was a sulphate

- To test for Bromide, Iodide or Chloride ions, add dilute nitric acid followed by silver nitrate solution 

- A chloride gives a white precipitate of silver chloride

- A bromide gives a cream ppt of silver bromide

- An iodide gives a yellow ppt of silver iodide

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Tests for Organic Compounds

- Tests for nitrates 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

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Instrumental Methods

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 more reliable than the 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 G/L

Mass spectrometry - Can be used for elements and compounds. tells you the mass of each molecule/particle

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