Chemistry unit 3

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  • Created by: Megan
  • Created on: 15-04-13 15:36

History of the periodic table (1800s)

Until quite recently, there were two obvious ways to categorise elements: 

1. Their physical and chemical properties  2. Their relative atomic mass 

1. Remember, they had no idea of atomic structure or of protons or electrons, so there was no such thing as atomic number to them. (It was only in the 20th century after protons and electrons were discovered that it was realised the elements were best arranged in order of atomic number

2. Back then, the only thing they could measure was relative atomic mass, and so the known elements were arranged in order of atomic mass. When this was done, a periodic pattern was noticed in the properties of the elements. This is where the name periodic table comes from. 

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Newlands law of octaves

A chap called Newlands had the first good stab at arranging things more usefully in 1864. He noticed that every eighth element had similar properties, and so he listed some of the known elements in rows of seven. 

These sets of eight were called Newlands' octaves. Unfortunately the pattern broke down on the third row, with transition metals like titanium and iron messing it up. 

It was because he left no gaps that his work was ignored. But he was getting pretty close, as you can see.

Newlands presented his ideas to the Chemical Society in 1865. But his work was criticised because: 1. His groups contained elements that didn't have similar propertiesm e.g. carbon and titanium. 2. He mixed up metals and non-metals, e.g. oxygen and iron. 3. He didn't leave any gaps for elements that hadn't been discovered yet. 

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Dmitri mendeleev

1. In 1869, Dmitri Mendeleev in Russia, armed with about 50 known elements, arranged them into his table of elements - with various gaps. 

2. Mendeleev put the elements in order of atomic mass (like newlands). But Mendeleev found he has to leave gaps in order to keep elements with similar properties in the same vertical columns (known as groups) - and he was prepared to leave some very big gaps in the first two rows before the transition metals come in on the third row. 

3. The gaps were the really clever bit because they predicted the properties of so far undiscovered elements. When they were found and they fitted the pattern it was pretty smashing news for old Dmitri. 

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Importance of the periodic table

1. When the periodic table was first released, many scientists thought it was just a bit of fun. At that time, there wasn't all that much evidence to suggest that the elements really did fit together in that way - ideas don't get the scientific stamp of approval without evidence. 

2. After Mendleev released his work, newly discovered elements fitted into the gaps he left. This was convincing evidence in favour of the periodic table. 

3. Once there was more evidence, many more scientists realised that the periodic table could be a useful tool for predicting properties of elements. It really worked

4. In the late 19th century, scientists discovered protons, neutrons and electrons. The periodic table matches up very well to what's been discovered about the structure of the atom. Scientists now accept that it's a very important and useful summary of the structure of atoms.

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Modern periodic table

1. The elements in the periodic table can be seen as being arranged by their electronic structure. Using the electron arrangement, you can predict the element's chemical properties.        2. Electrons in an atom are set out in shells which each correspond to an energy level.        3. Apart from the transition metals, elements in the same group have the same number of electrons in their highest occupied energy level (outer shell).        4. The group number is equal to the number of electrons in the highest occupied energy level - e.g. group 6 all have 6 electrons in the highest energy level.        5. The positive charge of the nucleus attracts electrons and holds them in place. The further from the nucleus the electron is, the less the attraction.                        6. The attraction of the nucleus is even less when there are a lot of inner electrons. Inner electrons get in the way of the nuclear charge, reducing the attraction. This effect is known as shielding.        7. The combination of increased distance and increased shielding means that an electron in a higher energy level is more easily lost because there's less attraction from the nucleus holding it in place.        

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Trends

As you go DOWN group 1, the alkali metals: 

1. become MORE REACTIVE        ... because the outer electron is more easily lost, because it's further from the nucleus. 

2. Have LOWER MELTING AND BOILING POINTS 

The alkali metals have LOW DENSITY. In fact, the first three in the group are less dense than water

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The alkali metals

 1. They are: Lithium, sodium, potassium, rubidium and caesiium.    2. The alkali metals all have one outer electron.          3. The alkali metals form ionic compounds with non-metals

  • They are keen to lose their one outer electron to form a 1^+ ion
  • They are so keen to lose the outer electron there's no way they'd consider sharing, so covalent bonding is out of the question
  • So they always form ionic bonds - and they produce white compounds that dissolve in water to form colourless solutions

4. Reaction with water produces hydrogen gas

  • When lithium, sodium or potassium are put in water, they react very vigorously
  • They float and move around the surface, fizzing furiously. 
  • They proudce hydrogen. Potassium gets hot enough to ignite it. A lighted splint will indicate hydrogen by producing the notorious squeaky pop as H2 ignites. 
  • They form hydroxides that dissolve in water to give alkaline solutions. 
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The halogens

1. Trends: as you go down group 7, the halogens have the following properties:        - Less reactive... because it' harder to gain an extra electron, because the outer shell's further from the nucleus.          - Higher melting and boiling points.                  2. Tha 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, red-brown volatile liquid.        - Iodine is a dark grey crystalline solid or a purple vapour.        3. The halogens form ionic bonds with metals.        - The halogens form 1-ions called halides when they bond with metals.        4. More reactive halogens will displace less reactive ones.        - A more reactive halogen can displace a less reactive halogen from an aquesous solution of its salt

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Transition elements

1. Transition elements make up the big clump of metals in the middle of the periodic table. Transition elements ar typical metals, and have the properties you would expect:     - They're good conductors of heat and electricity.        - They're very dense, strong and shiny.        - Transition metals are much less reactive than Group 1 metals - they don't react as vigorously with water or oxygen, for example.        - They're also much denser, stronger and harder than the Group 1 metals, and have much higher melting points. (except for mercury, which is a liquid at room temperature).   2. Transition metals often have more than one ion, e.g. Fe^2+, Fe^3+             - Two other examples are copper: Cu^+ and Cu^2+ and chromium: Cr^2+ and Cr^3+.     - The different ions usually form different-coloured compunds too: Fe^2+ ions usually give green compounds, whereas Fe^2+ ions usually form red/brown compounds.        3. The compounds are very colourful.          - The compounds are colourful due to the transition metal ion they contain.        - The colours in gemstones, like blue sapphires and green emeralds, and the colours in potery glazes are all due to transition metals. And weathered copper is green.               4. Transition metals and their compounds all make good catalysts.

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Hardness of water

1. Hard water makes scum and scale.        - With soft water, you get a nice lather with soap. But with hard water you get a nasty scum instead - unless you're using a soapless detergent. The problem is dissolves caclium and magnesium ions in the water reacting with the soap to make scum which is incoluble. So to get a decent lather you need to use more soap - and because soap isn't free, that means more money going down the drain.         - When heated, hard water also forms furring or scale on the inside of pipes, boilers and kettles. Badly scaled-up pipes and boilers reduce the efficiency of heating systems, and may need to be replaced - all of which costs money. Scale can even block pipes.        - Scale is also a bit of a thermal insulator. This means that a kettle with scale on the heating element takes longer to boil than a clean non-scaled-up kettle (less efficient).       2. Hardness is caused by Ca^2+ and Mg^2+ ions.                - Most hard water is hard because it contains lots of calcium and magnesium ions.        - Rain falling on some types of rocks can dissolves compounds like magnesium sulfate, and calcium sulfate (both soluble).        - BUT... Ca^2+ ions are good for healthy teeth and bones.                - People who live in hard water areas are at less risk of developing heart disease

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Making hard water soft

There are two kinds of hardness - temporary and permanent. Temporary hardness is caused by the hydrogencarbonate ion, HCO3-, in Ca(HCO3)2. Permanent hardness is caused by dissolved calcium sulfate (among other things).                  1. Tempoary hardness is removed by boiling. When heated, the calcium hydrocarbonate decomposes to form calcium carbonate which is insoluble. The solid is limescale in your kettle.        2. Both types of hardness can be softened by adding washing soda (sodium carbonate) to it. The added carbonate ions react with the Ca2+ and Mg 2+ ions to make an isoluble precipitate of calcium carbonate and magnesium carbonate. The Ca2+ and Mg2+ ions magnesium carbonate. The Ca2+ and Mg2+ ions are no longer dissolved in the water so they can't make it hard.             3. Both types of hardness can also be removed by running water through ion exchange columns which are sold in shops. The columns have lots of sodium ions (or hydrogen ions) and exchange them for calcium or magnesium ions in the water that runs through them. 

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Drinking water needs to be good quality

1. Water's essential for life but it must be free of poisonous salts and harmful microbes. Microbes in water can cause diseases such as cholera and dysentery.        2. Most of our drinking water comes from resevoirs. Water flows into resevoirs from rivers and groundwater - water companies choose to build resevoirs where there's a good supply of clean water. Government agencies keep a close eye on pollution in resevoirs, rivers and groundwater.        - Water from resevoirs goes to the water treatment works for treatment.        - Some people still aren't satisfied. They buy filters that contain carbon or silver to remove substances from their tap water. Carbon in the filters removes chlorine taste and silver is supposed to kill bugs. Some people in hard water areas buy water softeners which contain ion exchange resins.        - Totally pure water with nothing dissolved in it can be produced by distillation - boiling water to make steam and condensing the steam. This process is too expensive to produce tap water - bags of energy would be needed to boil all the water we use. Distilled water is used in chemistry labs

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Adding fluoride and chlorine

1. Fluoride is added to drinking water in some parts of the country because it helps to reduce tooth decay. Chlorine is added to prevent disease. However...        2. Some studies have linked adding chlorine to water with an increase in certain cancers. Chlorine can react with other natural substances in water to produce toxic by-products which some people think could cause cancer.        3. In high doses fluoride can cause cancer and bone problems in humans, so some people believe that fluoride shouldn't be added to drinking water. There is also concern about whether it's right to mass medicate - people can choose whether to use a fluoride toothpaste, but they can't choose whether their tap water has added fluoride.        4. Levels of chemicals added to drinking water need to be carefully monitored. For example, in some areas the water may already contain a lot of fluoride, so adding more could be harmful. 

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Reversible reactions

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants

Reversible reactions will reach an equilibrium.           - If a reversible reaction takes place in a closed system then a state of equilibrium will  always be reached.            - Equilibrium means that the amounts of reactants and products will reach a  certain balance and stay there.         - The reactions are still taking place in both directions, but the overall effet is nil because  he forward and reverse reactions cancel each other out. The reactions are taking place  at exactly the same rate in both directions.  

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Changing temperature and pressure

1. In a reversible reaction the position of equilibrium (the relative amounts of reactants and products) depends very strongly on the temperature and pressure surrounding the reaction.        2. If you deliberately alter the temperature and pressure you can move the position of equilibrium to give more product and less reactants. 

TEMPERATURE: All reactions are exothermic in one direction and endothermic in the other.       - If you raise the temperature, the endothermic reaction will increase to use extra heat.    - If you reduce the temperature, the exothermic reaction will increase to give out heat. 

PRESSURE: Many reactions have a greater volume on one side, either of products or reactants (greater volume means there are more gas molecules and less volume means there are fewer gas molecules).        - If you raise the pressure it will encourage the reaction which produces less volume.         - If you lower the pressure it will encourage the reaction which produces more volume

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Ammonia

Nitrogen and hydrogen are needed to make ammonia - 

N2 + 3H2 ---><--- 2NH3  (+ heat) 

1. The nitrogen is obtained easily from the air, which is 78% nitrogen (and 21% oxygen).   2. The hydrogen comes from natural gas or from other sources like crude oil.        3. Some of the nitrogen and hydrogen reacts to form ammonia. Because the reaction is reversible - it occurs in both directions - ammonia breaks down again into nitrogen and hydrogen. The reaction reaches an equilibrium

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The reaction is reversible

1. Higher pressures favour the forward reaction (since there are four molecules of gas on the left-hand side, for every two molecules on the right).      2. So the pressure is set as high as possible to give the best % yield, without making the plant too expensive to build (it'd be too expensive to build a plant that'd stand pressures of over 1000 atmospheres, for example). Hence the 200 atmospheres operating pressure. 3. The forward reaction is exothermic, which means that increasing the temperature will actually move the equilibrium the wrong way - away from ammonia and towards N2 and H2. So the yield of ammonia would be greater at lower temperatures.     4. The trouble is, lower temperatures mean a lower rate of reaction. So what they do is increase the temperature anyway, to get a much faster rate of reaction.      5. The 450C is a compromise between maximum yield and speed of reaction. It's better to wait just 20 seconds for a 10% yield than to have to wait 60 seconds for a 20% yield. 6. The ammonia is formed as a gas but as it cools in the condenser it liquefies and is removed.      7. The unused hydrogen and nitrogen are recycled so nothing is wasted

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Alcohols

 1. Alcohols have an -OH functional group and end in -ol. - The general formula of an alcohol is C(n)H(2n+1) OH. - All alcohols contain the same -OH group.              

2. The first three alcohols have similar properties. - The alcohols are flammable. They burn in the air to produce carbon dioxide and water. - The first three alcohols all dissolve completely in water to form neutral solutions. - They also react with sodium to give hydrogen and alkoxides. - Ethanol is the main alcohol in alcoholic drinks. It's not as toxic as methanol but it still damages the liver and brain.       

3. Alcohols are used as solvents. - Alcohols such as mehanol and ethanol can dissolve most compounds that water dissolves, but they can also dissolve substances that water can't dissolve.  - e.g. hydrocarbons, oils ad fats. - Ethanol is the solvent for perfumes and aftershave lotions. It can mix with both the oils and the water.

4. Alcohols are used as fuels - Ethanol is used as a fuel in spirit burners. - Ethanol can also be mixed in with petrol and used as fuel for cars. Since pure ethanol is clean burning, the more ethanol in a petrol/ ethanol mix, the less pollution is produced. - Some countries that have little or no oil deposits but plenty of land and sunshine grow loads of sugar cane, which they ferment to form ethanol.

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Carboxylic gases

1. Carboxylic acids have the functional group -COOH. - Their names end in -anoic acid

2. Carboxylic acids react like other acids. - They react just like any other acid with carbonates to produce carbon dioxide. - The salts formed in these reactions end in -anoate - e.g. methanoic acid will form a methanoate, ethanoic acid an ethanoate. - Carboxylic acids dissolve in water to produce acidic solutions. When they dissolve, they ionise and release H+ ions which are responsible for making the solution acidic. But, because they don't ionise completely, they just form weak acidic solutions. This means that they have a higher pH that aqueous solutions of strong acids with the same concentration

3. Some carboxylic acids are fairly common. - Ethanoic acid can be made by oxidising ethanol. Microbes, like yeast, cause the ethanol to ferment. Ethanol can also be oxidised using oxidising agents. - Ethanoic acid can then be dissolved in water to make vinegar, which is used for flavouring and preserving foods. - Citric acid is present in oranges and lemons, and is manufactured in large quantities to make fizzy drinks. It's also used to get rid of scale

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Esters

1. Esters are formed from an alcohol and a carboxylic acid. - An acid catalyst is usually used. - Their names end in -oate. The alcohol forms the first part of the ester's name, and the acid forms the second part. 

2. Esters smell nice but don't mix well with water. - Many esters have pleasant smells - often quite sweet and fruity. They're also volatile. This makes them ideal for perfumes. - However, many esters are flammable. So their volatility also make them potentially dangerous. - Esters don't mix very well with water. - But esters do mix well with alcohols and other organic solvents

3. Using esters - Inhaling the fumes from some esters irritates mucous membranes in the nose and mouth. - Ester fumes are heavier than air and very flammable. - Some esters are toxic, especially in large doses. Some people worry about health problems associated with synthetic food additives such as esters. - BUT... esters aren't as volatile or as toxic as some other organic solvents - they don't release nearly as many toxic fumes as some of them. In fact esters have replaced solvents such as toluene in many paints and varnishes. 

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Moles

1. THE MOLE is simply the name given to a certain number. - 6.023 x 10^23. - It's just a number, the reason it's so long is because when they get precisely that number of atoms of carbon-12 it weighs exactly 12g. So, get that number of atoms or molecules, of any element and compound, and conveniently, they weigh exactly the same number of grams as the relative atomic mass, Ar (or Mr) of the element or compound. This is arranged on purpose of course, to make things easier. - So, you can use moles as a unit of measurement when you're talking about an amount of substance. 

2. Concentration is a measure of how crowded things are. - The concentration of a solution can be measured in moles per dm3. So 1 mole of stuff in 1 dm3 of solution has a concentration of 1 mole per dm3. - The more solute you dissolve in a given volume, the more crowded the solute molecules are and the more concentrated the solution. 

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Titrations

1. Titrations also allow you to find out exactly how much acid is needed to neutralise a quantity of alkali. 

2. You put some alkali in a flask, along with some indicator - phenolphthalein or methyl orange. You don't use universal indicator as it changes colour gradually - and you want a definite colour change

3. Add the acid, a bit at a time, to the alkali using a burette - giving the flask a regular swirl. Go especially slowly when you think the alkali's almost neutralised. 

4. The indicator changes colour when all the alkali has been neutralised - phenolphthalein is pink in alkalis but colourless in acids, and methyl orange is yellow in alkalis but red in acids

5. Record the amount of acid used to neutralise the alkali. It's best to repeat this process a few times, making sure you get the same answer each time. 

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Calculate the concentration

Concentration = moles / volume 

n / c x V 

n = number of moles 

c = concentration in mol/dm3

V = volume in dm3 (1 dm3 = 1 litre) 

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Energy

1. You can measure the amount of energy released by a chemical reaction by taking the temperature of the reagants, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction.

2.The biggest problem with energy measurements is the amount of energy lost to the surroundings

3. You can reduce it a bit by putting the polystyrene cup into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation

4. This method works for the reactions of solids with water as well as for neutralisation reactions. 

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Endothermic and Exothermic

An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature

An endothermic reaction is one which takes energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature

ENERGY MUST ALWAYS BE SUPPLIED TO BREAK BONDS... AND ENERGY IS ALWAYS RELEASED WHEN BONDS FORM.

-During a chemical reaction, old bonds are broken and new bonds are formed. - Energy must be supplied to break existing bonds - so bond breaking is an endothermic process. - Energy is released when new bonds are formed - so bond formation is an exothermic process. - In an endothermic reaction, the energy required to break old bonds is greater than the energy released when new bonds are formed. - In an exothermic reaction, the energy released is bond formation is greater than the energy using in breaking old bonds. 

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Energy from hydrogen

1. Hydrogen and oxygen give out energy when they react. - Hydrogen and oxygen react to produce water - which isn't a pollutant.  - The reaction between hydrogen and oxygen is exothermic - it releases energy. - Put these two facts together, and you get something useful: you can get energy by reacting hydrogen and oxygen - either in a combustion engine or in a fuel cell

2. Hydrogen gas can be burnt to power vehicles. - Hydrogen gas can be burnt in oxygen as a fuel in the combustion engines of vehicles. - Pros: Hydrogen combines with oxygen in the air to form just water - so it's very clean. - Cons: You need a special, expensive engine. Although hydrogen can be made from water, which there's plenty of, you still need to use energy from another source to make it. 

3. Fuel cells use fuel and oxygen to produce electrical energy. - A FUEL CELL IS AN ELECTRICAL CELL THAT'S SUPPLIED WITH A FUEL AND OXYGEN AND USES ENERGY FROM THE REACTION BETWEEN THEM TO GENERATE ELECTRICITY. - Hydrogen can be used in a hydrogen-oxygen fuel cell. - Fuel cells were developed in the 1960s as part of the space programme, to provide electrical power on a space craft. - Unlike a battery, a fuel cell doesn't run down or need recharging from the mains. 

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Tests for positive ions

FLAME TESTS IDENTIFY METAL IONS: Compounds of some metals burn with a characteristic colour. So, you can test for various metal ions by putting your substance in a flame and seeing what colour the flame goes. LITHIUM = CRIMSON, SODIUM = YELLOW, POTASSIUM = LILAC, CALCIUM = RED AND BARIUM = GREEN. To flame test a compound in the lab, dip a clean wire loop into a sample of the compound, and put the wire loop in the clear blue part of the bunsen flame. First make sure the wire loop is really clean by dipping it into hydrochloric acid and rinsing it with distilled water.

SOME METAL IONS FORM A COLOURED PRECIPITATE WITH NAOH: 1. Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have a characteristic colour. 2. So in this test you add a few drops of sodium hydroxide solution to a solution of your mystery compound - all in the hope of forming an insoluble hydroxide. 3. If you get a coloured insoluble hydroxide you can then tell which metal was in the compound. CALCIUM = WHITE, COPPER = BLUE, IRON (2) = GREEN, IRON (3) = BROWN, ALUMINIUM = WHITE AT FIRST AND THEN A COLOURLESS SOLUTION AND MAGNESIUM = WHITE

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Tests for negative ions

1. TESTING FOR CARBONATES: - You can test to see if a gas is carbon dioxide by bubbling it through limewater. If it is carbon dioxide, the limewater turns cloudy. - You can use this to test for carbonate ions since carbonates react with dilute acids to form carbon dioxide. - acid + carbonate ---> salt + water + carbon dioxide

2. TESTS FOR HALIDES AND SULFATES: - You can test for certain ions by seeing if a precipitate is formed after these reactions... HALIDE IONS - To test for chloride, bromine or iodine ions, add dilute nitric acid, followed by silver nitrate solution. - A chloride gives a white precipitate of silver chloride. - A bromine gives a cream precipitate of silver bromine. - An iodine gives a yellow precipitate of silver iodine. SULFATE IONS - To test for a sulfate ion, add dilute HCL, followed by barium chloride solution. - A white precipitate of barium sulfate means the original compound was a sulfate. 

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