Chemistry Unit 3

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  • Created by: Chris
  • Created on: 09-05-13 14:31

History of the Periodic Table

Until recently there were two ways that you could OBVIOUSLY categorise elements:

1) Their physical and chemical prperties 

2) Their relative atomic masses

This was because people had no idea anout protons, electrons, neturons etc.

In 1864 NEWLAND noticed every 8th element had similar properties, so he made rows of seven. It was a good attempt but his work was criticised because:

  • His groups contained elements that didn't contain similar properties (eg Titanium and Carbon)
  • He mixed up metals and non-metals (eg Oxygen and Iron)
  • He left no gaps for undiscovered elements 

In 1869 MENDELEEV put the elements in order of atomic mass - just like Newland did. But Mendeleev left gaps in order to keep elements with similar properties in the same groups. 

The gaps predicted the properties of the so far undiscovered elements - and when they were discovered they did in fact fit the pattern.

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Atomic Structure

The positive charge of the nucleus attracts the electrons and holds them in place. The further the electron is from the electron the less teh attraction.

The attraction of the nucleus is even less where there are a lot of inner electrons. Inner electrons get in the way of the nucleus charge, reducing the attraction. This effect is known as SHIELDING.

The increased shielding means that an electron in higher energy levels are more easily lost because there's less attraction from the nucleus holding it in place.

Group 1 gets MORE REACTIVE the further down the group you go because the shielding means it is EASIER to lose an electron.

Group 7 gets LESS REACTIVE the furhter down the group you go because the more shielding means the HARDER it is to gain and electron.

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Alkali Metals

As you go DOWN GROUP 1, the alkali metals:

  • Become more reactive (previous card)
  • Lower melting and boiling points
  • Low density 

ALL HAVE ONE OUTER ELECTRON - ONLY FORM IONIC COMPOUNDS - DO NOT FORM COVALENT BONDS EVER!

When Lithium, Sodium and Potassium are put in water they react vigourously. The float, moving around the surface fizzing. They produce hydrogen, and potassium becomes hot enough to ignite it.

THEY TURN THE WATER ALKALINE 

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The Halogens

As you DOWN GROUP 7 the halogens:

  • Get less reactive 
  • Higher melting point
  • Higher boiling point 

They exist as molecules in pairs: F2, Cl2, Br2.....

Halogens form 1- ions called halides when they bond with metals.

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Transition Elements/Metals

Tranistion elements are just typical metals:

  • They're good coductors of heat and electricity 
  • They're very dense, strong and shiny 
  • Transition metals are much less reactive than Group 1 metals - do not react vigorously with water or oxygen 
  • They are much more dense, stronger and harder than Group 1 metals
  • High melting points

Transition metals and their compounds make very good catalysts. Catalysts increase the rate of reaction.

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Water

SOFT WATER = nice lather of soap 

HARD WATER = scum! Needs more soap to create a good lather, which means more expense 

When heated hard water creates scale (mostly calcium carbonate) on the insides of pipes, boilers, kettles etc. Scaled pipes reduces the efficiency of heating systems - replacing them costs money.

Scale is a thermal insulator.

HARDNESS OF WATER IS CAUSED WHEN THERE ARE A LOT OF CALCIUM AND MAGNESIUM IONS 

Rain falling on some types of rocks (limestone/chalk/gypsum) can dissolve compounds like magnesium sulfate and calcium sulfate.

Hard water isn't all bad - Ca2+ ions are good for healthy teeth and bones. People who live in hard water areas are at a less of a risk of developing heart disease than people living in soft water areas.

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Hard Water

There are TWO types of hardness - TEMPORARY AND PERMANENT

Temporary hardness is due to HYDROGENCARBONATE

Permanent hardness is due to CALCIUM SULFATE 

Temporary hardness can be removed by heating the water. The hydrogencarbonate will decompose to calcium carbonate (limescale).

CALCIUM HYDROGENCARBONATE -----> CALCIUM CARBONATE + WATER + CO2

Both types of hardness can be softened by adding washing soda

Both types of hardness can be removed by running water through 'ion exchange columns'. The columns have lots of sodium ions and exchange them for calcium or magnesium.

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Water Quality

How water is cleaned:

  • Water is passed through a mesh to remove things like twigs
  • Chemicals are added to make microbes and solids stick together and fall to the bottom
  • The water is filtered through gravel beds to remove all the solids 
  • Water is chlorined to kill any harmful microbes left

Totally pure water with nothing dissolved in it can be produced by distillation - boiling water to make steam and condensing the steam. THIS IS VERY EXPENSIVE.

Fluorine and Chlorine are added to water. Fluorine helps reduce tooth deacy and chlorine helps prevent disease.

HOWEVER

Is is right to mass medicate people without them having a choice?

High doses of Fluoride can cause cancer

Chloride can react with some substances which may cause cancer.

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Reversible Reactions

A reversible reaction is where the products of a reaction can in turn be reacted to create the original reactants. Essentially the reaction can go BOTH ways.

EQUILIBRIUM MEANS THAT THE AMOUNTS OF REACTANTS AND PRODUCTS WILL REACH A CERTAIN BALANCE AND STAY THERE

In a reversible reaction the position of the equilibrium depends very strongly on the TEMPERATURE AND THE PRESSURE. If you move the position of the equilibrium you can get more reactants and less products.

All reactions are exothermic in one direction - endothermic the other direction.

If you raise the temperature the endothermic reaction will increase becuase it will want to take in the extra heat.

If you lower the reaction then the exothermic reaction is increased because there isn't an endothermic reaction to take it back up.

The same pattern for pressure.

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Haber Process

You want a low temperature because a high temperature will favour the endothermic reaction therefore less heat energy will be produced. But a higher temperature increases the rate of the yeild. So as a comprimise it is set at 450*C.

The pressure wants to be as high as possible to favour the forward reaction. But setting it at a high pressure is expensive and dangerous, so it is set at 200 atmospheres. 

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Energy

Making bonds is an EXOTHERMIC process because heat is given out when a bond is formed.

Breaking bonds is an ENDOTHERMIC REACTION because the process requires more energy than what is released.

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Tests for Positive Ions

Flame test for metal ions:

Lithium = Crimson flame 

Sodium = Yellow flame

Potassium = Lilac flame 

Calcium = Red flame

Barium = Green flame 

If you add sodium hydroxide to the solution:

Calcium = white precipitate

Copper = blue precipitate

Iron 2 = green precipitate

Iron 3 = brown precipitate 

Aluminium = white precipitate but then dissovles into a colourless solution 

Magnesium = white precipitate

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Tests for Negative Ions

To test for CARBONATES add a weak acid to the solution. The reaction will take place and if the gas produced turns limewater cloudy then the compound was originally a CARBONATE.

To test for HALIDES by adding nitric acid and silver nitrate. CHLORINE produces a WHITE precipitate, BROMIDE produces a CREAM precipitate and IODINE produces a YELLOW precipitate.

To test for SULFATES add dilute hydrochloric acid and barium chloride solution. A WHITE precipitate mens the original compound was a SULFATE.

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Energy Calculation

ENERGY TRANSFERRED =

MASS OF WATER x 4.2 x TEMP CHANGE

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Bond Energies

What you need to know about bond energies:

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Alcohols

-OH functional group and end in -ol

They have similar properties to each other:

  • They are flammable 
  • They dissolves completely in water to form neutral solutions 
  • Can damage the liver and the brain 

Alcohols are used as solvents to dissolve things that water can't, for example hydrocarbons and oils. They can be used as fuels.

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Carboxylic Acids and Esters

 Carboxylic Acids have the -COOH functional group and the names end in -anoic acid.

React with substances to produces '-anotes'. Dissolve in water to produce acidic solutions.This is becuase they ionise in the water.

Esters have the -COO- functional group and their names end in '-oate'. Esters have a nice smell but don't mix well with water. They are VOLATILE and smell nice so are ideal for perfumes.

ALCHOL + CARBOXYLIC ACIDS -------> ESTER AND WATER

Inhaling the fumes of esters will irritate the mucous membranes in the nose and mouth. Ester fumes are heavier than air and are flammable. In large doses Esters are toxic. 

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