The Early Periodic Table
By the early 1800s, less than 40 elements had been discovered, although, new elements were being discovered reguarly. Scientists began noticing patterns in the way they reacted - their properties. They worked out properties like the 'atomic weight'.
In 1863, John Newlands proposed his law of octaves.
He arranged elements in order of atomic mass. He believd that similar properties were repeated in every eigth element. However, his table didn't work as some groups had a mixture of metals and non-metals, it only worked for the first few elements, and he failed to take into account that elements were still being discovered.
In 1869, Mendelleev produced a better table.
He arranged elements in a periodic way, he left gaps where he thought there were undiscovered elements, and he predicted the atomic masses and properties of the undiscovered elements.
Mendellev's table became the basis for the modern Periodic Table.
The Modern Periodic Table
When protons, neutrons and electrons were discovered in the early 20th century, the periodic table was arranged in order of atomin(proton) numbers. When this was done, all elements were placed in appropriate verticle 'groups'.
Elements in the same group have the same number of electrons in their highest occupied energy level, therefore the groups have similar chemical properties.
The number of electrons in an atom's outermost shell determines it's properties.
Group 1 - The Alkali Metals
* Low Density - Lithium, Sodium and Potassium all float on water
* React with non-metals to form ionic compounds - The all have a +1 charge. The compounds are white solids that dissolve in water to form colourless solutions.
* They are all metals
* React with water, releaseing Hydrogen
* Form Hydroxides that dissolve in water to give alkaline solutions
* The further down the group you go;
- The more reacive the element
- The lower the melting a boiling point
The Transition Elements
Compared with Group 1 Metals, the Transition Metals:
* Have much higher melting and boiling points
* Are harder and stronger
* Much less reactive as they don't react with water or oxygen
* Maluble and Ductile - good conductors of heat and electricity
Also, the Transition elements;
* For ions with different charges
* Form brightly coloured compounds - e.g. Fe2+ --> Green, Fe3+ --> Reddish Brown
* Useful as catalysts
Group 7 - The Halogens
* Poor conductors
* Produced coloured vapours
* React with metals to form ionic compounds in which the Halide ions carried a charge of -1
* The further down you go;
- The less reactive the element
- The higher the melting and boiling points
* A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt
e.g. Br2 (aq) + 2 NaI (aq) ----> 2 NaBr (aq) + I2 (aq)
Hard and Soft Water
* Soft water form a rich amount of lather with a small amount of soap.
* Hard water requires a large amount of soap to form a suitable amount of lather.
* Hard water contains dissolved magnesium and calcium compounds. These compounds are picked up when the water flows through rivers and streams, and it flows over certain types of rock containing the magnesium and calcium compounds. The compounds dissolve into the water from the rock.
* Scum is formed when the calcium ions water react with the soap. The scum is an insoluble percipitate. You have to react all the calcium ions in the water with the soap to form lather, therefore more soap is needed.
* Soapless detergents do not form scum. They are substances that do not contain the sodium stearate that reacts with calcium to form scum.
* Scale is an insoluble solid that reduces the efficiency of appliences in the home, such as kettles and boilers. The scale is a bad conductor so the kettle has to work harder and use more energy to heat the water.
Benefits: The calcium ions in the water are good for bone and teeth development, and reduce heart disease.
* Hard water can be either temporary or permanent. Temporary can be softened by heating or boiling, and permanent remains hard even when heated or boiled.
Softening Hard Water
Soft Water doesn't contain salts that produce scum or scale. Hard water can be softened by removing the salts that produce the scum and scale.
Adding Sodium Carbonate ('washing soda')
* The calcium ions in the water react with the carbonate ions in the washing soda to form calcium carbonate and sodium ions which remain in the water. The calcium carbonate is a percipitate/solid.
Ca2+ (aq) + CO 2-/3 (aq) ---> CaCO3 + Na+ (aq) * This take less energy so is cheaper.
Calcium Carbonate + Carbonate ions ---> Calcium Carbonate + Sodium Ions
The same reaction can be done for the magnesium ions in the water too.
Using Commercial Softeners
* Ion exchange columns contain hydrogen and sodium ions which, when the hard water travels through the column, are replaced with the calcium and magnesiun ions in the water.
Screens - Removes debris, Settlement tank - Sand, soil settles out, * Aluminium Sulphate - Makes particles of dirt clump together and settle, * Filter Bed - Fine sand filters remaining particles (insoluble solid), Chlorine - Added to kill bacteria,
In some areas, Flourine is added to harden teeth.
+ Reduces tooth decay, only added in small amounts. - Possible links with learning difficulties, unethical (you don't get a choice)
* Carbon - Reduce/remove Chlorine levels and possibly other chemicals (e.g. traces of perstcides)
* Ion Exchange Resin - Removes calcium, magnesium and possibly lead, copper and alluminium
* Silver - Discourages bacterial growth on the resin
Distillation - 100% pure water can be produced by distillation, but this requires large amounts of energy and is expensive. Water is heated, and then evaporated. The evaporated water is passed through tubes. It is cooled down, and then condenses as water again.
The relative amounts of energy released when substances burn can by measured by the experiment calorimerty. The method can be used to compare the amounts of energy released by fuels and foods.
* Measure cold water into a calorimeter (metal/glass)
* Record starting temperature of the water
* Heat water using the flame from the burning fuel
* Record the final temperature of the water
> Q = mc ^T = Energy Transfered to the Water ---> c = Energy needed to raise the temperature of 1g of water by 1*C
Energy Released (J) = Mass of Water (g) x Specific Heat Capacity of Water (4.2 J/g*C) x Rise in Temperature (*C)
Errors: Lot of heat lost to the surrounings, Not all the food/fuel burns, Water should be stirred to distribute the heat evenly, some enegy heats the container rather than the water.
It is useful to measure the energy per gram so we can have a comparison.
The amount of energy released or absorbed by a chemical reaction in solution can be calculated from the measured temperature change of the solution when the reactants are mixed in an insulated container. This method can be used for reactions of solids and water or for nuetralisation reactions.
Energy Level Diagrams
When a reaction takes place, we need to supply energy to break the existing bonds. Then, when making bonds, energy is released to the environment.
Energy Level Diagrams can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of the reaction.
The activation energy is the energy needed to start off the reaction. We can lower the activation by using a catalyst, and this speeds up the reaction.
An Exothermic reaction gives off heat to it's surroundings, therefore, on a graph, the products line is lower than the reactants line. An Endothermic reaction takes in heat from it's surroundings, an so on a graph the products line is higher than the reactants line.
'Bump' = Activation Energy
Difference between reactants and products = Energy Change
Difference between activation energy and products (Exo) / reactants (Endo) = Energy Released
Hydrogen for Fuel
The most widely used fuels are Petrol, Diesel and Ethanol. The problem with these type of fuels is they produce Carbon Dioxide, Sulfur Dioxide and Nitrogen Oxide. These contribute to pollution, acid rain and the greenhouse effect. Although Ethanol is slightly less harmful, we have to use land to grow the crop plants, so there is less land available for the growth of plants.
A safer alternative to these fuels is Hydrogen: + Less polutants, Less need for fossil fuels, Burns easily, Releases large amounts of energy per/g - Expensive to produce, Difficult to provide the hydrogen, Safety (it is flammable and explosive)
Ways we can use the hydrogen
* Combustion Engine - As it's a gas, it's difficult to store (Held in tanks, High pressure, Kept liquid): This is Expensive, and it is dangerous to keep gases at high pressure.
* Fuel Cells - A chemical reaction goes on inside them, and that produces electricity, heat and water. The elcetricity is used to power the car, and the heat and water are given out.
+ Reduces pollution, Have higher efficiency, Maintenance is more simple
- Production, transportation, distribution and storage of hydrogen is difficult, Driving range of cars is shorter, Refueling and starting times are longer, Expensive to produce, and uses expensive materials e.g. platinum.
Testing for Positive Ions, Flame Tests
Flame Tests - We have a wire loop made of an unreactive substance called Nicrome. The loop is dipped in concentrated Hydrochloric Acid, heated, then dipped in the hydrochloric acid again to ensure there is no other substances on the loop. We then dip the loop into the unknown solution to test for positive ions. Next, we put the loop into a blue bunson burner flame and we can identify the positive ions present in the solution by the colour of the flame.
* Lithium (Li+) - Crimson Flame
* Sodium (Na+) - Yellow Flame
* Potassium (K+) - Lilac Flame
* Calcium (Ca2+) - Red Flame (Brick Red)
* Barium (Ba2+) - Green Flame
Sodium Hydroxide solution can be used to identify different positive ions, depending on the percipitate that is formed. We begin by adding the sodium hydroxide solution to the unknown solution. Then, either a coloured or white percipitate is formed. If it is white, we then add excess sodium hydroxide solution, and if the percipitate dissolves, we know we have Alluminium ions. If the percipitate doesn't dissolve, then we either have calcium or magnesium ions. We can then do a flame test to identify them. If we get a red colour, then it's Calcium, and if there is no colour, then it is magnesium. If we get a coloured percipitate, then it is either, Copper (ii), Iron (ii) or Iron (iii).
Testing for Negative Ions
Carbonates (CO 2-/3)
To test for Carbonates, we add some dilute acid to the carbonate, and we should get a fizz if a carbonate is present. This is the release of Carbon Dioxide. We can test this by taking the gas and putting it through a tube with some limewater. When the CO2 is added to the limewater, it produces a white percipitate (calcium carbonate) which turns it cloudy.
Halides (Chloride, Bromide, Iodide)
To test for Halides, we add some dilute Nitric acid. We then add Silver Nitrate Solution. (We can't add any other acids, as they would produce a percipitate with the silver nitrate solution). Once we've done this, a percipitate is formed.
* If we have a white percipitate - Silver Chloride (presence of Cl-)
* If we have a cream percipitate - Silver Bromide (presence of Br-)
* If we have a yellow percipitate - Silver Iodide (presence of I-)
Sulfates (SO 2-/4)
To test for sulfates, first, we add dilute Hydrochloric Acid, then add Barium Chloride solution. Then all we do is look out for a white percipitate that is formed, as, if this is produced, we know there is a presence of sulfate ions. We must add hydrochloric acid to the test, not sulfuric acid, otherwise we would be adding the sulphuric ions to the test, which would defeat the object.
Titration, The Method
A Titration is used to measure the volume of one solution that exactly reacts with another solution.
The volumes of acid and alkali solutions that react with each other can be measured by Titration, using a suitable indicator.
Burette, Conical Flask, Blass bulb pipette, Phenolphthalein Indicator (NOT universal as it has a gradual change, and won't show the change at a specific enough point)
1) Add a known volume of alkali into a conical flask, using the glass bulb pipette
2) Add a few drops of Phenolphthalein Indicator to the flask
3) Pour some acid into a burette, and record a reading
4) Open the tap, add a small amount of acid and swirl the flask to mix
5) Add acid until the solution is neutral, as shown by the colour change in the indicator, (with the Phenolphthalein indicator, it goes from pink to colourless)
6) Repeat at least three times to refine your measurements
Making Ammonia, The Harber Process
Ammonia is important for making other chemicals, such as fertiliser.
Nitrogen (g) + Hydrogen (g) <------> Ammonia
The two raw materials we need to make Ammonia are Nitrogen and Hydrogen. Nitrogen comes from the air, and Hydrogen comes from natural gas (Methane).
The two gases are placed into a reaction vessel, and Ammonia comes out at the other end. We liquify the Ammonia by condensing it, and then it is ready to be taken off and used. At the end of the process, any Nitrogen and Hydrogen that didn't react is recycled and goes back into the vessel to be used again. The reason some of the hydrogen and nitrogen didn't react is because the reaction is reversible.
Conditions Required to Make the Ammonia: These help us to increase the rate of the reaction, and the yield of Ammonia
Catalyst - Iron
Preassure - 200 Atmospheres
Temperature - 450*C
Organic Compounds contain carbon, and are the basis of all living things. Examples of organic compounds include; Alcohols, Carboxylic Acids and Esters.
The Alcohols are a family of organic chemicals. They all contain the functional group -OH (Hydroxyl). We call them a Homologous series as they are members of the same group of an organic compound, with the same functional series.
Methanol - CH3OH
Ethanol - CH3CH2OH
Propanol - CH3CH2CH2OH
- Dissolve in water to form a neutral solution
- React with sodium to produce hydrogen
- Burn in air to give Carbon Dioxide and Water
- Can be used as a fuel, solvents and in alcoholic drinks
* Ethanol can be oxidised (either by chemicals or microbes) and become Etanoic acid, which is the main acid in vinegar.
The functional group for the Carboxylic acids is -COOH
Methanoic Acid - HCOOH
Ethanoic Acid - CH3COOH
Propanoic Acid - CH3CH2COOH
- Dissolve in water to produce acidic solutions
- React with carbonates to produce carbon dioxide
e.g. Ethanoic Acid + Sodium Carbonate ---> Sodium Ethanoate + Water + Carbon Dioxide
- React with alcohols (and acid catalyst) to make esters
- Do not ionise completely in water - weak acids
- They are made when alcohols react with carboxylic acids
- They have the functional group -COO-
- They are smell and taste fruity so can be used as scents and flavourings
- They are volatile so they can easily evaporate
* This is made by reacting a Carboxylic Acid with an alcohol
Ethanoic Acid + Ethanol <---> Ethyl Ethanoate + Water
CH3CH2OH (aq) + CH3CHOOH (aq) <---> CH3CH2OOCCH3 (aq) + H2O (l)
Other types of Esters include:
Methyl Ethanoate, Ethyl Methanoate and Propyl Ethanoate