Non-Metals can share electron pairs between atoms. This is known as covalent bonding.
Atoms can be made stable by transferring electrons. This is called ionic bonding.
The attraction between carbon dioxide molecules is called intermolecular forces.
The weak intermolecular forces in substances that have a simple molecular structure, such as carbon dioxide and water, meaning that these substances are easy to break apart as they have low melting points. (as there are no free electrons, carbon dioxide and water do not conduct electricity.)
The group number is the same as the number of electrons in the outer shell. If an atom of an element has has electrons in only one occupied shell it will be found in the first period; if it has electrons in two occupied shells it will be in the second period and so on.
1865-Newlands put 56 elements into groups and saw that every eighth one behaved similar. 1869-Mandeleev arranged the elements in a table, and saw the changes in properties. 1891-Mandeleev's table still do not containnoble gases.
Group 1 Elements
Caesium and rubidium both are group 1 metals, and therefore have similar properties to lithium, sodium and potassium: -They react vigorouslywith water. -Hydrogen gas is given off. -The metal reacts with water to form an alkali- the hydroxide of the metals.
It is possible to predict how an alkali metal, such as caesium and rubidium will behave by looking at the patten of reactivity in other alkali metals. e.g: Potassium reacts more vigorously than sodium, and Sodium reacts more vigorously than lithium. . The balanced symbol equation for litium and water is: . 2Li + 2H2O ----> 2LiOH + H2 Atoms of group 1 alkali metals have similar properties because they have one electron in the outer shell.
If electrons are lost this is called oxidation.
If electrons are gained this is called reduction.
Group 7 Elements
They have seven elements in the outer shell. It is possible to predict the properties of halogens because they follow trends. Halogens have similar properties because when they react each atom gains one electron to form a negative ion with a stable electronic structure. The nearer the outer shell is to the nucleus, the easier it is for an atom to gain one electron. The easier it is to gain the electron, the more reactive the halogen.
When halogens react with alkali metals, a metal halide is made. For example, when potassium reacts with iodine, the metal halide produced is potassium iodide.
If halogens are bubbled through solutions of metal halides there are two possible outcomes: no reaction or a displacement reaction. A displacement reaction is shown in the example below: -Chlorine displaces bromine to form a bromine solution.
Chlorine + potassium bromide ---> Potassium chloride + bromine
Compounds that contain ransition elements are often coloured. e.g. Copper is often blue. Iron(II) are often pale green and Iron(III) are often orange/brown.
Transition elements are often used as catalysts, e.g. Iron is used in the Haber process and Nickel is used in the manufacture of margarine to harden oils.
If a transition metal carbonate is heated it undergoes thermal decomposition to form a metal oxide and carbon dioxide.The metal carbonate changes colour during this reaction.
Sodium hydroxide solution reacts with compounds of each transition metal to make a solid of a particular colour. The addition of sodium hydroxide solution identifies the presence of the transition metal ion in solution(these form metal hydroxide precipitates):
-Cu2+ ions form a blue solid
-Fe2+ ions form a grey/green solid
-Fe3+ ions form an orange/brown 'gelatinous' solid.
Metal Structure and Properties
Metals have specific properties in order to make them suitable for different uses. -An example of a physical property is the high thermal conductivity of copper. Saucepan bases need to be good conductors of heat. Other properties include being lusterous, malleable or ductile. -An example of a chemical property is the resistance to attack by oxygen or acids shown by gold. Copper is also resistant, which is another reason why it is used for saucepans.
Metals have high melting points and high boiling points, due to their strong metallic bonds. The bonds between the atoms are very hard to break and alot of energy is needed. Copper, silver and gold conduct electricity very well. A metallic bond is a strong electrostatic force of attraction between close-packed positive metal ions and a sea of delocalised electrons. Metals often have high melting points because a lot of energy is needed to overcome strong attractions between delocalised electrons. Superconductors are materials that conduct electricity with little/no resistance. The potential benefits of superconductors are: Loss free power transmission, Super fast electronic circuits and powerful electromagnets. However, the disadvantages are that they only work at very low temperatures and that superconductors that function at 20'c need to be developed.
The water in a river is cloudy and cannot be drunk. To turn it into clean water it has to be purified before being passed through the taps. Some pollutants such as nitrates from fertilisers and pesticides from crop spraying get in to the water before purification, and some, such as lead from old water pipes get in after treatment. There are three main stages in water purification: -Sedimentation- Chemicals are added to make solid particles and bacteria settle out. -Filtration- A layer of sand on gravel filters out the remaining fine particles , also moves out microbes. -Chlorination- Chlorine is added to kill microbes.
These steps cost money so water must be conserved to provide clean drinking water, e.g. it takes energy to pump and purify, which increases global warming and also water is a renewable resource but the supply is not endless.
Sea water has so many substances dissolved in it that is undrinkable. Techniques such as distillation must be used to remove the dissolved substances. Distillation takes huge amounts of energy, so it is very expensive and only used when there is no fresh water.
Molar Mass and Moles
Metal Carbonates will thermally decompose.
Metal Carbonate ---> Metal Oxide + Carbon Dioxide
Number of Moles= Mass of chemical . Molar Mass
Mass of chemical= Number of moles x molar mass .
1- Write down the mass of each element
2- Look up the relative atomic mass of each element
3- Work out how many moles there are of each element (divided by atomic mass)
4- Choose the element present in the smalled amount
5- Divide the moles of each of the other elements in the compound by the moles of the one in the smallest amount
% mass of element= Total mass of the element in the compound x100 . Relative formula mass of compund
As concentration increases the solute particles become more crowded.
If a solution is diluted three times, it will take up three times more volumes.
A relationship exits between the amount of moles, concentration in mol/dm3 and volume in dm3
Amount of moles= concentration x volume
Concentration= amount in moles (divided by) volume
Volume= amount in moles (divided by) concentration
The government has introduced a colour coded traffic light system for food labels showing the types of food such as sugar, salt and fat and their quatities.
Sodium ions are essential in our diet for water balance and nerve responses, but too much can cause high blood pressure and heart disease.
In a titration an acid and alkali neutralise.
They are used to find a concentration of an alkali.
Acid + Alkali ---> Salt + Water
Normally only an alkali is present at the beginning of the experiment so the pH number is high. As acid is added it starts to neutralise the alkali and the pH drops slowly at first, the point at which all the acid reacts with all the alkali is the end point.
Indicators are chosen as they give sharp colour changes at specific pH. Indicators such as phenolphthalein, screened methyl orange and litmus give a sudden colour change. Mixed indicators such as universal indicator comes in a range of colous.
There are three ways to collect a gas given off in a reaction so that the gas volume can be measured:
1- If an upturned burette or measuring cylinder is used it must be filled with water before it is turned upside down. The volume is read off the scale on the side.
2- The scale on a burette goes the opposite way to that on a measuring cylinder.
3- If a balance is used, a loose plug of cotton woolin the neck of the flask prevents a spray. The volume of gas produced can be calculated from the mass of gas produced.
E.g. At room temperature and pressure, 1 mole of a gas takes up 24dm3. If 3 moles were given off in a reaction the volume would be: Volume of gas= number of moles x 24 Volume of gas= 3 x 24= 72
The number of moles can then be found id the gas volume in known.
Gas Volumes (2)
When the amount of magnesium ribbon reacts with acid, the amount of hydrogen given off varies with the amount of the limiting reactant. If the magnesium is used up first this is the limiting reactant. This is the reactant that is used up first.
A reaction is at equilibrium when a balance of amounts of reactants and products is reach ed. The rate of the forward reaction equals the rate of the backwards reaction. The concentrations of reactants and products do not change. When the backward and forward rates balance, the concentration of reactants and products are equal. if the concentration of reactants is greater than that of the products the equilibium position is left. If the concentration of the the reactants is less than that of the products, the equilibrium position is right.
Changing temperature, pressure or concentration can all alter the position of the equilibrium. As the temperature increases in this reaction, less product is moving to the left.
Strong and Weak Acids
Acids such as HCl contain hydrogen atoms. In water, acid molecules ionise. The hydrogen atoms form hydrogen ions.
Stong acids ionise completely when they are in water, There are lots of hydrogen atoms so the water seems very acidic and lots of collisions occur, so the acid and water react quickly:
Strong acid---> Hydrogen Ions + Other Ions
In weak acids, such as ethanoic acid, only a few of the acid molecules ionise. A reversible reaction forms an equilibrium mixture, the equilibrium position is on the left. The mixture contains lots of acid molecules but not many H+ ions, so weak acids do not seem to be so acidic. This means fewer collisions occur.
Weak Acid ---> Hydrogen Ions + Other Ions
Strong and Weak Acids (2)
Acid reactions are caused by hydrogen ions, H+. The pH scale is used to show the concentration of H+ ions.
High concentration means a low pH number Low concentration means a high pH number Rate is a measure of the number of collisions every second. This is the collision frequency. In strong acid is the concentration of H ions is higher and the collision frequency is greater. In weak acids the H ion concentration is lower and therefore the collision frequency is lower.
Ionic substances form ionic lattices. The ions are fixed in position within the solid lattice. The lattice of ionic substances breaks apart in water and the seperated ions can moves. In precipitation reacions, the reatant ions can move in solution but must collide to react. In a precipitation reactin involving mixing two different ionic solutions, there is an extremely high collision frequency between the ions of the two different solutions, so the reacion is extremely fast.
Precipitation reactions can be summarised as: AB + CD ---> AD + CB E.g. Barium Chloride + Sodium Sulfate ---> Barium Sulfate + Sodium Chloride
Preparing a clean and dry sample of an insoluble salt: Step 1- Mix the two solutions to make barium sulfate and sodium chloride. Step 2- Filter the precipitate of barium sulfate, which stays in the filter paper. Step 3- Wash with distilled water to remove traces of the sodium chloride solution. Step 4- Dry by leaving the precipitate in a warm place for the water to evaporate.
When lead nitrate and sodium iodide react, only lead ions react with iodide ions as they are not precipitated out of the solution. The other ions do not directly take part in the reaction. They are called spectator ions.
Electrolytes are ionic. The charge moves through the molten liquid by the movement of ions. If the electrolyte solidifies then the ions are in fixed positions and cannot move and the current cannot flow. Electrolysis is the flow of charge by moving ions. These moving ions discharge at electrodes.
Positive ions move to the cathode. Negative ions move to the anode.
The reaction at the cathode can be written as: Na+ + e- ---> Na The sodium chloride splits up into ions, so the ions are free to move. The position of Na+ ions migrate towards the negative cathode. Each Na+ ion gains one extra electron from the cathode.
The reaction at the anode can be written as: 2Cl - 2e ---> Cl (2) The negative Cl ions are discharged as chlorine gas. Two Cl ions each gain one elctron and combine to form a chlorine molecule.
The amount of substance formed during electrolysis varies with time and current- it increases with increasing time and with incrasing current. The substances produced at the electrodes are discharged ions. The only thing that affevts the number of ions discharged is the amount of charge transferred.
- The water produced is not wasted
- They are lightweight: normal batteries are heavier
- They are compact
- There are no moving parts
- No carbon dioxide emissions are released when they are used for cars.
- Fossil fuels are non-renewable so this is an alternative (for cars)
- The main product of a hydrogen-powered fuel cells is water, which is not a main pollutant.
- Especially useful in mobile energy sources.
(Hydrogen-Oxygen fuel cells)
- There is direct energy transfer.
- Fewer production stages, this makes it more efficient.
- Less polluting.
- Weigh less
- Poisonous catalysts are used that have to be disposed of at the end of the life-time of the cell.
-Fossil fuels are burnt to produce the hydrogen and oxygen needed.
The two processes are oxidation and reduction.
Rusting is a redox reaction. ( Iron + Water ---> Hydrated Iron (lll) Oxide )
Galvanisng protects iron from rusting by covering it with a leyer of zinc, the thin layer stops water and oxygen from reaching the surface of the iron, zinc also acts as a sacrificial metal as it is more reactive than iron.
When iron rusts, the oxygen is a oxidising agent as it takes electrons from the iron. Iron loses electrons (oxidises) and the oxygen gains electrons (reduction).
Redox reactions can be forced in either direction. One direction is oxidisation and the other direction is reduction.
When a word equation is written for a displacement reaction is written, the more reactive metal swaps place wth the less reactive metal. E.g
Magnesium + Zinc Sulfate ---> Magnesium Sulfate + Zinc
Displacement happens with reactive metals and compounds of a less reactive metal. All metals react by pushing off electrons and turning in to ions. This is oxidation.
Ethanol can be made from glucose solution by a process called fermentation. Carbon dioxide is also produced as a by-product:
glucose → carbon dioxide + ethanol
C6H12O6 → 2CO2 + 2C2H5OH
a moderate temperature of 25°C to 50°C
water (provided by the glucose solution)
an absence of oxygen (air must be kept out)
Fermentation is a slow process and stops when the ethanol concentration reaches about 15%. The ethanol is separated from the reaction mixture using fractional distillation.
The ozone layer
The ozone layer is the part of the upper atmosphere where ozone is found in the highest concentrations. The ozone there absorbs ultraviolet radiation, preventing most of it from reaching the ground. This is important because ultraviolet radiation can lead to skin cancer.
Near the end of the last century, scientists discovered that ozone levels over the Antarctic were reduced. This discovery was unexpected. Chemists knew that reactive chlorine atoms could destroy ozone. They also knew that chemicals called chlorofluorocarbons - CFCs - break down in ultraviolet light to release reactive chlorine atoms. Scientists used these ideas to explain the low ozone levels.
CFCs were once used widely in insulating foam and aerosol spray-cans. Once released, they gradually spread through the atmosphere, eventually reaching the ozone layer. Once there, they destroy ozone. CFCs have now been almost completely replaced by chemicals that do not cause this damage.
Hard and soft water
Rainwater is naturally weakly acidic because it contains carbonic acid, formed by the reaction between water and carbon dioxide in the atmosphere. When the rain falls, it flows over rocks or soaks into the ground and then passes through layers of rock. Compounds from the rocks dissolve into the water.
Hard water contains dissolved compounds, usually calcium or magnesium compounds. For example, limestone contains calcium carbonate, CaCO3. Carbonic acid in rainwater reacts with this to produce soluble calcium hydrogencarbonate:
carbonic acid + calcium carbonate + water→ calcium hydrogencarbonate
H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq)
The presence of calcium ions and magnesium ions in the water makes it hard. Soft water readily forms lather with soap, but it is more difficult to form lather with hard water. The dissolved calcium ions and magnesium ions in hard water react with the soap to form scum, so more soap is needed. Soapless detergents do not form scum with hard water.
The types of rocks found in different regions determines how hard or soft the water will be.
The water in some parts of the country is soft because it has low levels of dissolved calcium and magnesium compounds, while the water in other parts of the country is hard because it has higher levels of dissolved calcium and magnesium compounds.
Fats and oils
Natural fats and oils are important raw materials. They are used in the chemical industry to make soap and biodiesel. Biodiesel is an alternative fuel to diesel made from crude oil. It is a renewable fuel, whereas normal diesel is a non-renewable fuel. Natural fats and oils from plants are also used to make margarine.
Natural fats and oils can come from plants or from animals. At room temperature:
fats are solids
oils are liquids
However, fats and oils have very similar chemical structures. They are esters (with a glycerol part joined to fatty acid parts).
Unsaturated fats and oils
saturated fats and oils contain no carbon-carbon double bonds
unsaturated fats and oils contain one or more carbon-carbon double bonds
The displayed formula for an unsaturated fat or oil will show C=C in its structure, but the displayed formula for a saturated fat or oil will not.
Testing for unsaturation
Orange bromine water can be used to test for unsaturation. When it is added to a sample of the fat or oil:
the bromine water stays orange with a saturated fat or oil
the bromine water goes colourless with an unsaturated fat or oil
Soap is made by reacting natural fats and oils with hot sodium hydroxide solution. Ester bonds are broken in the reaction, producing:
soap (sodium salts of the fatty acid parts)
This process is called saponification.
Here is the general word equation for saponification:
fat + sodium hydroxide → soap + glycerol
The type of reaction involved is called a hydrolysis reaction.
In a hydrolysis reaction, a substance reacts with water and becomes split as a result. The hydroxide ions present in saponification act as a catalyst, with the water needed supplied by the sodium hydroxide solution.
Detergent molecules are long and thin. It is good for the environment to wash clothes at 40'c or below. Washing machines need lots of energy and therefore contribute to greenhouse gases. Good for coloured clothes as many dyes are damaged at high temperatures.
One end is hydrophilic - it loves water but hates oily liquids.
The other end is the opposite - hydrophobic - it likes oil but hates water.
When clothes are washed dirt is lifted off the clothes and put in to the water. If the dirt is made from oil or grease it will not dissolve in water. Detergents remove fat or oil stains by forming stong intermolecular bonds with both the oil and the water. The molecules of a stain are similar to each other. They cling together (by intermolecular forces) and so they're difficult to remove. A dry cleaning fluid consists of molecules similar to the stain molecules so the intermolecular forces between stain and solvent are similar to those between stain and stain. This enables the stain molecules to separate from each other and dissolve in the solvent. They can then be carried away.