1) They have low melting and boiling points (many are liquids or gases).
2) They tend to be soft and/or have little strength.
3) They do not conduct electricity
Simple molecular substances are nearly always non-metallic elements or their compounds.
e.g. Water (H2O), Iodine (I2), Carbon dioxide (CO2), Sugar (C12H22O11).
Giant Covalent Substances
In these substances, strong covalent bonds join atoms together in large numbers to make giant structures. Sand (silicon dioxide), diamond and graphite (both forms of carbon) are examples.
Diamond has a 3 dimensional structure where each carbon is bonded to 4 others. Diamond is extremely hard, does not dissolve and does not conduct electricity.
Graphite has a layer structure. In each layer the carbons are bonded to 3 other carbon atoms. The layers can slip over each other making graphite soft and slippery (it is used in pencil leads)
Silicon Dioxide is similar to diamond in the arrangement of its atoms.
Metals
Metals have atoms in layers. These layers can slide over each other so that metals can be bent or shaped. Metals are also strong and hard.
Giant Ionic Structures
Compounds of metals, such as sodium chloride (NaCl) or copper sulphate (CuSO4) are made up of positive metal ions (e.g. Na+ ) and negative non-metal ions (e.g. Cl-).
The ions are arranged in a regular lattice so positive and negative ions are arranged alternately in 3 dimensions.
1) They have high melting and boiling points.
2) They are hard but brittle (shatter easily)
3) They conduct electricity only when melted or dissolved in water.
4) Many ionic substances are soluble in water.
Calculations Summary (F)
Formula masses (Ar and Mr)
The Ar is the relative atomic mass from the periodic table. The Ar values of the atoms in a formula are added to get the formula mass or Mr
H2SO4 = 2x1 + 32 + 4x16 = 98
Mg(NO3)2 = 24 + (14 + 3x16)x2 = 148
Percentage Calculations
If you need to find the percentage of an element E in a compound, you use formula
Percentage = Ar of E x No of atoms of E x 100
Mr of compound
e.g. Find the percentage of nitrogen in ammonium nitrate (NH4NO3)
- Mr of N = 15
- No of N atoms = 2
c) Mr of ammonium nitrate = 14 + 4 x 1+ 14 + 3 x 16 = 80
Percentage of nitrogen = 2x14x 100 = 35%
80
Atom Economy
This is given as Formula Mass of all molecules of Useful Product x 100
Formula Mass of all molecules of Reactants
Eg. Atom economy for making iron from the equation
Fe2O3 + 3CO = 2Fe + 3CO2
(Mr : Fe2O3 =160, CO = 28, Fe = 56, CO2 =44)
Mass of useful product (iron atoms) = 2x56 =112
Mass of all reactants = 160 + (3x28) = 244
Atom economy = 112 x100 =45.9%
244
Percentage Yield.
To find the percentage yield, divide the mass obtained in an experiment by the maximum mass which could have been obtained and multiply by 100.
e.g. the maximum mass of alcohol which could be obtained by fermenting some sugar is 4.6g of alcohol. The actual mass obtained in an experiment was 3.5g. Find the % yield.
Answer = 3.5 x 100 = 76.1%
4.6
Rates of Reaction (F)
There are 4 main ways of controlling the rate of a reaction:
Catalysts
Catalysts are substances which increase the rate of chemical reactions without being chemically changed. They do this by providing a different route for the reaction with a lower activation energy.
Enzymes are biological catalysts.
Other examples of catalysts are:
Catalysts are very valuable in chemical industry, since they can be reused and they provide a much cheaper way of speeding up a reaction than heating, so providing cheaper goods.
Temperature
Another way of controlling the rate of a reaction is by temperature. A rise of 10oC will roughly double the rate of a reaction. This is because at higher temperature, the particles move faster, they therefore collide more often and with more energy This makes the reaction faster.
Cooling down a reaction will make it slower, this means that food will deteriorate more slowly at low temperatures (e.g. in a fridge or freezer)
Remember that raising temperature does not make any difference to the amount produced, only to the speed of the reaction.
Concentration
The concentration of a solution is the amount of substance dissolved in a solution..
Reactions happen more quickly when concentrated solutions are used because the particles are closer and so are more likely to collide and react.
Pressure
When gases are at high pressure, their particles are closer, and so are more likely to collide and react.
Surface Area
Solids in powder form will react much more quickly than those in lump form. This is because the powder particles have much more surface area exposed to the other reactant. If a lump is used, only the outside of that lump can react.
Summary
If we take a reaction such as the reaction between zinc an sulphuric acid:
ZINC + SULPHURIC ACID = ZINC SULPHATE + HYDROGEN
The rate of this reaction can be measured by seeing how much hydrogen gas is given off in one minute. This rate can be increased by:
1) Raising the temperature of the reaction.
2) Using more concentrated sulphuric acid.
3) Using powdered zinc rather than lumps.
4) Adding some copper, which acts as a catalyst.
The rate of a reaction can be measured and followed on a graph, e.g.
Curve A represents a reaction between powdered zinc and sulphuric acid, while curve B represents a reaction between lumps of zinc and the same amount of sulphuric acid. You should note:
a) That a steeper curve (A) represents a faster reaction.
b) That the curves start steeply, but then level off. This is because the concentration of the acid is high at the start of the reaction, but gets less as the acid is used up and eventually stops.
c) That the amount of gas made by the end of the reaction is the same as long as the amounts of reactants are the same. The rate of a reaction does not affect the final amount of product made.
Measuring the Rate of a Reaction
The rate of a reaction is defined as
Rate = Amount of Reactant Used up or Amount of Product Formed
Time
A reaction can be followed in various ways, e.g. by seeing how much mass is lost in a certain time, or how much gas is produced in a certain time.
For instance, a number of similar experiments can be done under similar conditions, e.g. when reacting limestone with hydrochloric acid, different concentrations of hydrochloric acid could be used and the time taken to collect a boiling-tube of gas (50cm3) could be compared.
Also, the rate of a reaction changes as it proceeds. When the reaction starts, the reactants are quite concentrated, but as it goes on, the reactants become less concentrated, and the reaction slows down and eventually stops.
Energy changes in reactions (F)
Reactions which give out energy are called Exothermic. They cause the temperature to rise. Many reactions are exothermic, including all burning (combustion) reactions
e.g. 2Mg + O2 = 2MgO
Reactions which take in energy are called Endothermic. They may cause the temperature to fall. Thermal decompositions are always endothermic
e.g. CaCO3 = CaO +CO2
Some reactions require an initial input of energy to start the reaction (called the activation energy) but are exothermic. E.g. the match to light the fire.
Reversible reactions
If a reaction is exothermic in one direction, it will be endothermic in the other direction and the amount of heat given out by the exothermic reaction will be equal to the amount of heat taken in by the endothermic reaction.
e.g. HYDRATED COPPER SULPHATE = ANHYDROUS COPPER SULPHATE + WATER
(Blue) (White)
If the hydrated copper sulphate is heated, it turns white as the water is given off. This requires energy so the forward reaction is endothermic
When water is added to anhydrous copper sulphate, it turns blue and heat is given out so the reverse reaction is exothermic. This reaction is sometimes used as a test for water.
In any industrial process it is necessary to minimise the amount of energy used. This makes the process cheaper, and does less harm to the environment. In general non-vigorous conditions should be used.
ELECTROCHEMISTRY (F)
Conductors
Metals and graphite are the only solids which conduct electricity, but no chemical change is involved. Liquid (melted) metals also conduct, but again there is no chemical change.
Electrolytes
These are liquids which conduct electricity, and are decomposed by it. They are ionic substances which are dissolved in water or have been melted. This includes all acids and metal compounds.
Examples: Copper sulphate solution, iron chloride solution, molten sodium chloride, dilute sulphuric acid.
Non-electrolytes are covalent substances, e.g. pure water, sugar solution, alcohol, petrol.
Electrolysis
This is when an electric current passes through an electrolyte.
Electrons enter the solution through the negative electrode (cathode), cause a chemical change and leave by the positive electrode (anode).
Molten electrolytes are split into their elements by electrolysis.
The metal is produced at the cathode (-), while the non-metal is produced at the anode (+)
e.g. Lead Bromide (molten) = Lead (at the cathode) + Bromine (at the anode)
Aluminium Oxide (molten) = Aluminium (cathode) + Oxygen (anode)
With aqueous electrolytes, the electrolyte is also split up, but if the metal is reactive, then hydrogen from the water is produced at the cathode in place of the metal.
e.g. Copper chloride (aq) = Copper (at the cathode) + Chlorine (at the anode)
e.g. Sodium chloride (aq) = Hydrogen (at the cathode) + Chlorine (at the anode)
The lower in the reactivity table a metal is (including hydrogen), the more likely it is to be released at the cathode.
Electrolysis of Sodium Chloride Solution
When sodium chloride solution is electrolysed a number of useful products are formed.
- Hydrogen is given off at the cathode. (cathode is -, hydrogen ions are +)
(Not sodium as reactive metals cannot be released when solutions are electrolysed)
- Chlorine is given off at the anode (anode is + chloride ions are -)
- Sodium hydroxide is left in solution
Uses of products:
Chlorine: Purifying water, making PVC plastic.
Hydrogen: Making margarine or ammonia.
Sodium Hydroxide: Making soap.
Electrolysis of Copper Sulphate Solution using Copper Electrodes
If copper sulphate (CuSO4) solution is electrolysed using copper electrodes instead of carbon electrodes.
- Copper dissolves away from the positive anode and
- Copper is deposited at the negative cathode
- The solution remains the same as it is losing copper ions at the cathode but gaining them at the anode.
This is used industrially to purify copper. A lump of impure copper is used as the anode and a rod of pure copper as the cathode. Copper is dissolved away from the anode, as above, leaving behind any impurities. Pure copper is deposited on the anode.
Other Uses of Electrolysis
Many of the more reactive metals (e.g. sodium, magnesium, calcium) can only be extracted from their ores by electrolysis.
Electroplating allows a thin layer of one metal to be deposited on another.
e.g. Iron can be protected from rust by coating with chromium or nickel, or a cheap metal can be coated with silver or gold to make it look expensive.
The object to be plated is made the cathode, while the metal to coat it is made the anode. The coating metal is also present in the solution.
e.g. to nickel plate a piece of iron, the iron would be the cathode, the nickel would be the anode and the solution would be nickel sulphate.
Acids and alkalis (f)
Acids
1) They are corrosive.
2) They have a pH of less than 7
3) They react with and neutralise bases.
4) They produce hydrogen (H+) ions in water.
Examples
Sulphuric acid H2SO4
Hydrochloric acid HCl
Nitric acid HNO3
Bases
1) Bases react with and neutralise acids.
2) Metal oxides and metal hydroxides are bases
3) ALKALIS are soluble bases
4) Alkalis produce OH- ions in water.
Examples
1) These are soluble and are therefore alkalis as well as bases
Sodium hydroxide NaOH
Calcium hydroxide Ca(OH)2
Potassium hydroxide KOH
Ammonia NH3
2) These bases are insoluble
Magnesium oxide MgO
Copper oxide CuO
Salts
These are crystalline substances, generally compounds of metals. They are formed when an acid neutralises a base. The salt is formed when hydrogen in an acid is replaced by metal atoms.
Sodium chloride NaCl (A salt of hydrochloric acid, HCl)
Copper sulphate CuSO4 (A salt of sulphuric acid H2SO4)
Potassium nitrate KNO3 (A salt of nitric acid HNO3)
Reactions of acids
1) Acids react with and neutralise bases
ACID + BASES = SALT + WATER
The name of the salt comes from the metal in the base and a word derive from the acid
- Sulphuric acid gives sulphates
- Nitric acid gives nitrates
- Hydrochloric acid gives chlorides
e.g. Sodium hydroxide + sulphuric acid = sodium sulphate + water
Ammonia reacts with acids to give ammonium salts such as ammonium nitrate. These are valuable fertilisers
2) Acids react with metals (except for the unreactive metals such as copper, silver and gold)
ACID + METAL = SALT + HYDROGEN
e.g. Zinc + sulphuric acid = zinc sulphate + hydrogen
Neutralisation
To find out the neutralisation point for an acid-alkali reaction, various techniques can be used:
1) When Universal Indicator becomes pale green (pH7)
2) When a pH meter indicates that the pH is 7
Therefore when an acid reacts with an alkali, H+ ions from the acid react with OH- ions from the alkali to make water
H+ + OH- = H2O
Making salts
Method 1. Used to make sodium, potassium & ammonium salts. An acid is to react with an alkali.
1) The alkali is measured out using a pipette, indicator is added, and enough acid is added from a burette to just change the colour of the indicator.
2) The volumes of acid and alkali used are noted, and the experiment is repeated using the same volumes, but no indicator.
3) The solution is evaporated to leave the salt.
Method 2 Where an acid is reacted with an insoluble substance (base or metal).
1) Some acid is measured into a beaker and warmed.
2) The insoluble base is added a little at a time until no more will dissolve.
3) The solution is filtered to remove any unreacted material, and the filtrate is left to evaporate, leaving the pure salt.
Method 3. Used to make an insoluble salt
- We find solutions which contain the two halves of the salt.
- These solutions are mixed and form a solid precipitate, which can then be filtered off.
- The precipitate is washed and dried
e.g. to make silver , we mix solutions of silver nitrate and sodium chloride solutions.
Silver Nitrate + Sodium Chloride = Silver Chloride + Sodium Nitrate
This method can also be used for purifying water. A chemical is added to the impure water to precipitate out the impurities.