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WJEC GCSE Chemistry 2
Transcript of WJEC GCSE Chemistry 2
Atomic Structure and the Periodic Table
REACTIONS OF ALKALI METALS & HALOGENS
CHEMICAL BONDING, STRUCTURE & PROPERTIES
RATE OF CHEMICAL CHANGE
BASIC ORGANIC CHEMISTRY
All Atoms contain three things
in the nucleus
in orbits around the nucleus
Atoms have no charge because the number of electrons = number of protons
This number is the atomic number. This is the number of protons in the nucleus
This number is the mass number. The number of protons and neutrons in the nucleus
These are group numbers, this is the number of electrons in the outer shell
Periods run horizontal, this is the number of shells
This is how the electrons are arranged in the first 20 atoms. Atoms with full outer shells are unreactive.
Atoms can have different versions called
. These have the
They will have the same atomic number but different mass numbers
Group 1 Reactions
Group 1 + Water
M + H O MOH + H
Group 1 + Oxygen
4M + O 2M O
Group 1 + Group 7
M + X 2MX
M = Any group 1 metal
X = Any group 7
Group 1 reactions get more violent down the group because the electron lost gets further away from the nucleus so is more easily lost
Group 1 with Water
Floats on the surface, fizzes around.
Floats on the surface, fizzes around, forms a ball
Sinks at first, then rises up and fizzes. Forms a ball and bursts into a lilac flame.
Group 7 Reactions
Group 7 are more reactive at the top of the group.
They get smaller up the group so the nucleus is closer to the outside and has a larger attractive force on the extra electron.
Group 7 with Iron
3X + 2Fe 2FeX
Remember X is any halogen (Cl, Br, I)
Solution goes from colourless to orange-red
Solution goes from colourless to red
Solution goes from colourless to orange-yellow
Solution goes from colourless to orange-yellow
Solution goes from colourless to orange-yellow
Solution stays colourless
HALOGENS - DISPLACEMENT REACTIONS
Group 7 with Halide
Cl will displace any halide (Br and I
Br will only displace I
I will not displace any halide
Cl + 2I 2Cl + I
Cl + 2Br 2Cl + Br
Br + 2I 2Cl + I
Identifying Group 1 and 7
Identify Group1 by flame colour. Burn the sample and examine the flame.
Lithium - Red
Sodium - Yellow- Orange
Potassium - Lilac
Group 7 identified using Silver Nitrate
Add Silver Nitrate to a solution of your halide ion
Observe the precipitate (solid) formed.
chloride - white precipitate
bromide - cream precipitate
iodide - yellow precipitate
X + Ag Ag X where X is a halide
Three Types of bonding
Formed when atoms lose/gain electrons to form Ions
Formed by metal and non metals generally
Metals lose electrons and become positive
Non Metals gain electrons and become negative
Group 1 - lose 1 electron and become +1
Group 2 - lose 2 electrons and become +2
Group 6 - Gain 2 electrons and become -2
Group 7 - Gain 1 electron and become -1
How to draw this type of bonding
When a solid they form ionic lattices, this explains many of the properties of ionic compounds.
Lattices with +2 and -2 ions will be stronger than +1 and -1. As there is a stronger attraction between the ions
Conduct when dissolved or molten
Ions free to move when lattice is broken down
High melting points
To melt the lattice you have to overcome the electrostatic attraction of the ions
Formed when atoms share electrons
Generally formed by two non metals
Two types - Simple molecular and Giant covalent
Dot cross diagrams for common covalent compounds
1 electrons shared is 1 bond
4 electrons shared is a double bond
Have very week bonds between each molecule
So these have low melting points
No free electrons or charges so never conduct electricity
These form giant compounds of covalently bonded atoms.
The ones you need to know are
- carbon nanotubes
4 Strong covalent Bonds
So very hard but brittle
Does not conduct (no free electrons)
Very high melting point
3 strong bonds (in the layer) and 1 weak (between layer)
So the layers can slide long each other - Graphite excellent solid lubricant
Each carbon has 1 free electron so graphite conducts along the layers.
High melting point as the bonds in the layers are very strong
Made of rolled up sheets of graphite
Very strong but light - ideal for carbon fibers
Conduct along their length but not across so can be used as wires
This is due to bonding (structure made of carbons with 3 covalent bonds and 1 free electron)
Metals their own set of bonds
Formed from sea of free electrons and positive ions
More free electrons then the stronger the metallic bond (So aluminum with its three electrons, is stronger than sodium with is 1)
Free electrons allow metals to conduct electricity
The structure allows the metal atoms to move around each other so metals are malleable (hammered in to sheet) and ductile (drawn in to wires) as the free electrons stabilise the ions.
These materials have properties that change when a
change occurs in their environment;
This change is reversible.
The ones you must know are
thermochromic pigments – change colour with
photochromic pigments – change colour with changing light intensity
hydrogels – absorb/expel water and swell/shrink (up to 1000 times their volume) due to changes in pH or temperature
shape memory alloys – regain original shape when heated
shape memory polymers - regain original shape when heated
Rate of reaction is always the change of something over a give time
Ways to measure rate
We can measure rate of reaction in a number of ways.
If a gas is given off we can collect that gas
If a precipitate is made (solid) we can measure the time to go cloudy.
This method can be improved using a light sensor to determine end point as deciding when the cross is obscured is personal choice
steeper the curve the faster the rate
if the line goes horizontal the reaction has finished
Factors effecting Rate
Higher the Temperature the faster the rate because the particles move around faster and with more energy. So they collide with the high enough energy to react more frequently. More frequent collisions = faster rate
Many more molecules in region 2 than in region 1
For a reaction to happen particles must collide with enough energy
The higher the concentration the faster the rate because higher concentrations have more particles in the same volume. More tightly packed particles will collide more often so will have a faster rate
Smaller Particles have a larger surface area to volume ratio. More surface area means more particles are available to collide
A catalyst increases the rate of a chemical change while remaining chemically unchanged itself. They reduce the energy needed for a successful collisions
Lower energy means more molecules have enough energy to react. More molecules mean faster rate.
Catalysts are used in industry because they
reduce energy costs
reduce the need for raw materials
Although Catalysts do not get used up, they can become affected by impurities and need to be replaced.
New Catalysts are always being sought to improve their effect
hydrocarbon – compounds containing
carbon and hydrogen only
Most hydrocarbons are found in crude oil and an are separated using fractional distillations, where hydrocarbons with similar chain lengths have similar boiling points.
Most basic hydrocarbons
Contain only singly bonded carbons and hydrogens
The are saturated hydrocarbons
They burn well but otherwise are unreactive
General formula C H .
So for an number of carbons there will be 2 x that number plus 2 hydrogens
Next family of hydrocarbons
Contain one double bonded carbon with singly bonded hydrogens and carbons attached
The are unsaturated hydrocarbons
The double bond makes them quite reactive.
General formula C H .
So for an number of carbons there will be 2 x that number hydrogens
We can test for these using bromine water. They decolourise bromine water from yellowy brown to colourless
Alkenes with hydrogen and bromine
Alkenes react with hydrogen and bromine in the same way.
In both hydrogens and bromines add across the double bond.
you dont need to know this name
The exam board says you need to know about 4 polymers
They are all addition polymers.
They are made by opening the double bond of an alkene up and joining them all together, making long chains.
Uses of our polymers
- most common plastic, used for plastic bags
- Brand name is Teflon, used in non-stick surfaces and also in Gore-Tex waterproofing
- moulded in to hard plastics such as windows or door frames
- used in ropes and textiles
Thermosetting and Thermoplastics
Plastics come in two types
Ones which soften when heated and those that dont.
If they soften they are called thermoplastics
If they dont they are called thermoset
All the polymers we have seen are thermoplastics
Bakelite and melamine are thermosets
Empirical formulas are the simplest ratio of atoms in a compound.
Molecular formulas are the scaled up ratio. In the exam any empirical formulas will also be the molecular formula.
Steps to calculate Empirical formula
1. Write out each element
2. Underneath write the mass/percentage of each element
3. Divide each mass/percentage by the Ar of each element
4. Divide the numbers from step 3 by the smallest to give a ratio 1:
5. Multiply to give whole number rations. (Can round up or down 0.3 or less)
6. Check compounds make sense (e.g. check charges on ions)
A sample of magnesium oxide was found to contain 0.19g of magnesium and 0.1g of oxygen. What is the empirical formula?
1. Mg O
2. 0.19 0.1
3. 0.19/24 0.1/16
4. 1.27 1
5. 1 1
6. MgO (Mg is +2, O -2 so charges balance)
From % composition can also be used to find the mass of each element in a compound.
What is the percentage composition of NaOH?
Stage 1: find the relative atomic mass (RAM) numbers of the elements
Mass number sodium = 23
Mass number oxygen = 16
Mass number hydrogen = 1
Stage 2: add these RAM numbers together to get the formula mass:
23 + 16 + 1 = 40
Stage 3: work out each elements percentage composition using their RAM numbers and the formula mass
Na = 23/40 X 100 = 57.5%
O = 16/40 X 100 = 40%
H = 1/40 X 100 = 2.5%
Stage 4: Add up the individual percentages and check that the total equals 100%
We can calculate the expected mass of a product or reactant from the balanced equation.
CH + 2O 2H O + CO
4 2 2 2
When methane burns it produces water and carbon dioxide as shown below.
If 12g of methane are burnt, what mass of water is produced?
Step 1- Calculate Mr
Step 2- divide both Mr by the Mr of the compound with the given mass (methane in our example)
Step 3- multply both sides by mass given in the question
methane = 12+4 = 16 water = 2 (2+16) = 36
methane = 1 water = 36/16 = 2.25
methane = 12g water = 27g
No reaction produces 100% of the product it should. There is always wastage. Using reacting masses we can calculate the % yield.
How to calculate % yield
1. Write balanced symbol equation
2. Use reacting masses to find theoretical yield (how much you should have made)
3. Divide actual by theoretical and x 100
% = 770/ 896 x 100 = 86 %
Exo and Endo thermic
When a reaction happens the products have a different energy to the reactants
Exothermic - the reaction gives out heat, the energy of the products is lower than the reactants so this is a negative energy change
Endothermic- the reaction takes in heat, the energy of the products is higher than the reactants so this is a positive energy change
The energy required to break a bond is the same as is released when they are made
Breaking Bonds= Endothermic
Making Bonds= Exothermic
So is more energy is needed to break the bonds in the reactants then is released when the bonds in the product are made then the overall reaction is endothermic
If more energy is released when the bonds of the product are made than needed to break the reactant bonds then the reaction is exothermic.
To calculate bond energies
1. Write all the bonds of reactant and product down.
2. Add up the energies of the reactant bonds.
3. Add up the energies of the product bonds.
4. Energy change is total of reactant bonds - total of product bonds
Our drinking water must be treated before it can be drunk.
It under goes a 4 step process
- in reservoirs/tanks, larger solid particles settle under gravity
– through layers of sand and gravel, remove smaller particles
– chlorine added to kill bacteria, prevents disease/makes it safe to drink
- Stored in towers or reservoirs until it is pumped to homes and industry
chlorine added here
Sedimentation in our exam
Water is vital for life. Without it we cannot live.
It is also vital for agriculture and industry.
So increase in population and industrialization increases demand on water.
Climate change is changing the amount of fresh water available
Both of these will mean demand will out strip supply and prices will rise.
Ways to conserve water therefore will have great economic benefits.
Desalination is the removal of salt from sea water to make fresh drinking water.
Simplest way of doing this is distillation.
However this is very costly as large amounts of energy are needed to boil lots of sea water.
This means until know only countries with a dire need of desalination with lots of "cheap" energy have invested in it.
There are other methods like reverse osmosis or using membranes
Distillation separates fluids by their boiling point, pure substances have defined boiling points (water 100 C, ethanol 80 C). So the mixture is heated to 80 C and mostly ethanol is evaporating off, this is condensed in the condenser and collected, leaving the water behind
5 S's of solutions
Solute - the substance that dissolves
Solvent - the substance that does the dissolving
Solution - mixture of solvent and solute
Saturated - a solution where no more solute can dissolve
Solubility - how soluble something is
Solubility is temperature dependent. For solids the hotter the solvent the more soluble. For gases the opposite is true. We can plot this information on a graph called a solubility curve
Always given in terms of 100g of Solvent.
So take care in exams if they ask for 50g (ml) of solvent then you halve the solubility.
both salts have the same solubility here
cooling down a saturated solution of potassium nitrate will mean this amount of potassium nitrate will be as a solid in the bottom of the beaker
Depending on the dissolved ions in the water it can be classed as hard or soft. Hard Water contains dissolved magnesium and calcium ions. Soft does not.
There are two types of hard water depending on how they are softened and what they contain.
You can tell how hard your water is with a soap solution. Hard water will make a scum and a poor lather requiring more soap.
Soft water will make a lather quickly with little soap.
More soap needed for a lather then the harder the water.
Temporary Hard Water - contains hydrogen carbonate ions (HCO ) upon heating these convert to insoluble carbonate ions (CO )
Permanent Hard Water - contains sulphate ions, heating doesn't change these. The only way to remove them is ion exchange, swapping calcium and magnesium for sodium.
Both types of water can be softened using an ion exchange column. Sodium is exchanged for the magnesium and calcium ions. Eventually the sodium is all used up and the column is refreshed with sodium chloride solution.
Chromatography can separate dyes by their differing solubility in a solvent. The more soluble travel quicker up the page.
To set up a chromatogram, draw a pencil line about 2cm from the bottom of the paper, add a small dot of your unknown, then place the paper into the solvent of choice making sure the dot does not rest in the solvent.
Remember not all solvents work for all dyes, some might be insoluble in water but soluble in ethanol!!!
dyes a similar colour and height are the same dye
Rf = distance moved by dye
distance moved by solvent
Rf are always less than 1!!
Here the Rf is 4/10 = 0.4
Gas Chromatography uses gas instead of a solvent to carry the substance. They are useful to tell us the amount of substances are in a sample
Diagram of the GC machine
Sample GC output