Different atomic forms of the same element with the same amount of protons, but different amount of neutrons.
Must have the same atomic number but different mass number.
Eg: Carbon 12 | Carbon 14
Ionic bonding - transferring electrons
Atoms lose or gain electrons to form charged particles which are then strongly attracted to one another.
Charged particles are known as ions.
Giving an electron makes the atom a positive ion.
Taking an electron makes the atom a negative ion.
Sodium and chlorine is very common.
Sodium gives an electron out, chlorine takes.
Compound - A substance where the atoms or 2 or more elements are chemically combined.
Chemical bond - Transferring or sharing electrons in the outer shell.
Ionic bonding continued
Ionic compounds always have giant ionic lattices
The ions form a closely packed regular lattice arrangement.
There are very strong electostatic forces of attraction.
Sodium chloride is one giant ionic lattice.
Ionic compounds have:
- High melting points/boiling points.
- Strong attraction.
- Dissolve in water easily
Atoms that have lost or gained electrons are ions.
Ions have the electronic structure of noble gases.
Ionic compounds are made up of a positively charged part and a negatively charged part.
The overal charge is 0
Eg: Sodium chloride
Na+(+1), and Cl-(-1)
Eg2: Magnesium chloride
Mg2+(+2), and Cl-(-1)
2 Cl ions are needed to balance.
Sometimes atoms like to make convalent bonds, which is sharing electrons with other atoms.
Both atoms have a outer shell.
Convalent bond is a shared pair of electrons.
- Hydrogen (H2)
- Methane (CH4)
- Hydrogen chloride (HCl)
- Ammonia (NH3)
- Water (H2O)
- Oxygen (O2)
Convalent: Simple molecular
Atoms form very strong convalent bonds to form small molecules of several atoms, with intermolecular bonds.
Very weak force of attraction.
Intermolecular forces means boiling point is very low, and also because its an intermolecular force, it can be broken easily.
Convalent: Giant convalent
Giant convalent structures are macromolecules.
Similar to giant ionice lattices, but there are no charged particles.
All the atoms are bonded with strong convalent bonds.
Very high melting point.
Don't conduct electricity.
Main examples: Graphite and diamond.
Giant convalent: Diamond + Graphite
- Each carbon atom has 4 convalent bonds and is very rigid
- This structure makes diamond the hardest natural substance.
- Each carbon atom has 3 convalent bonds.
- Layers are held together loosely.
- Only non-metal which is a good conductor of heat and electricity.
- Sometimes called silica, this is what sand is made of.
- Each grain of sand is one giant structure of silicon and oxygen.
- Metals have free electrons
- Metals consist of a giant structure
- The free electrons(delocalised) come from the outer shell of every metal atom.
- These electrons are free to move.
- Strong electrostatic fore of attraction.
- Metals can be bent and shaped.
- Alloys are a mixture of 2 or more metals.
- Alloys are harder than pure metals
Different elements have different sized atoms, causing a distort in layers
- Giant ionic
- Simple molecular
- Giant convalent
- Giant metallic
Nitinol - A shape memory alloy.
Made of: Nickel and titanium
When it is cool, you can bend it to a certain position, but when you heat it up it goes back to it's remembered position.
- Glasses frames
Nanoparticles are really tiny particles.
Nanoparticles include fullerenes.
Fullerenes are molecules of carbon, shaped like hollow balls.
Fullerenes can be joined together to form nanotubes.
- Can be used in tennis rackets
They have a huge surface area to volume, so could be used to make catalysts.
Use nanoparticles to make sensors to test water purity.
Nanotubes can make stronger, lighter building materials.
New cosmetics, such as sun cream, which don't leave white marks on the skin.
Fullerenes could be used to deliver drugs to the right cells in the body.
Nanotubes conduct electricity, so can be used in electric circuits.
Polymers - Thermosoftening/Thermosetting
Weak forces: Tangled chains of polymers, held together with a weak intermolecular force.
Thermosoftening polymers don't have cross linking between chains.
Easy to melt the plastic.
When cools the polymer hardens into a new shape.
You can melt these plastics and remould anytime.
Strong forces: Strong, due to crosslinks holding chains firmly together.
Thermosetting polymers have crosslinks.
These hold chains together in a solid structure.
Polymer doesn't soften when it's heated.
It is strong, hard and rigid.
The starting materials and reaction conditions both affect the properties of a polymer.
Low density polyethene is made by heating ethene to about 200°c, under high pressure.
- Its flexible and used for bags and bottles.
High density polyethene is made at a low temperture and pressure (with a catalyst).
- Its more rigid and used for water tanks and drainpipes.
Relative atomic mass/formula mass/moles
1) Relative atomic mass(Ar) Average mass number of the isotopes.
2) Relative formula mass (Mr) - Adding up the relative atomic mass.
- H20 = 1+1+16=18
- CH4 = 12+1+1+1+1=16
Moles = Mass/Mr ,,,,,,,,,, 6.02x10²³
- How many moles in 42g of Carbon?
42 ÷ 12 = 3.5 moles.
4) Formula mass calculations - Percentage mass
Ar x no of atoms (of that element) ÷ Mr(of whole compound) x 100 = Percentage mass
Calculating masses in reactions
1)Balance the equation.
2)Work out the Mr.
3)Apply the rule.
What mass of magnesium oxide is produced when 60g of magnesium is burned in air?
48g of Mg reacts to give 80g of MgO
1g of Mg reacts to give 1.67g of MgO
60g of Mg reacts to give 100g of Mgo
Percentage yield = (actual yield ÷ predicted yield) x 100
Yields are never 100% as some product or reactant always gets lost in some way.
Reasons why never 100%
1) The reaction is reversible
- This means that some of the reaction will go both ways.
- When filtering liquids to remove solids, nearly all the time a bit of solid of liquid is lost.
3) Unexpected reactions
- Sometimes a reaction that was not meant to occur happens, so some reactant is lost.
Product yield is important for sustainable development
This is about making sure we don;t use recources faster than they can be replaced.
Chemical analysis - Chromatography
Artificial colours can be seperated using paper chromatography.
1) Fill a cup with solvent.
2) Put a drop of the solution being tested on the filter paper.
3) Put it in the cup of solvent, with the bottom of it just a little in the solvent.
4) Then the solvent will rise, seperating the dye.
Chemical analysis - Machines
Advantages of machines
- Very sensitive
- Very fast
- Very accurate
Gas chromotography can be used to identify substances
1) Gas is used to carry substance trough column.
2) The substances travel at different speeds so they are seperated.
3) The time it takes to reach the detector is called the retention time. And can be used to identify the substance.
4) The recorder draws a gas chromograph.
5) It can be linked up to a mass spectrometer, and can measure accurately.
6) Using this allows you to work out the relative molecular mass.
1) List elements
2) Divide each mass by the Ar.
3) Divide each number by the lowest
4) Multiple to the chosen number.
Rate of reaction
The rate of reaction deppends on 4 things:
- Suface area of solids
Measuring rates of reaction
Measuring rates of reactions
Rate of reaction = Amount of reactant used or product formed ÷ Time
1) Precipitation - Measure how long it takes to disappear.
- Very subjective - dffierent people may not agree on the exact point when it dissapears.
2) Change in mass - Compare the mass before and after the reaction.
- Most accurate as the mass is measured accurately.
3) The volume of gas given off - Use a gas syringe to measure the volume of gas given off.
- Quite accurate as small resolution.
Rate of reaction experiments
1) Reaction of hydrochloric acid and marble chips
- This experiment is used to demonstrate the effect of breaking the solid up into small bits.
2) Reaction of magnesium metal and dilute HCl
- This reaction is good for measuring tje effects of increased concentration
3) Sodium thiosulfate and HCl produce a cloudy precipitate
- 2 clear solutions react to form a cloudy precipitate
4) The decomposition of hydrogen peroxide
- This reaction is good to show the effect of differnet catalyst
The collsion theory just say that the rate of reaction simply depends on how often and how hard the reacting particles collide with each other.
Particles have to colide to react.
More collisions increase the rate of reaction.
1) Higher temperture increase collisions.
- Temperture increases speed of particles so will colide more often.
2) Higher concentration increase collisions.
- More particles to bounce off, so more chance of the important particles colliding.
3) Larger surface area increases collisions.
- When the surface area is larger, there is more area to collide, so more collisions
Collision theory More
Faster collisions increase the rate of reaction.
Reactions only happen if the particles colide with enough energy.
Activation energy is the minimal amount of energy needed for a particle to react.
Cataysts are substances than increase the speed of a reaction, without being changed or used up.
Increases the number of successful collisions.
Catalysts help reduce costss in industrial reactions.
- They are very important for commercial reasons.
- Increase the rate of reaction, which saves a lot of money.
- Allows a reaction to work at a much lower temperture.
Disadvantages of catalysts:
- Can be very expensive to buy, and often need to be removed from the product.
- There are different catalysts for different reactions, so you may need to buy more than 1 catalyst for serveral reactions.
- They also can be poisoned by impurities, which causes them to stop working.
Energy transfer - Exothermic
An exothermic reaction is one that gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperture.
What are exothermic?
1) Burning fuels.
2) Neutralisation reactions (Acid and alkali)
3) Oxidation reeactions. (eg: adding sodium to water)
- Hand warmers.
- Self heating cans.
Energy transfer - Endothermic
An endothemic reaction is one that takes in energy from the surroundings, usually in the form of heat and is usually shown by a fall in temperture.
A lot less common.
1) Thermal decomposition. (Heat must be supplied)
- Sport injury packs.
Reversible reactions can be endothermic and exothermic
- An example is hydrated copper sulphate.
If you heat it leaves powder, however when you add water it turns back to crystals.
Acids and alkalis
Ph scale is from 0-14
Universal indicator is used to see the ph.
An acid is a substance with a ph less than 7.
- Acids for H+ ions in water.
A base is a substance with a ph greater than 7.
An alkali is a base that disssolves in water.
- Alkalis form OH- ions in water.
Acids and base neutralise each other (Neutralisation). Acid + Base --> Salt + Water
(s) = Solid, (l) = liquid, (g) = gas, (aq) = dissolved in water,
Acids reacting with metal
Metal react with acids to give salt.
acid + metal --> salt + hydrogen
1) Hydrochloric acid will always produce chloride salts.
2) Sulfuric acid will always produce sulfate salts.
3) Nitric acid produces nitrate salts when neutralised
Oxides, hydroxides and ammonia
Metal oxides and metal hydroxides are bases.
acid + metal oxide --> salt + water
Acid + metal hydroxide --> salt + water
The combination of metal and acid decides the salt.
Ammonia can be neutralised with HNO3 to make fertiliser
- This is different to most reaction as no water is produced, just salt.
- Good as a fertiliser, as it has nitrogen from 2 sources, ammonia and nitric acid. So a double dose. Plants need nitrogen to make protein.
Making soluble salts using a metal or an insoluble base.
1) Need to pick the right acid, plus a metal or insoluble base.
2) You add the metal, metal oxide or hydroxide to the acid, which will dissolve in the acid as it reacts.
3) Filter out the excess material to get the salt solution. Then let it evaporate, to get pure solid crystals of salt. (crystalisation).
Making soluble salts using an alkali.
1) In this reaction, you have to add exactly the right amount of alkali to just neutralise the acid, using a indicator. Then repeat using exactly the same volume of alkali and acid so the salt isn't contaminated.
2) Then just let it evaporate to crystalilise the salt as normal
Making salts (2)
Making insoluble salts - precipitation reactions.
1) You just need to pick 2 solutions that contain the ions you need.
eg: sodium chloride - needs sodium ions and chloride ions
2) Once the salt has precipitate, and is at the bottom of the flask, filter it, then wash and dry it.
3) The precipitation reaction can be used to remove poisonous ions from drinking water. Calcium and magnesium ions can be removed from water this way.
Electrolysis means "Splitting up with electricity"
If an electric current is passed through a ionic substance thats molten or in a solution, it breaks down into the element its made of. This is called electrolysis.
The liquid which conducts electricity is called the electrolyte.
Electrolytes contain free ions.
Electrolysis always involves an oxidation and a reduction.
Oxidation is the loss of electrons.
Reduction is the gain.
Electrolysis - Lead bromide
Negative electrode gains electrons (reduction)
Positive electrode loses electrons. (oxidation)
1) The lead ions move to the negative electrode.
2) The bromide ions move to the positive electrode.
Electrolysis - Sodium chloride
Produces are - Hydrogen, chlorene, sodium hydroxide.
1) At the negative electrode, hydrogen is produced.
2) At the positive electrode, chlorene is produced.
3) Sodium ions stay in the solution because there more reactive than hydrogen.
Hydroxide ions are also left behind.
2H+ + 2e- --> H2
2Cl- - 2e- --> Cl2
Chlorene - bleach and plastics, sodium hydroxide - soap
Electrolysis - Aluminium
Used to remove aluminium from its ore
- Very abundant metal.
- Main ore is bauxite.
- This is pure aluminium oxide Al203.
- Aluminium has to be extracted from this using electrolysis.
Cryolite is used to lower the temperture.
- This is because aluminium oxide has a high melting point (2000).
- Using cryolite brings the temperture down (to about 900), which makes it cheaper and easier.
- The electrodes are made of carbon, which is a good conductor of electricity.
Aluminium forms at the negative electrode.
Oxygen forms at the positive electrode.
The oxygen reacts with carbon in the electrode to form carbon dioxide.
So the positive electrode gets eaten away and has to be replaced every so often.
Electroplating using electrolysis.
- The negative electrode is the metal object that you want to be plated.
- The positive electrode is the pure metal which you want it to be plated with.
- The electrolyte will need to contain ions of the plating metal.
The ions in the solution are the ones that plate the metal, and positive electrode keeps the solution supplied with these ions.
- Decoration: Silver is attracted, however expensive, so it's a lot cheaper to plate brass with silver, than it is to make a solid silver object.
- Conduction: Metals like copper conduct electricity well - so often used to plate metals in electronic circuits and computers.