Ionic Bonding and the Basics
Chemistry Revision (AQA Additional Science)
Atoms, Compounds and Isotopes
The Mass number is the top number next to an element on the periodic table, this says how many protons AND neutrons there are in total, the atomic number tells what the total number of protons (therefore also electrons) there are in an atom. Compounds are chemically bonded.Formed when 2 or more elements chemically combine. Difficult to seperate.
Isotopes are: Different atomic forms of the same element where the number of neutrons differs
This is the transfer of electons so that both elements get a full outer shell Group 7 and 1 react, 6 and 2, 5 and 3 e.t.c. (metals ALWAYS give and non-metals gain) Ions are produced, the metal is always posotive and non-metals are negative, they form a regular lattice structure and they all have similar properties
Covalent Bonding and Substances
This is where electons are shared between atoms. Remember all atoms want to be 'noble'. Each bond (single, double, triple) is another atom being shared so if 2 were being shared it's a double covalent bond.
Simple Molecular Substances: The atoms have VERY strong covalent bonds to form small molecules of different atoms.The forces of attraction between these molecules are VERY weak! They therefore have low melting and boiling points although this is of the molecules, not the covalent bonds, an example is water! The bonds between 'H2O' is super strong but the bonds holding them all together is easily broken.
Giant - Covalent Substances: Bonded by strong covalent bonds but have no charge. High melting and boiling points, there are 3: Diamond and Graphite (made only from carbon) and Silicon Dioxide. Diamond is the strongest natural substance, with 4 carbon to carbon bonds so cant conduct...Graphite has 3 carbon to carbon bonds so has one spare electron to help conduct, Forms in layers that can slide over eachother... Silicon Dioxide (silica) is sand.
Metals and New Materials
Metals all have a free electron which allows the to conduct heat and energy. They have a giant structure and it is the free electrons that hold the structure together. The intermolecular forces allow the metal to be shaped and bent.
Smart materials have super weird properties. They can behave differently depending on the conditions to which it is exposed. Some can remember shapes, braces, glasses are some examples of this.
Nanoparticles are really really really really really really really really tiny. They contain a few hundread atoms.There are lots of uses for nanoparticles and they're currently being used in nanoscience. They could be industrial catalysts cause of their huge surface area, can make stronger bulding materials, cosmetics, can conduct electricity, LOADS of stuff tbh.
Polymers and Relative Formula Mass
Polymers (oh no)
Forces between the molecules determine the properties of the plastic.
Weak forces: individual tangled chains, weak intermolecular forces, allow them to slide, thermosoftening polymers dont have cross links between chains, forces easy to overcome so it's easy to melt, can be remoulded lots. Strong Forces: They have crosslinks, which make it look like a brick wall, hold it firmly together. The crosslinks hold it in a solid structure. Polymer doesn't soften they're toughies.
Relative Formula Mass
Relative mass is just the mass number, that's it, if it's a compound, you need to add these together to get the relative mass. One mole is just the relative mass, in grams. Simple.
Number of moles = mass in grams (of element or compound) / Mr of that substance
^red highlighting are equations, you need to learn them
Two Formula Mass Calculations
Two Formula Mass Calculations
Calculating % mass:
%mass of an element in a compound = Ar x number of that element / Mr of whole thing x100
Finding the empirical formula:
1) list all the elements in the compound
2) Underneath write their experimental masses
3) Divide each by the Ar for that particular element
4) Turn the answer into a simple ration by multiplying by any well-chosen number ( e.g. 10)
5) Simplify by dividing both by their highest factor
Calculating Masses in Reaction and Yield
Calculating Masses in Reactions
There are 3 important steps...
- 1) write out the balanced equation
- 2) work out the Mr for the 2 bits that you want
- 3) Divide to get one then multiply to get all
Percentage yield = actual yield (grams) / theoretical yield (grams) x 100
Yields are always less than 100% because there is always waste from the reaction eg, gases may escape e.t.c.
A reversible reaction is where the products can themselves react to produce the original reactants again (There is a symbol for a reversible reaction with half of an arrow going one way and the other half, the opposite way, page 52 in the revision guide)
Chemical Analysis and Instrumental Methods
Artificial colours can be seperated using chromatography
- Pen and dyes may contain more than one dye, so it's a mixture
- Put some of the dye in a small cup with a few drops of solvent (e.g. water)
- Put spots of the solution on a pencil baseline on filter paper
- Roll up the sheet and put it in a beaker with some solvent (keep baseline above solvent)
- Solvent seep up the paper taking the dyes which stop at different places (seperate)
- This show the minimum dyes, not total
Machines can alse analyse unknown substances, they're sensitive, fast and accurate
Gas chromotography can be used to identify substances by seperating them.
- gas is used to carry a substance through a column packed with a solid material
- they travel at different speeds so are seperated
- The time they take to reach each detactor is the retention time
Rate Of Reaction
Rate of Reaction
Reactions happen at different rates, the rate of a reaction depends on: Temperature, Catalyst, Concentration and Surface area of a solid
Meausuring rates of reactions: Rate of reaction=amout of reactant used or product formed / time
- Percipitation is where a solution goes cloudy and is measured by lengh of time a certain point takes to disappear from sight although this varies depending on individuals.
- Change in mass (usually lighter) when the gas escapes when it is produced, the quicker the drop, the faster the reaction and it is the most accurate method although gas can be released straight into a room.
- The volume of gas given off using a gas syringewhich is pushed by the gas and then is measured, if the reaction is too vigorous the plunger can fly out
Rate of Reaction Experiments and Collision Theory
Rate of Reaction Experiments
- Reaction of hydrochloric acid and marble chips, shows that the larger the surface area (smaller the chips) then the quicker the reaction
- Reaction of magnesium matal and Dilute Hcl shows the effect of using concentrated acid
- Reaction of sodium thiosulfate and HCl (produces a cloudy percipitate) can be used to test the effects of concentration
- The decomposition of hydrogen peroxide shows the effects of using catalysts
More collisions increase the speed of the reaction
- Higher temperature (more energy) increases collisions and makes them faster
- Higher concentration (or pressure) increases collisions
- Larger surface area increases the collisions
- Catalysts also speed up reactions
Energy Transfer and Acids and Alkalis
An exothermic reaction is one where energy is given to the sorroundings, usually in the form of heat (temperature rise)
An endothermic reaction is one which takes in energy from the sorroundings, usually heat (shown as a temperature loss)
Reversible reactions swap the type of reaction (endo / exo) that it began with.
Acids and Alkalis
pH scale goes from 0 to 14, 0-6 are acids with 0 being the strongest, 7 is neutral and 8-14 are alkalis with 14 being the strongest.
Universal Indicator can change colour depending on the liquids pH.
An acid and an alkali can neutralise eachother (Acid + Base = Salt + Water)
Acids and Metals and Making Salts
Acids and Metals
- Acid + Metal = Salt + Hydrogen
- Acid + Metal Oxide = Salt + Water
- Acid + Metal Hydroxide = Salt + Water
Hydrochloric acid produces a chloride salt, Sulphate acid produces a sulphate salt, Nitric acid produces a nitrate salt
- Making soluble salts using a metal or insoluble base: also produces water, or using an alkaline but it has to be perfect balance and then crystalise the salt as normal after filtirisation.
- Making insoluble salts: percipitation reaction; pick two solutions using the ions that you need and mix, then filter and crystalise
Electricity means "splitting up with electricity"
When a current is passed through an ionic substance that's molten or in a solution, it breaks the elements that it's made of. It requires liquid to conduct the electricity, electrolytes contain free ions which allow it all to work, the anode (posotive) attracts all thenegative matal ions (how you can plate something, e.g. silver plating) and the posotive non-metals are attracted to the cathode (negative)
They are OIL RIG reactions; oxidation is loss, reduction is gain
Look in the revision guide, pages 67 - 68 for electroylsis of sodium chloride solution and extraction of aluminium and electoplating (briefly mentioned above). All are quite easy but you do need to learn them