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External Exam
Thursday 17h November, 2:00pm
5 literacy Credits
Best resources/ past papers can be found on the NZQA website: https://www.nzqa.govt.nz/ncea/assessment/view-detailed.do?standardNumber=91164
Learn
Learn
This exam topic will build on the knowledge you learnt in yr11 with regards to atomic science. We will be investigating the 4 different structures of atoms can arrange themselves into and their properties.
Foundational chemistry
Foundational chemistry
The first section is recapping everything you did in yr 11 which is relevant to this yr12 topic. We need to recall the following topics:
What makes up an atom
Electron arrangement
How and why atoms form ions
Writing chemical formulae (swap and drop)
Ions introIonic substances
Ionic substances
REMEMBER THE FOLLOWING SENTENCE OFF BY HEART- An ionic substance is a cation (positive ion) and anion (negative ion) bonded together by strong electrostatic forces of attraction in a 3D lattice structure.
Properties of Ionic Substances
Properties of Ionic Substances
There are 4 main properties that we need to understand. Melting points (M/P) and boiling points (B/P), solubility, conductivity and brittleness.
M/P and B/P
Due to the very strong electrostatic attraction between the cation and anion, large amounts of energy are required to break this bond. This means that that ionic substances will have a high M/P and B/P.
Conductivity
They can only conduct when they are dissolved in a solvent or molten (liquid). Ionic solids are made of ions so the ions can hold an electrical charge. However when in the solid state these ions cannot move. In order to conduct electricity the ions must be free to move to carry the charge through the substance, hence they must be dissolved or molten.
Solubility
Ionic substances can dissolve in water due water being polar (it has a slightly negative Oxygen and slightly positive Hydrogen present). The ions will dissociate and be stabilized by the respective polar ends of water.
Brittleness
Ionic substances are made up of alternating positive and negative ions in a 3D lattice structure. When a force is applied then this can cause the same charged ions close to each other. Like charges repel so when this happens the ions will quickly and violently repel from one another- this will physically be seen as the solid shattering.
Molecular substances
Molecular substances
REMEMBER THE FOLLOWING SENTENCE OFF BY HEART- A molecular substance is two non metals (both from the right hand side of the periodic table) bonded together. They share electrons to form a strong covalent bond.
Lewis Diagrams
Lewis Diagrams
We can draw diagrams to show the location of electrons. We calculate the number of valence electrons an atom has by the group number (group 13 has 3e, group 17 has 7e etc). This is going to help us identify the shape of the molecule.
Shapes
Shapes
Depending on the number of bonds and lone pairs around the central atom, we can assign a particular shape to the molecule. We also need to remember bond angles.
We are going to learn a template to explain the shape. Through repetition this will become second nature for you!
Electronegativity and polarity
Electronegativity and polarity
Adding to our shape description is including these two factors. Electronegativity is the ability of an atom to attract a pair of bonding electrons to itself- how strongly it holds on to the bonding electrons. If you can remember that fluorine is the big bully and has the highest value then you can remember that as you go across the period electronegativity increases and as you go down a group it decreases.
A difference in electronegativity means that the stronger atom in a bonding pair will be slightly negative and the other will be slightly positive- this causes a polar bond.
If the molecule is symmetrical then the polar bonds will cancel out and we say the molecule will not have a molecular dipole so it is non-polar. If it is asymmetrical then the polar bonds will not cancel and there will be a molecular dipole- so the molecule is polar.
Recite the following template, then you are guaranteed an excellence mark because this is always in the exam:
Properties of molecular substances
Properties of molecular substances
The three main properties we need to understand are M/P and B/P, solubility and conductivity.
M/P and B/P
Molecules generally have low M/P and B/P. This is because even thought the covalent bonds between atoms is very strong. The intermolecular bonding is very weak. It requires very little energy to break these bonds, hence a low M/P and B/P..
Solubility
Once you have described the shapes of the solute (solid) and solvent (liquid). You just need to state that like dissolves in like. This means that only polar solids will dissolve in polar liquids, and the same will be said for non-polar solids and liquids.
Conductivity
Due to the molecules not containing any ions, there is nothing capable of carrying a charge. This is why molecules are known to be good insulators not conductors.
Metallic substances
Metallic substances
REMEMBER THE FOLLOWING SENTENCE OFF BY HEART- A metallic substance is made up of metal cation nuclei in a sea of delocalized electrons. There are strong electrostatic forces of attraction between the positive nuclei and negative electrons.
Properties of Metals
Properties of Metals
Conductivity
Metals have a sea of electrons which are able to carry a charge. These are electrons are delocalized which means they are not fixed in position and can travel freely. This allows metals to be great conductors of electricity.
Malleability/ Ductility
When a force is applied to a metal its particles are able to move. They do not lose their strong bonds as the positive nuclei make new bonds to nearby electrons as they move, allowing metals to be bent into shapes.
Hardness
Scientifically speaking most metals are hard as they have very strong metallic bonds. We do not need to discuss this property further.
Solubility
Most metals are insoluble in solvents as the bonds are too strong to be overcome by the attractive force between a solvent and the particles in the metal
M/P and B/P
Due to the strong metallic bond, metals will require a lot of energy to break these bonds. Hence a high M/P and B/P.
Giant Covalent Networks
Giant Covalent Networks
REMEMBER THE FOLLOWING SENTENCE OFF BY HEART- Giant covalent networks are large molecules made up of non metal atoms bonded together with covalent bonds. They can be linear, 2D or 3D shapes.
Properties of GCN
Properties of GCN
These are dependent on the type of GCN. However you can use your understanding of properties from the previous substances to explain. For example if you are told it has a high M/P you know this is because of the strength of the bonds or it it can conduct it must have free electrons to carry the charge.
Energy Changes
Energy Changes
Chemical reactions involve either the making or breaking of bonds. A reaction is exothermic if overall there is energy being released. It is endothermic if overall there is energy being absorbed.
When you break a bond (eg solid to liquid), that will absorb energy.
When you make (eg gas to liquid) a bond that will release energy.
Energy Diagrams
Energy Diagrams
Depending on the amount of energy the reactants and products have. We can see if overall energy has been lost to the surroundings or gained by the system.
The activation energy is the energy required for a successful reaction to occur.
Bond Enthalpies
Bond Enthalpies
If you are provided with the data outlining the energy required to break each bond then you can calculate the energy either lost of gained by the reaction and classify it as either endothermic or exothermic. Make sure you take in to account every single bond!
Energy= (Bonds broken - Bonds formed)
Thermochemical Equations
Thermochemical Equations
We are going to use the following formulae to calculate the enthalpy change of a reaction, using experimental data. This is tricky and is best to go through this in class.
And H= -Q/ n
Revision
Revision
A3 revision sheets to be made up and then used to help you revise and initially complete past papers.
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