Tuesday, 30 April 2013

1.57 calculate the amounts of the products of the electrolysis of molten salts and aqueous solutions

One faraday is 96500 coulombs. It is also one mole of electrons.

If current of 0.2 Apms is passed through copper(ll) sulphate for tow hours, how much copper do you get?

  • Write out the half equation
Cu2+ + 2e > Cu
  • Work out coulombs of electrons flowing
Coulombs= current x time
time is 2x60x60 (times 60 makes minutes, times 60 again makes it seconds)
Q= 0.2 x 7200= 1440 coulombs
  • Convert C into moles of electrons
Moles= C/Faraday
Mol= 1440/96500
Mol= 0.015
  • Work out scale factor
Cu2+ + 2e > Cu
For every 2 moles of electrons, there will be one Cu
Sf= Moles of product/ moles of electrons
Sf= 1/2
Sf= a half
  • Work out moles of product using Scale factor
so we do the moles of electrons times the scale factor
0.015x1/2= 0.0075 Moles of Cu
  • Convert moles into mass
Moles x Mr
0.0075 x 63.5= 0.48g of copper

Sunday, 28 April 2013

1.55 write ionic half-equations representing the reactions at the electrodes during electrolysis

At the positive electrode, electrons will be lost: to show this we write the lost electrons as products:
2Br- > Br2 + 2e-
Make sure the charges are equal on both sides: 1- > 1-.

At the negative electrode, electrons will be gained so we write them as reactants:
2H+ + 2e- > H2
And to make sure the charges are the same on both sides: 0 > 0.

1.54 describe experiments to investigate electrolysis, using inert electrodes, of aqueous solutions such as sodium chloride, copper(II) sulfate and dilute sulfuric acid and predict the products

Place inert electrodes (ones that wont react) into an aqueous solution.

At the positive electrode the negatively charged ion from the compound will form an atom. At the negative electrode the atom of the positive ion will form.

sodium chloride: Hydrogen at the negative; chlorine at the positive
copper(II) sulfate: copper at the negative; oxygen at the positive
dilute sulfuric acid: Hydrogen at the negative; oxygen at the positive

If the metal in the solution is more reactive then hydrogen, the hydrogen from the water will be a product, as the metal will bond with the oxygen.

Test the products using known methods: eg damp blue litmus paper turned red by chlorine.

1.53 describe experiments to investigate electrolysis, using inert electrodes, of molten salts such as lead(II) bromide and predict the products

Inert electrodes are ones that don't react with any other substances, but only play a role in the transfer of electrons.

To describe an experiment you need to be able to draw and label an electrolysis experiment. As an example:


Lead bromide will make lead and bromine, you can use chemical tests to see is you got these products from electrolysis.

1.52 understand that electrolysis involves the formation of new substances when ionic compounds conduct electricity

In electrolysis ionic compounds conduct electricity. Positively charged ions move to one end, negatively to the other, these are then turned into atoms (by losing their charge) and so new substances are formed.

1.51 describe experiments to distinguish between electrolytes and nonelectrolytes

Set up an electric circuit with an LED and a break in the wire, put both ends of wire into a solution/molten substance. If the LED lights up then there is a current flowing, this will only be able to happen if the solution is conducting: so it must be an electrolyte. Conversely if the LED does not light up then there is no current flowing, and so the solution has not conducted electricity meaning it must be a nonelectrolyte.

1.50 understand why ionic compounds conduct electricity only when molten or in solution

When ionic compounds are molten or in solution, the positive and negative ions separate  this means that there are ions free to flow, and so they can conduct electricity.

1.49 understand why covalent compounds do not conduct electricity

In covalent compounds there are no electrons free to move, this means there can be no transfer of electricity through a covalent compound

1.48 understand that an electric current is a flow of electrons or ions

An electric current is a flow of electrons, although it can also be a flow of ions (as they have a charge.)

1.47 explain the electrical conductivity and malleability of a metal in terms of its structure and bonding.

Metals have delocalised electrons, electrons carry electricity; so because there are free electrons charge can pass easily through a metal.

The structure of a metal is with rows of atoms on top of one another, in pure metals as all the atoms will be the same size, the layers can slide easily over one another making them easy to bend.

1.46 understand that a metal can be described as a giant structure of positive ions surrounded by a sea of delocalised electrons

In a metal atoms come together into a lattice, the electrons become detached from their atoms- delocalised- making the atoms positive ions.

1.36 describe an ionic crystal as a giant three-dimensional lattice structure held together by the attraction between oppositely charged ions

An ionic crystal is a lattice of electrons in a 3D structure, the ions are alternate positive and negative and their opposing charges hold the structure together.

1.37 draw a diagram to represent the positions of the ions in a crystal of sodium chloride.

In a lattice structure with negative touching positive and visa versa.

Wednesday, 24 April 2013

1.45 explain how the uses of diamond and graphite depend on their structures, limited to graphite as a lubricant and diamond in cutting.

In Graphite the atoms from layers, these layers can slide over each other, this makes it very slippery and so can be used as a lubricant.

Diamond is extremely hard because it has a many bonds in it, this means it is great for cutting as it can cut anything.

1.44 draw diagrams representing the positions of the atoms in diamond and graphite

Carbon atoms in Graphite (left), are each bonded to three other atoms.

Diamond (right) is formed of carbon atoms each joined to four others.

1.43 explain the high melting and boiling points of substances with giant covalent structures in terms of the breaking of many strong covalent bonds

A giant covalent structure is one with many atoms bonded together. To melt or boil them you are not separating intermolecular bonds (between molecules), you are separating intramolecular bonds that keep the molecule together. These bonds are strong covalent bonds which take a lot of energy to break, so a lot of heat energy is required before the bonds will break to boil or melt; meaning they have high melting and boiling points.

1.42 explain why substances with simple molecular structures have low melting and boiling points in terms of the relatively weak forces between the molecules

A substance with a simple molecular structure is one that contains only a few atoms in a molecule.
The intermolecular forces (between the molecules) are weak, so it doesn't take much energy- or heat- to break them- this means they will melt and boil under low heats, as even small amounts of heat energy are enough to break the bonds.

1.41 understand that substances with simple molecular structures are gases or liquids, or solids with low melting points

A simple molecule (one with only a few atoms) will have a low melting point.

1.40 explain, using dot and cross diagrams, the formation of covalent compounds by electron sharing

To draw a dot and cross diagram for a covalent bond, you need to draw the outer shells of the two atoms involved with an overlap, in this overlap should be the electrons they share. Half the electrons in the overlap should be dots and half crosses, because one electron in every pair comes from each atom. The rest of the electrons for the outer shell should be drawn on, one atom with dots, the other with crosses.

For example, H + Cl = HCl:
Inline images 1

You need to be able to do dot and cross for the following substances (comment if you need help with any):
i hydrogen
ii chlorine
iii hydrogen chloride
iv water
v methane
vi ammonia
vii oxygen
viii nitrogen
ix carbon dioxide
x ethane
xi ethene

1.39 understand covalent bonding as a strong attraction between the bonding pair of electrons and the nuclei of the atoms involved in the bond

Electrons, being shared by atoms in a covalent bond, are attracted to the nucleus of each atom in the bond. Remember that electrons are negative and protons- in the nucleus- are positive.

1.38 describe the formation of a covalent bond by the sharing of a pair of electrons between two atoms

A covalent bond is a bond formed between atoms by sharing a pair of electrons (one from each atom.)

1.20 understand the term molar volume of a gas and use its values (24 dm3 and 24,000 cm3) at room temperature and pressure (rtp) in calculations.

At standard temperature and pressure, one mole of any gas will occupy 24000 cm3; also known as 24dm3.

A really good way to do calculations with this information is by using this triangle:

In a recent past paper I did this was one of the questions: calculate the amount, in moles, of carbon dioxide gas collected if you collect 144cm3.

The correct way to find this out is by doing 144 / 24000 which gives you 0.006 Mol

Sunday, 21 April 2013

1.35 understand the relationship between ionic charge and the melting point and boiling point of an ionic compound

The bigger the difference in charge, the stronger the attraction: if you have + and - ions they will have a weaker bond than +3 and -3 ions. The stronger the attraction, the harder it is to break the bonds, this means that the melting and boiling points will be higher. So the bigger the charge of an ion, the higher the melting and boiling point.

1.34 understand that ionic compounds have high melting and boiling points because of strong electrostatic forces between oppositely charged ions

To melt or boil anything, heat is used to break bonds. The stronger the bonds, the more heat needed. Ionic compounds have strong bonds, so they don't melt or boil unless there is a considerable amount of heat, this means the have high melting and boiling points.

1.33 understand ionic bonding as a strong electrostatic attraction between oppositely charged ions

Ionic bonding happens between two ions: they are attracted to each other due to their opposite charges, so we say the ions have electrostatic attraction. This attraction bonds them together into an ionic compound.

1.32 explain, using dot and cross diagrams, the formation of ionic compounds by electron transfer, limited to combinations of elements from Groups 1, 2, 3 and 5, 6, 7

Dot and cross diagrams represent electron transfer. One atom will have dots as electrons, the other crosses.

Here we begin with sodium and chlorine. Sodium looses one electron, so is drawn on the right with one less. Chlorine gains the electron that sodium looses so is drawn with one extra electron: because the electron came from sodium it is a cross instead of a dot.

The ions are drawn in brackets with their charge written outside.

Sorry the picture is hard to see.

1.31 deduce the charge of an ion from the electronic configuration of the atom from which the ion is formed

This is a way of working it out:
  • How many electrons are on the outer shell?
  • How many shells does it have?
  • To fill up its outer shell how many electrons will it take?
  • Now see weather it will take more transferring to loose electrons (to go down an shell,) or gain electrons (to fill the shell)
  • Which ever one takes the least transferring will be the route that was taken
  • If it lost, it will have a positive charge of the number of electrons lost to empty the shell
  • If it gained, it will have a negative charge of the number of electrons it gained to fill the shell

Most of the time the atoms will be on the second orbital in which case its simpler to think of it like this:

An atom with less then four electrons on its outer shell will want to loose electrons because that is the quickest way for it to have a full outer shell: if all the current electrons on the outer shell go, then the next shell- which will be full- will become the outer one. So we know they will loose electrons to make positive ions. If you have a group 2 atom (2 electrons on outer shell) it will loose two electrons when it becomes an ion, so you know it will have a +2 charge.

Similarly atoms with more than four electrons will gain electrons to fill their outer shell. This means they will make negative ions: as an example a group 7 electron has to gain one electron to fill its outer shell and so will become -1.

1.29 understand oxidation as the loss of electrons and reduction as the gain of electrons

The name given to loosing electrons is oxidation; the name given to gaining electrons is reduction.

One way to remember this is oil rig:

1.28 describe the formation of ions by the gain or loss of electrons

Electrons are transferred from one atom to another (this is in an effort to either fill or empty the outer shell to become stable.) An atom has no charge because the electrons and protons have equal and opposite charges. But an ion will have a charge: an electron has a charge of -1, so loosing an electron looses one negative charge, making the ion +1. So gaining one electron will make an atom a -1, gaining two will make it -2.

The atoms the gain or loose electrons to each other will have opposite charges: for example if a gives away 1 atom and b gains it, a is +1 and b is -1. These charges mean that the ions are attracted to each other (ionic bond), so they form an ionic compound.

1.27 carry out mole calculations using volumes and molar concentrations.

Moles / volume = concentration

1.26 calculate percentage yield

Percentage yield is: (actual yield / theoretical yield) x 100

Theoretical yield is what you expect to get, if a reaction doesn't finish you may end up with a lower yield than the expected theoretical yeild.

To work out theoretical yield it is important to understand that an equation gives you a ratio of moles, e.g.
Fe2O3 > 2Fe
tells us that every one mole of iron oxide makes two moles of iron. In this equation the weight of Fe will be your yield.

If we are told the weight of the Fe2O3 is 100g we can easily work out the theoretical yield:
Work out the moles of Fe2O3 by doing the weight (g) divided by the atomic mass: 100/160= 0.625Mol
We know that for every one mole of Fe2O3 there are two of Fe so we do: 0.625 x 2= 1.25 Mol
Now we have moles of Fe we can work out weight by Mol x Ar: 1.25 x 56= 70g

If you in fact got 62g of Fe you'd do (62/80)x100= 77.5%

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