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OCR F321 - 23rd May

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Original post by Dave_Beeston
That is a nasty equation that I found out we have to memorise:

3Cl2 + 6NaOH -> 5NaCl + NaClO3 + 3H2O.

I just remember it because there's 365 days in a year :smile: Then balance it from there!

What effect does going down group 2 have on the ease of decomposition?


Harder to decompose as you go down the group.:smile:

Describe and explain the trend in atomic radii of the elements Li to F across period 2. (4)
Original post by greenford
is there negative marking for chem.. because I remember my chemistry teacher ( she's an examiner) that there is negative marking... really confused:confused::confused:


Yes there is, it is a Chemistry exam not an English Literature or History exam. So don't write too much. Bullet points are advised. If it asks for 3 points, only the first 3 points will be marked, even if you have given 4 or 5 points and all are correct, they can deduct marks in that way. Also if the examiners can't read your handwriting you won't get marked on that either. So presentation of answers is really important too.
Original post by Dave_Beeston
That is a nasty equation that I found out we have to memorise:

3Cl2 + 6NaOH -> 5NaCl + NaClO3 + 3H2O.

I just remember it because there's 365 days in a year :smile: Then balance it from there!

What effect does going down group 2 have on the ease of decomposition?


The ease of decomposition decreases as you go down the group.

BTW they're really harsh on this and you have to word it quite accurately to get the mark.
Reply 143
Avatar for 786girl
786girl
spec point:
explain why C-12 is used as the standard
Original post by The medjai
Harder to decompose as you go down the group.:smile:

Describe and explain the trend in atomic radii of the elements Li to F across period 2. (4)


The Atomic Radii decreases as you go across period 2. The nuclear charge increases as the proton number increases across the group. There is the same level of shielding, so all the electrons are being put in the same shells. Therefore the increased nuclear charge pulls the electrons in to the nucleus more tightly, as the increased nuclear charge outweighs the increased repulsion between electrons.

(I think that's 4? 1 for stating and 3 for justifying?)

For Graphite and NaCl(s)/(l) state the following properties in comparison:
Boiling point
Electrical Conductivity
Bonding

(6) Just made that up, but pretty sure you can easily make 6 points for comparison!
Original post by ilovedubstep
The ease of decomposition decreases as you go down the group.

BTW they're really harsh on this and you have to word it quite accurately to get the mark.


It decreases I think.
Original post by The medjai
Harder to decompose as you go down the group.:smile:

Describe and explain the trend in atomic radii of the elements Li to F across period 2. (4)


The atomic radii decreases because the number of shells of electrons remains the same, this means electron shielding is constant, but the number of protons increases. This means that the nuclear attraction increases as you go along the period which explains why the atomic radii decreases.
Original post by Dave_Beeston
The Atomic Radii decreases as you go across period 2. The nuclear charge increases as the proton number increases across the group. There is the same level of shielding, so all the electrons are being put in the same shells. Therefore the increased nuclear charge pulls the electrons in to the nucleus more tightly, as the increased nuclear charge outweighs the increased repulsion between electrons.

(I think that's 4? 1 for stating and 3 for justifying?)

For Graphite and NaCl(s)/(l) state the following properties in comparison:
Boiling point
Electrical Conductivity
Bonding

(6) Just made that up, but pretty sure you can easily make 6 points for comparison!


Thats a perfect answer, 4 marks in the bag.
-As you go across the period the atomic radius decreases
this is because electrons are added the the same shell and so experience similar shielding from the nucleus
Nuclear charge increases as you go across as the number of protons increases
so the increasing positive charge pulls the electrons closer to the nucleus= atomic adius decreases
Original post by ilovedubstep
The atomic radii decreases because the number of shells of electrons remains the same, this means electron shielding is constant, but the number of protons increases. This means that the nuclear attraction increases as you go along the period which explains why the atomic radii decreases.


Yep full marks dude.
Original post by voices1
it is the regular periodic variation of elements and their chemical properties based on their atomic number and their position in the periodic table


This might come up :wink:..Trying to get my head around these defi
Original post by The medjai
It decreases I think.


i think it increases
Original post by arvin_infinity
This might come up :wink:..Trying to get my head around these defi


yeah only just revised periodicity today!

Um define the following:

- mole
- isotope
- water of crystallisation
- covalent bonds
Reply 153
Avatar for timkench
timkench
2
I've gone through past papers and summarised what i found hard by using the mark scheme.

Hope this helps anyone :smile:
CHEMISTRY F321.docx97.2KB
Original post by ilovedubstep
yeah only just revised periodicity today!

Um define the following:

- mole
- isotope
- water of crystallisation
- covalent bonds


A Mole is the amount of any substance containing as many particles as there are atoms in exactly 12g of the Carbon-12 isotope.

An isoptope is an atom of the same element with the same number of protons and electrons, but a different number of neutrons.

Water of Crystallisation is the water that is used to hydrate compounds. (No idea about this one really :s-smilie:)

A Covalent bond is a shared pair of electrons.

State the colour changes seen when adding Chlorine solution to (separate) aqueous solutions of Bromide ions and Iodide ions. Write an ionic equation for one of the reactions that has taken place. A student adds an organic solvent to the two test tubes, state what would be seen. (5)
Anyone memorised the differences between the properties of metallic, ionic and covalent bonding? solubility, conductivity, melting and boiling points, atoms or ions...
Original post by Dave_Beeston
A Mole is the amount of any substance containing as many particles as there are atoms in exactly 12g of the Carbon-12 isotope.

An isoptope is an atom of the same element with the same number of protons and electrons, but a different number of neutrons.

Water of Crystallisation is the water that is used to hydrate compounds. (No idea about this one really :s-smilie:)

A Covalent bond is a shared pair of electrons.

State the colour changes seen when adding Chlorine solution to (separate) aqueous solutions of Bromide ions and Iodide ions. Write an ionic equation for one of the reactions that has taken place. A student adds an organic solvent to the two test tubes, state what would be seen. (5)


Cl2 + 2Br- => Br2 + 2Cl- Solution turns Orange
Cl2 + 2I- => I2+2Cl- Turns brown.

The solution with iodine turns violet. The other solution doesnt change.

(Or does it form two layers? Help!)

btw the definition for water of crystalisation is: the water molecules that form an essential part of the crystalline structure of a compound. The rest were good! (perfect)
(edited 12 years ago)
Reply 157
Avatar for voices1
voices1
Why are the 1st and 2nd ionisation energies of potassium different??
Original post by Dave_Beeston
A Mole is the amount of any substance containing as many particles as there are atoms in exactly 12g of the Carbon-12 isotope.

An isoptope is an atom of the same element with the same number of protons and electrons, but a different number of neutrons.

Water of Crystallisation is the water that is used to hydrate compounds. (No idea about this one really :s-smilie:)

A Covalent bond is a shared pair of electrons.

State the colour changes seen when adding Chlorine solution to (separate) aqueous solutions of Bromide ions and Iodide ions. Write an ionic equation for one of the reactions that has taken place. A student adds an organic solvent to the two test tubes, state what would be seen. (5)


Bromine - Orange
Iodine - Brown

Cl2 + 2Br--->2Cl- + Br2.

Not sure about the organic solvent bit...are you talking about adding NH3?
If so then: Bromine - Cream, Iodine - Yellow
Reply 159
Avatar for timkench
timkench
2
Original post by ilovedubstep
Cl2 + 2Br- => Br2 + 2Cl- Solution turns Orange
Cl2 + 2I- => I2+2Cl- Turns brown.

The solution with iodine turns violet. The other solution doesnt change.

(Or does it form two layers? Help!)


only forms layers when cyclohexane is added, the bonded halogen (the ion in the equation) forms the water colour whilst the Br2 Cl2 I2 react with the cyclohexane to form the more prominant version of the colours
(edited 12 years ago)

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