ocr a f325 revision thread

Magnesium oxide has a high melting point and boiling point this is because of the strong electrostatic attraction between the oppositely charged ion and hence mgo is stable And is placed to line the furnaces

okay

Yes, correct

Yeah So i'll ask 1st:

whats the enthalpy change of sln:? (2)
What is the rate determining step: ? (2)
And name 3 properties of transition element? (3)

1)Enthalpy change of solution is the enthalpy change when 1 mole of an ionic compound is completely dissolved in solution under standard conditions, 100KPa, 298K and 1 mol dm-3

2) Rate determining step is the slowest step mechanism of a reaction in a multi-step reaction

3) Catalysts, form coloured compounds that are coloured, variable oxidation states

Describe the difference between geometric and optical isomerism with examples (5)

YES, your turn/

Oh no, not geometric :P

I may have geometric isomers confused with something else, but I believe that geometric isomers are isomers with the same molecular formula but with different structural arrangements of atoms. For example, butane and methylpropane are optical isomers.

Optical isomers are non-superimposable mirror images of each other. For example, complex ions like [Cu(NH3)4(H20)2]2+ form optical isomers.

Why is the pH scale used?

1) How is the rate of reaction measured from a concentration-time graph (2)

2) Suggest why industrially many companies use Transition metals as catalysts (3)


3) Suggest which of any are stronger acid, HCL or H2SO4 and explain your reasoning in detail including equations (4)

1) by drawing a tangent at particular point on the graph.
2) wow, good question.... is it because they can be recovered at the end of the reaction. and be reused.. ( vague i think )?

3)i'm not really sure again. but i think its h2so4 because it has a higher conc of H+ ions
Eq) H2SO4 -----> H+ + HSO4-
HSO4- ---------> H+ + SO4- ??

Your optical isomerism is correct, but your geomteric I was looking for cis/trans, and the mentioning of the word arragement of atoms in space and an example could have been the cis and trans platin but good overall!


1) PH is used because it deals with a wide range /variety of negative indices and hence is used most often

1) by drawing a tangent at particular point on the graph.
2) wow, good question.... is it because they can be recovered at the end of the reaction. and be reused.. ( vague i think )?

3)i'm not really sure again. but i think its h2so4 because it has a higher conc of H+ ions
Eq) H2SO4 -----> H+ + HSO4-
HSO4- ---------> H+ + SO4- ??

Ooh I see, thank you very much!

Exactly, you're on a roll

thanks you too , you are doing very good as well!

try these questions , I made them my self

1) How is the rate of reaction measured from a concentration-time graph (2)

2) Suggest why industrially many companies use Transition metals as catalysts (3)


3) Suggest which of any are stronger acid, HCL or H2SO4 and explain your reasoning in detail including equations (4)

Thank you very much!

1) On a concentration time graph, a tangent is drawn at a particular point on the graph at the time you want to measure the rate at (e.g. t=10). The gradient of the tangent is measured, and the tangent is equivalent to the rate.

2) Hmm...I would say because they can form ions such as ions with a plus 7 oxidation state (MG7+), so this means that transition metal ions are able to gain and lose electrons quite readily, and this could help them catalyse reactions. Also, some transition metals are easily available, like iron.

3) HCl is a weaker acid then H2SO4, because for every dm3 of HCl, one dm3 of H+ is produced, whereas in H2SO4, for every dm3, 2 dm3 H+ is produced. As there's more H+ for H2SO4 in a given volume, it is a stronger acid.
HCl -> H+ + Cl-
H2SO4 -> 2H+ +SO42- (overall equation - there are 2 dissociations).

Good questions!

1) Why does Kw vary with temperature?
2) Describe an example of a biological buffer.
3) From a concentration-time graph, how can you tell if a reaction is first order?

Excellent answers!


1) Kw varies with temperature because as the temperature increases Kw increases as more water (ionic product of water) dissociates and hence, the concentration of H+ and OH- increases , however because more H+ concentration increases the PH neutrality decreases and this is why Scientists do not accept the 1.00x10-14 figure as it is inaccurate and can vary depending on the extent of dissociation of water.

2) An example is the controlling OF the blood PH, the PH of blood has to be around 7.35

The plasma(blood) contains water and co2 in which can react to form carbonic acid catalysed by an ezyme called Carbonic anhydrase.

CO2 + H2O ---> H2CO3

the carbonic acid can dissociate

H2CO3 equilibrium sign 2H+ + CO32-

The addition of more hydrogen ion reacts with the alkali and causes the equilibrium to shift to the left in order to remove most of its H+ ion concentration.

The addition of more alkali reacts with the carbonic acid which causes equilibrium to shift to the right in order to restore the concentration of H+.

This takes place because some acid remains in the solution/blood

the co2(aq) ---> co2(g) which can be expelled out of the body as h2co3 can form co2(aq) and to co2(g) and hence, PH of blood is controlled

3) Constant half lives and as the time increases the concentration decreases . Two half lifes

Thank you, you're very kind

Wow, they're brilliant answers too! Lots of detail, especially with question 2, good stuff

I'm sorry for my late reply, just come back from work!

No worries


Write an ionic equation between mno4- and the iron(3)


Which indicator is used (2)

For MnO4- and Iron:

MnO4- + 5e- + 8H+ -> Mn2+ + 4H2O
Fe2+ -> Fe3+ + e-

MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ + 4H2O

No indicator is used for this because as MnO4- is reduced to Mn2+ when it is titrated against Fe2+, the solution changes from colourless to pale pink when the Mn2+ is in slight excess.

Hope that's right xD

Perfect

My turn