what is given during exam (1 Viewer)

[Ragerunner]

During chemistry exam what are given ?

Periodic table and anything else ?

because i seem to have trouble determining where a substance is a strong acid/base or weak acid/base. Is there any easy way to tell without just remembering off by hard ?

And same goes for valencies of certain acud such has sulfuric acid or ethanoic acid.

 

[spice girl]

no there isn't

remembering them off by heart is the best option.

this is all i remember you need:

strong acids: HNO3, HCl, H2SO4
strong bases: NaOH, KOH
all others are weak.

diprotic: H2SO4
triprotic: H3PO4, citric acid
all others are monoprotic.

 

[chunder]

you could use the periodic table to determine whether oxides are acidic or not. i believe the aciditiy of oxides increases as you move across the period. For example compare Na20 with S03. Oxides of transitional elements are amphiprotic. Proving the concept involes some smart implementation of Brownsted Lowry principles.

 

[abdooooo!!!]

Originally posted by Ragerunner
And same goes for valencies of certain acud such has sulfuric acid or ethanoic acid.

valencies of acids? do you mean the valencies of the sulfate ion? if that is what you mean, you can work it out by using the periodic table.

for example:
SO4 has one sulfur and four oxygen. by looking at the periodic table sulfur is a group six element and it is written first, so it is a positive ion in the formula, which means it has a valencie of +6.

oxygen is also a group six element but it is written second in the formula because of its higher electronegativity, that means its a negative ion. 6-8= -2, therefore oxygen has a valencie of -2

add the valencies of the sulfur and the 4 oxygen together: 6+4(-2)= -2.
so the sulfate ion has a valencie of -2.

you can work out most the valencies for common polyatomic ions like CO3, NO3, PO4... in the same way by using your periodic table if you can't remember it. and for ethanoic acid, just remember that all alkanoic acids donate one hydrogen ion only, which means they're all monoprotic, and its always the last H in R-COOH that gets donated.

 

[spice girl]

just a note - you CAN'T work out the charge on polyatomic anions by using the periodic table. This is because atoms like N and S have multiple oxidation states. e.g. you can't explain just using the periodic table why NO2- and NO3- both exist, as with SO3_2- and SO4_2-, using abdoooo's method.

as i said, the best way is to remember it off by heart. the reasons why this is diprotic or why this ion is -1 instead of -2, stretches to uni chemistry.

 

[Akira_Tikira]

Couldn't have said better myself,
Remember the damn thing....

 

[abdooooo!!!]

Originally posted by spice girl
just a note - you CAN'T work out the charge on polyatomic anions by using the periodic table. This is because atoms like N and S have multiple oxidation states. e.g. you can't explain just using the periodic table why NO2- and NO3- both exist, as with SO3_2- and SO4_2-, using abdoooo's method.

you're right, but you can pretty much workout the common ones. and for element like nitrogen you should know that it commonly has a valencie of 3 or 5 (it can have other valencies) depending on what it bond with. but for someone to learn all these if they don't know it, it'll be even harder then just remembering by heart, although its good to know the actual theory behind it...

 

[Twintip]

Originally posted by Ragerunner
During chemistry exam what are given ?

Periodic table and anything else?

A table of standard reduction potentials as well I assume.

 

[McLake]

Re: Re: what is given during exam

Originally posted by Twintip
A table of standard reduction potentials as well I assume.

Yes, you get one of those, along with some standard no's such as Avagadros and the gas constants.

"Log" eqation for pH is also includued.

 

[mannnnndy]

and
(delta)H = mC(delta)T