CHEM AND BIO HELP AND TIPS HERE

[WARRIOR]

you guys when they tell us to describe a way to make a salt like lead nitrate, how do we know which one to put in excess, the lead oxide or the nitric acid??

lead oxide afcourse , because we want aall the nitric acid to be used up , thne we filter to remove the excess lead oxide  and we evaporate the filtrate for obtaining crystals

[Raywin]

That makes sense, thank u

No Probs, Glad to help..

[MasterMath]

Its always the insoluble substance which is the lead oxide in your case .. so you can filter it off .. its just a way of making sure all nitric acid reacted

[Dana]

lead oxide afcourse , because we want aall the nitric acid to be used up , thne we filter to remove the excess lead oxide  and we evaporate the filtrate for obtaining crystals

why don't we want all the acid to be used up?   plus sometimes they say add excess acid dont they?

[the_grim_reaper]

https://studentforums.biz/index.php/topic,54.msg253414.html#msg253414

Q 7-a (i)
In the MS it's written "an equation that gives: alkene + alkane or alkene + alkene + hydrogen"

Why is that. Cracking of alkanes --> alkene + hydrogen not alkene + alkane

https://studentforums.biz/index.php/topic,54.msg253414.html#msg253414

Q 7-a (i)
In the MS it's written "an equation that gives: alkene + alkane or alkene + alkene + hydrogen"

Why is that. Cracking of alkanes --> alkene + hydrogen not alkene + alkane

I can't really understand your question. Really ambiguous language. Well answering what I can grasp.
Lady lets put it like this. Here its decane and you have to give butene. Now after that, you've got six carbons and 14 hydrogens. So you can make anything in the world that you like from them. You can simply make it hexane or if you like hydrogen, you can make it hydrogen and hexene. That's why there are only two combinations, that you've already stated because you have to produce butene.
If butene wasn't stated, then you can make ethene, propane and pentene.

Basically, what I'm trying to state is that you must make an alkene from cracking. After that, you can make anything from the remaining stuff. Say if you want to crack ethane, then you can make ethene and hydrogen. Only 2 products are formed. You don't get an alkane here.

[MasterMath]

why don't we want all the acid to be used up?   plus sometimes they say add excess acid dont they?

Well if you add excess acid for example , how can you remove the excess ? its liquid you cant really remove it
And remember here we are making salts .. so we want to just have the right amount of solution to evaporate then cool and filter.. so we can use excess oxide and remove it , then we have  solution of equal moles of Lead ions and Nitrate ions.. but if you use excess acid you cant remove it so they wont be balanced.
And yeah sometimes they do use excess acid , thats when you want to react like a metal fully but not for getting salts

[Dana]

Its always the insoluble substance which is the lead oxide in your case .. so you can filter it off .. its just a way of making sure all nitric acid reacted

ahaa, well thanks alot

[Dana]

Well if you add excess acid for example , how can you remove the excess ? its liquid you cant really remove it
And remember here we are making salts .. so we want to just have the right amount of solution to evaporate then cool and filter.. so we can use excess oxide and remove it , then we have  solution of equal moles of Lead ions and Nitrate ions.. but if you use excess acid you cant remove it so they wont be balanced.
And yeah sometimes they do use excess acid , thats when you want to react like a metal fully but not for getting salts

mhmm i see. thanks!

[Dana]

lead oxide afcourse , because we want aall the nitric acid to be used up , thne we filter to remove the excess lead oxide  and we evaporate the filtrate for obtaining crystals

and thank you!

[J.Darren]

Got a doubt from the 2005 m/j paper 3 chemistry paper.
When 0.015 moles of iodine react with 0.045 moles of chlorine to form 0.030 moles of a single product. So, we have to complete the equation. The answer is:
I2 + 3Cl2 = 2ICl3                my question is how come theres 3 atms of Cl formed?

Oh and second question....when iodine reacts with zinc nd wen u decrease the concentration of iodine how come less of zinc is used up?

compare the mole ratio.

0.015 : 0.045

1 : 3

i : cl

[Dana]

oh and another qs, once i was solving past papers and they asked what the product would be when a metal (forgot which one) reacted with water. i thought it would give the hydroxide and hydrogen but instead it actually gave the oxide :S then with another metal, it gave the hydroxide. how can we tell which one forms with each metal?

[Raywin]

Guys, this has been bugging me. In some papers, the mole concept Q, sometimes the easy way to solve the problems is to use something like the volume ratio is same as the mole ratio or something. Umm, what is this and when can we use this? Thanks a lot in advance.

[MasterMath]

oh and another qs, once i was solving past papers and they asked what the product would be when a metal (forgot which one) reacted with water. i thought it would give the hydroxide and hydrogen but instead it actually gave the oxide :S then with another metal, it gave the hydroxide. how can we tell which one forms with each metal?

Hmm .. look at this
                 K
                 Na
                 Li
                 Ca
                 Mg
                 Al
                 C
                 Zn
                 Fe
                 Pb
                 H
                 Cu
                 Ag
Okay thats the reactivity series .. as you go down reactivity decreases .. ANY METAL ABOVE Ca .. reacts with COLD water to form hydroxide and H2
Anything under it forms oxide and H2
Hope i helped

[Dana]

Guys, this has been bugging me. In some papers, the mole concept Q, sometimes the easy way to solve the problems is to use something like the volume ratio is same as the mole ratio or something. Umm, what is this and when can we use this? Thanks a lot in advance.

thats when the equation involves only gases. then you can use mole volume ratio or whatever is called

[Dana]

Hmm .. look at this
                 K
                 Na
                 Li
                 Ca
                 Mg
                 Al
                 C
                 Zn
                 Fe
                 Pb
                 H
                 Cu
                 Ag
Okay thats the reactivity series .. as you go down reactivity decreases .. ANY METAL ABOVE Ca .. reacts with COLD water to form hydroxide and H2
Anything under it forms oxide and H2
Hope i helped

you did thanks loads what abt calcium does it form a hydroxide or oxide?