A lvl Chemistry Questions

Charmy SG

hey, need some help on some chem questions regarding grp 7 elements :D

Q1) The halides, Cl-, Br- and I- differ in their reactions with concentrated sulfuric acid. Describe what you would see if conc. sulphuric acid is added to seperate samples of the solids KCl, KBr and KI. Suggest an explanation for the differences in reaction, write equations illustrating the types of reaction undergone.

(I only know the reaction between hydrogen halides and conc H2SO4. Is it similar?)

Q2) When Chlorine gass is passed through a strongly alkaline solution of sodium bromide, a salt is produced which has the following composition by mass: Na, 15.2%; Br,53.0%; O, 31.8%.
(i) Calculate the formula of the salt (I calculated NaBrO3)
(ii) Calculate the change in oxdation number of bromine during this reaction( it's from -1 to +5 right?)
(iii) Construct an equation for the reaction (I have no idea)
all i could squeeze out for the equation was 2Cl2 + NaBr + 6OH- --> 3NaBrO3 + 3HCl which i think is wrong.

please help! many thanks :D

UltimaOnline SG

hey, need some help on some chem questions regarding grp 7 elements :D

(I only know the reaction between hydrogen halides and conc H2SO4. Is it similar?)

please help! many thanks :D

Different halide ions have varying reducing capacities (ie. different redox potentials), due to varying distances between the positively charged nucleus and the valence shell. Demonstrate their varying reducing capacities by stating the OS of the sulfur atoms in the sulfur containing product of the redox reaction. Eg. bromide ions can reduce sulfate(VI) ions only to sulfur dioxide (OS = +4), while iodide ions can reduce sulfate(VI) ions all the way to hydrogen sulfide (OS = -2).

Regarding the equation to generate the bromate(V) ion, you should balance the ionic redox equation (don't include spectator ions such as Na+) as follows :

1) balance the hetero-element by adding coefficients

2) balance the oxygens by adding water molecules

3) balance the hydrogens by adding protons

4) cancel out any protons by adding hydroxide ions

Charmy SG

So when they say write equations to show the types of reaction, i dont need to write out full equations?

which means i would just write: 2Br- + (SO4)2- --> Br2 + 2(SO2) to show the oxidation state change?

So the K in KBr doesn't matter and the reactions are still like HBr reactions?

and about the bromate(V) ion, i've got a final equation: Br- + 6OH- --> BrO3- + 3H2O + 6e-, right?

how do i include Cl2 inside the full equation? is it just adding the Cl2 half eqn (Cl2 + 2e- --> 2Cl-) to get:
3Cl2 + Br- + 6OH- --> BrO3- + 3H2O + 6Cl-?

dont i need to add the Na in? :\

Thanks!:D

UltimaOnline SG

So when they say write equations to show the types of reaction, i dont need to write out full equations?

which means i would just write: 2Br- + (SO4)2- --> Br2 + 2(SO2) to show the oxidation state change?

So the K in KBr doesn't matter and the reactions are still like HBr reactions?

and about the bromate(V) ion, i've got a final equation: Br- + 6OH- --> BrO3- + 3H2O + 6e-, right?

how do i include Cl2 inside the full equation? is it just adding the Cl2 half eqn (Cl2 + 2e- --> 2Cl-) to get:
3Cl2 + Br- + 6OH- --> BrO3- + 3H2O + 6Cl-?

dont i need to add the Na in? :\

Thanks!:D

>>> So when they say write equations to show the types of reaction, i dont need to write out full equations? which means i would just write: 2Br- + (SO4)2- --> Br2 + 2(SO2) to show the oxidation state change?

So the K in KBr doesn't matter and the reactions are still like HBr reactions? <<<

Depending on the question, in many cases it doesn't matter whether you give the ionic or chemical equation. But since the question specified potassium halides, no harm in writing the chemical equation (though you may find it easier to write the ionic equation 1st, and add in the spectator ions later).

If the question is ambiguous in what it wants, or you're unsure which form the question wants, go ahead and write both. You'll get all the marks as long as both are correct. I'm not just talking about ionic vs chemical equations, I'm talking about how to tackle ambiguous exam questions in general.

The relevant chemical equations are (for instance, if you use sodium halides) :

NaCl + H2SO4 --> HCl + NaHSO4

NaBr + H2SO4 --> HBr + NaHSO4

2HBr + H2SO4 --> Br2 + SO2 + 2H2O

NaI + H2SO4 --> HI + NaHSO4

8HI + H2SO4 --> 4I2 + H2S + 4H2O

>>> and about the bromate(V) ion, i've got a final equation: Br- + 6OH- --> BrO3- + 3H2O + 6e-, right?

how do i include Cl2 inside the full equation? is it just adding the Cl2 half eqn (Cl2 + 2e- --> 2Cl-) to get:
3Cl2 + Br- + 6OH- --> BrO3- + 3H2O + 6Cl-?
dont i need to add the Na in? :\ <<<

The problem is, if you add the Na+ spectator ions in, then it should be a full chemical equation (ie. either write ionic equation or write chemical equation, not some half-ionic-half-chemical equation). Meaning that you should also add in the spectator ions that were used to counterbalance the hydroxide ions in the alkali employed. But no mention of which alkali was used (eg. NaOH(aq), KOH(aq), etc), so writing the full chemical equation becomes difficult due to lack of data.

However, since the desired salt appears to be sodium bromate(V), the chemist carrying out the preparation of the salt should utilise sodium hydroxide alkali (ie. if he had used potassium hydroxide alkali instead, the salt obtained would be a mixture of sodium and potassium bromate(V).)

In which case, for the chemical equation, it is fine to add Na+ in not only for the counterbalancing of Br- ions, but also for the OH- as well.

When in doubt, the exam-smart thing to do, is to write both the ionic and the chemical equation.

In writing the ionic equation (or if you choose to write only the ionic equation), add on in words, "The above ionic equation describes the redox reaction between gaseous chlorine and a strongly alkaline solution of sodium bromide to generate the bromate(V) ions present in the sodium bromate(V) salt produced in the reaction."

Charmy SG

THANKYOU. :D