Kanon's Chemistry Questions!

[Kanon]

Now with added exclamation marks for excitement!!!!

I'll probably be maintaining this throughout the first term (6-ish weeks) so I guess it's good to have a consistent hub for it.
If anyone else has like one or two questions, feel free to post them here if you don't think the questions warrant a new thread

I'm not very good at Chemistry and i think it kinda shows, so expect a lot of noob questions!

If dichromate ions, Cr₂O₇^2-, are mixed with iodide ions, I^-, Iodine I₂ and chromium(III) ions are produced.Write the overall ionic equation for this reason.

So my first problem with this question is setting the equation up, from my understanding we've got  up, from my understanding we've got

Cr₂O₇^2- +  I₂ --> Cr³⁺

as the law of conservation of mass states, we cannot create nor destroy matter, so thus the Iodine must bond with Oxygen and be added to the equation?

Cr₂O₇^2- +  I₂ --> Cr₂³⁺ + I₂O₇?

At this point, I ask myself what the hell i'm doing, and realize i'm probably off on a tangent, so I check the answers and realize i'm completely wrong.  Could somebody please help me setup the equation? 

[Nobby]

It's like an ionic equation; no spectators, so you can have it exist as ions. Next bit is from my summary.

Reduction half equations and oxidation half equations can be combined to form an overall redox equation.
When writing half equations, any O’s must be balanced by adding H₂O’s, any H2O’s balanced by adding H⁺’s, and any H⁺’s balanced by adding e^-‘s. The overall charge on each the left side of the equation must equal the overall charge on the right.
When combining half equations, each half must be multiplied by the appropriate factor (if necessary) so as to cancel out any electrons. After this, cancel out any H₂O’s and/or H⁺’s.

Oxⁿ: 2I^- --> I₂ +2e^-
Redⁿ: Cr₂O₂^2- + 14H⁺ + 6e^- --> 2Cr³⁺ + 7H₂O
Ovr: Cr₂O₇^2- + 6I^- + 14H⁺ --> 2Cr³⁺ + 3I₂ + 7H₂O

[Phy124]

Just to add onto what Nobby said:

In the question it states, "Cr₂O₇^2-, are mixed with iodide ions, I^-" So these will be on the left side of the overall equation. It then says "Iodine I₂ and chromium(III) ions are produced" so they will have to be on the right hand side of your overall equation.

Understanding this will make it a lot easier to work out the overall ionic equation and the oxidation and reduction reactions when you are asked for that in the future.

[Kanon]

Sorry for the late reply, i've been packed with Physics, Meth(ods) and IT work.
Thank you!  I finally get it, I didn't realize it was so systematic.

Now for thou art second question,
In situations where we're asked to calcuate the volume occupied by a gas at STP, how do we know which conversion of pressure (P) we use?
If this was physics, you could generally deduct the units you use from the question and applying basic index laws, but with this it's very confusing.

 

[Aurelian]

In situations where we're asked to calcuate the volume occupied by a gas at STP, how do we know which conversion of pressure (P) we use?
If this was physics, you could generally deduct the units you use from the question and applying basic index laws, but with this it's very confusing.

 

Can you clarify what you mean here?

[Kanon]

Calculate the volume occupied
1. 2.01x10^23 molecules of O2 at STP (Ans: 7.48L)

So for this, we'd have to use a gas equation, my question is how do we know what unit
of pressure to sub into P?  Like we could use atm, mm Hg or Pa?
With Physics, atleast you can tell from the units, so like if you had ms^-2 you must use a quantity in meters multiplied by a quantity in seconds. 

[Nobby]

Calculate the volume occupied
1. 2.01x10^23 molecules of O2 at STP (Ans: 7.48L)

So for this, we'd have to use a gas equation, my question is how do we know what unit
of pressure to sub into P?  Like we could use atm, mm Hg or Pa?
With Physics, atleast you can tell from the units, so like if you had ms^-2 you must use a quantity in meters multiplied by a quantity in seconds.

It's almost always kPa, but it can be Pa, mPa or whatever so long as it is proportional to the volume units; kPa to L, Pa to kL, etc.
And for the 2.01 x 10²³ molecules of O₂, all you need to do is calculate the number of moles, then multiply by the molar volume, V_m (in the case of STP, 22.4L mol^-1).

[Panicmode]

Calculate the volume occupied
1. 2.01x10^23 molecules of O2 at STP (Ans: 7.48L)

So for this, we'd have to use a gas equation, my question is how do we know what unit
of pressure to sub into P?  Like we could use atm, mm Hg or Pa?
With Physics, atleast you can tell from the units, so like if you had ms^-2 you must use a quantity in meters multiplied by a quantity in seconds.

It depends on what "R" is. For VCE, the gas constant (R) will always be 8.31 J K^−1 mol^−1.

However, it is possible to use other pressures if other gas constants are used.

To make things simpler however, just always convert whatever;

- pressure to kPa
- volume to litres
- temperature to Kalvin

This is the accepted/preferred method for VCE.