Chemistry 3/4 2013 Thread

[chemdeath]

For KNO3, the energy absorbed is more than released, as the delta H is positive and you have to gain energy overall.
We need it in kJ/mol. So (2528.9/1000)kJ/(1.0M*0.05L)=50.58kJ/mol. As it is exothermic delta H is negative, and assuming the reaction is the normal 1:1 mol ratio then delta H=-50.58kJ/mol for HCl+NaOH-->NaCl+H2O

Thanks heaps your a true champ

[chemdeath]

One more urgent question....haha
1) 5.001g of NH4Cl was dissoluted in water to cause it (waters temp) to reduce by 12 degrees
a) find heat energy, mole of the solid and delta H.

Thanks

[clıppy]

One more urgent question....haha
1) 5.001g of NH4Cl was dissoluted in water to cause it (waters temp) to reduce by 12 degrees
a) find heat energy, mole of the solid and delta H.

Thanks

I feel like that question is missing something. Was there a calibration factor given? Or a mass/volume of water?

[chemdeath]

I feel like that question is missing something. Was there a calibration factor given? Or a mass/volume of water?

No CF factor, but the volume of water was 100ml Thanks

[Edward21]

One more urgent question....haha
1) 5.001g of NH4Cl was dissoluted in water to cause it (waters temp) to reduce by 12 degrees
a) find heat energy, mole of the solid and delta H.

Thanks

4.18=J/(mL*dT) dT being delta T solve for the J absorbed by substituting the 100mL, and the temperature change to get 5016J, 5.016kJ absorbed.
The mol of NH4Cl is n=m/M=5.001g/53.5gmol=0.09348mol, therefore delta H is 5.016kJ/0.09348mol=53.66kJ/mol, assuming normal equation with compound having a co-efficient of 1 (if it's something else you have to multiply the kJ/mol value by this, it's not literally kj per 1 mole, but kJ per mole of whatever's being described in the equation!!) as it's endothermic delta H is positive so dH=+53.66kJ/mol

[Edward21]

QUESTIONS: what reactions would occur at the anode and cathode with electrolysis of ZnCl2 solution, and KI solution? I think mine are wrong, the internet keeps saying other reactions 

[lzxnl]

QUESTIONS: what reactions would occur at the anode and cathode with electrolysis of ZnCl2 solution, and KI solution? I think mine are wrong, the internet keeps saying other reactions 

Strongest reductant present is iodide ion, strongest oxidant is water, so you'll form solid iodine, hydrogen gas and hydroxide ion.

[Edward21]

Strongest reductant present is iodide ion, strongest oxidant is water, so you'll form solid iodine, hydrogen gas and hydroxide ion.

Thanks! But what about the ZnCl2 solution??

[lzxnl]

You have water in the ZnCl2 solution, so the water will be reduced to form hydrogen gas and hydroxide ions.
The formation of the negative charge at this cathode will be balanced by the salt bridge.

[jgoudie]

I stand corrected this was in a the 2008 paper, Question 7 i think.

H+ is used when the solution is neutral or acidic, OH- is used when the conditions are alkaline, such as a alkaline battery.  I am pretty sure i have not seen this other than in a multiple choice practice paper.  But if you want to know how to do it normally check below: 

http://www.chemguide.co.uk/inorganic/redox/equations2.html

A very clear to follow set of instructions as always from chemguide.co.uk.

[Edward21]

The (SO4)2- ion can act as an oxidant, thus get reduced in electrolysis, how come this doesn't occur? I was thinking because the cathode is already negative, the ion wouldn't be electrostatically attracted? what do you think? 

[SocialRhubarb]

I think the reduction of sulfate only occurs in acidic solutions.

Also, even if the solution is acidic, it's not included on the electrochemical series in the VCAA data book, so we're not expected to use it in equations. The same is true of the nitrate ion.

[barydos]

In electrolysis, is the forward Z thingo more likely to happen for a pair furthest apart, or closest together?

[BasicAcid]

In electrolysis, is the forward Z thingo more likely to happen for a pair furthest apart, or closest together?

Closer. Strongest oxidant reacts with strongest reductant.

[barydos]

Closer. Strongest oxidant reacts with strongest reductant.

Thanks!

Is electrolysis occurring during recharge or discharge?

And I need help with this question: