Chemistry 3/4 2013 Thread

[lzxnl]

I gave an ionic equation above for the dissolution of sulfuric acid.

[achre]

Hey all.
Are there any suggested responses for the new study design sample exam on the VCAA website? (VCAA produced or otherwise)

[Edward21]

Is there a possibility we could be asked to determine avogadro's number from experimental data on a VCAA exam? I came across a checkpoints question and had NO idea on what to do 

[Jeggz]

Is there a possibility we could be asked to determine avogadro's number from experimental data on a VCAA exam? I came across a checkpoints question and had NO idea on what to do 

We had to determine the Avogadro's number in Faraday, if that's what you're referring to?

[lzxnl]

Is there a possibility we could be asked to determine avogadro's number from experimental data on a VCAA exam? I came across a checkpoints question and had NO idea on what to do 

What was the question?

[Edward21]

What was the question?

Et voila!

[Lasercookie]

Hey all.
Are there any suggested responses for the new study design sample exam on the VCAA website? (VCAA produced or otherwise)

By the Chemistry Education Assoc. http://www.cea.asn.au/sites/default/files/sample_paper_answers_2013_0.pdf
I don't remember if there was an AN user that produced their own set of solutions either, might be floating around somewhere (not sure).

Some of the questions on the sample exam were from previous VCAA exams too

[SocialRhubarb]

Is there a possibility we could be asked to determine avogadro's number from experimental data on a VCAA exam? I came across a checkpoints question and had NO idea on what to do 

Question doesn't seem to out there. I'm not familiar with the specifics of the course design though.

Avogadro's constant just describes the number of atoms per mole.

Calculate the number of moles of copper that are formed from the mass and molar mass of copper, then work out how many atoms of copper are produced, based on how many electrons are required, remembering that each copper atom requires two electrons. The ratio between the number of copper atoms and the number of moles of copper is Avogadro's constant.

[barydos]

Why do some redox reactions use the OH- electrolyte rather than H+?
And when do we know to balance using OH?

[jgoudie]

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.

Why do some redox reactions use the OH- electrolyte rather than H+?
And when do we know to balance using OH?

[chemdeath]

Can anyone help me with this attached question, the reaction occurred in a calorimeter?

[Edward21]

Can anyone help me with this attached question, the reaction occurred in a calorimeter?

I'm assuming they're exothermic? I know HCl and NaOH is, and also the Mg reaction producing heat? If you look at the graph for an exothermic reaction/energy profile, the energy released is greater the rise that you see is the energy required to break the bonds in the reactants, the Ea (activation energy), BUT! this is then released as the curve comes back down to the same level as you started, then it drops even further to produce a negative delta H/change in enthalpy. Recap: you absorb let's say 1 unit of energy as Ea, this is then released, so you're back to zero, but then the reaction, being exothermic, releases even more energy so in effect you release more energy than you take in.

[Edward21]

I'm reading the instructions for the hydrogen oxygen fuel cell prac I have coming up, in the textbook it implies fuel cells are a glorified electrochemical cell with spontaneous redox reactions occurring to produce electricity, why do we have to connect it to a DC power supply, if its purpose is to generate electricity as a fuel cell??? why are we using electricity here 

[chemdeath]

I'm assuming they're exothermic? I know HCl and NaOH is, and also the Mg reaction producing heat? If you look at the graph for an exothermic reaction/energy profile, the energy released is greater the rise that you see is the energy required to break the bonds in the reactants, the Ea (activation energy), BUT! this is then released as the curve comes back down to the same level as you started, then it drops even further to produce a negative delta H/change in enthalpy. Recap: you absorb let's say 1 unit of energy as Ea, this is then released, so you're back to zero, but then the reaction, being exothermic, releases even more energy so in effect you release more energy than you take in.

Thanks that makes sense, KNO3 is endothermic though?

And one more question for a reaction between 50ml 1M hcl and 50ml naoh the energy change was found to be 2528.9J. Find delta h
Cheers

[Edward21]

Thanks that makes sense, KNO3 is endothermic though?

And one more question for a reaction between 50ml 1M hcl and 50ml naoh the energy change was found to be 2528.9J. Find delta h
Cheers

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