VCE Chemistry Question Thread

[jyce]

But in my Chen SAC there was a question, 2 actually which wanted us to write "forward, backward and no change" for 3 reactions, 2 of which were (AQ)
What is correct then?
Thanks

In my assessment, aqueous solutions had no change when the pressure was altered. Maybe this is the same the sac? (Assuming your teacher got the QATs SAC?)

Ah, I didn't see you said there was a "no change" option - I probably would have gone with that, blacksanta62. As I said, we don't think of aqueous equilibrium systems in terms of pressure but rather in terms of concentration.

Sounds like a pretty lame question tbh, and you won't find any VCAA questions talking about pressure in an aqueous system.

Hope your SAC went well 

[Sine]

Hey guys, for a MSDS for a school SAC, does it have to be as detailed as the ones used professionally? I've looked up a few online and they have 16 sub sections with 5k + words O.o


and on a side note, congratz on 200 posts insanipi <3

Pretty sure the more "summarised" ones. Do you want me to send you a photo of one that I use?

[blacksanta62]

Thanks for the responses guys
Yeah my chem teacher used QATs 2015 SAC, didn't really like this SAC, the questions we were asked mainly.
I can see why it's "no change" now, thank you

Yeah, not much I can do about it now but I'll remember it if I ever come across a question like this again

[HasibA]

Thanks for the responses guys
Yeah my chem teacher used QATs 2015 SAC, didn't really like this SAC, the questions we were asked mainly.
I can see why it's "no change" now, thank you

Yeah, not much I can do about it now but I'll remember it if I ever come across a question like this again

ayy my school used the same QATs sac i think- i thought it was alright

[blacksanta62]

ayy my school used the same QATs sac i think- i thought it was alright

Great to hear 
Guess it's personal preference really
pm me, haven't chatted in a while

[HasibA]

hey guys, just wondering, im seeing questions that involve calculating initial rates of reactions and average rate of reactions, just curious, is it reasonable to assume these will appear on the VCAA exam? i.e they calculate the initial rate with rise/run and they have a formula for the 'average rate of reaction' - has this appeared on an exam before?

[blacksanta62]

hey guys, just wondering, im seeing questions that involve calculating initial rates of reactions and average rate of reactions, just curious, is it reasonable to assume these will appear on the VCAA exam? i.e they calculate the initial rate with rise/run and they have a formula for the 'average rate of reaction' - has this appeared on an exam before?

I have never heard of this before and my teacher has never brought it up, my inclination is that it won't be on the exam. Don't think I've seen it on the SD either.

Hopefully someone else can confirm

[mary1911997]

hi guys need help please as i am so confused and do not know what to do with this question

 A sample of biphenyl (C6H5)2  weighing 0.526 g was ignited with a bomb calorimeter containing 100g of water in order to determine an approximate value for the molar heat of combustion. The temperature of the calorimeter changed from 300K to 301.91K

a. Using the specific heat capacity of water only , determine the molar enthalpy of combustion of biphenyl?

b. In a separate calibration experiment , a 0.825g sample of liquid benzoic acid (C6H5COOH) was ignited , and the temperature of the calorimeter rose from 290K to 291.94K . The standard molar enthalpy of combustion of benzoic acid is -3227KJ/mol
Write a thermochemical equation for the combustion of benzoic acid

c. Calculate the calibration factor of this calorimeter

thanks

[mary1911997]

need help please to figure out this question

 In an electrolysis experiment, a student observed that an unknown metal anode with an oxidation number of +2 lost 0.208g while a total of 3.5mg of hydrogen gas was produced at the cathode. The temperature of the solution was 25C and the pressure was 740 mmHg
   a. Calculate the number of moles of hydrogen gas produced ?
   b. How many Faradays of electricity was passed through the cell?
        c. What is the metal in the anode?

thanks guys

[Sine]

I know that the exams from 2008 till now are still mostly relevant but is it worthwhile doing any vcaa exams before 2008?

[jyce]

need help please to figure out this question

 In an electrolysis experiment, a student observed that an unknown metal anode with an oxidation number of +2 lost 0.208g while a total of 3.5mg of hydrogen gas was produced at the cathode. The temperature of the solution was 25C and the pressure was 740 mmHg
   a. Calculate the number of moles of hydrogen gas produced ?
   b. How many Faradays of electricity was passed through the cell?
        c. What is the metal in the anode?

thanks guys

I'm at uni right now so I'm not going to actually give you the answers, but I'll briefly describe the process of how to answer them.

a. You have the mass of hydrogen gas, and the molar mass of hydrogen gas is just 2 g mol^-1. n = m/M. Make sure that you convert the mass to grams, though!

b. Find the relevant half-equation in the electrochemical series for the production of hydrogen gas and apply stoichiometry to calculate the moles of electrons that must have been consumed to produce the 3.5 mg of H₂. The Faradays is the same as the moles of electrons.

c. As the oxidation number for the anode material is 2+, the mole ratio of anode material reacted : electrons released is 1 : 2. Use this knowledge to calculate the moles of anode material/metal. You also have the mass of anode material that reacted, so you can calculate the molar mass of the metal (again, use n = m/M). Once you have the molar mass, it's a simple matter of finding the correct metal from the periodic table.

I can't see why you need the temperature or pressure; maybe I've missed something. Can have a closer look when I get home if my above method doesn't get you the answer.

Is it tin??

I know that the exams from 2008 till now are still mostly relevant but is it worthwhile doing any vcaa exams before 2008?

Yeah, you'll just have to make sure you know what's required of you according to the SS and only do the relevant questions. I'd say maybe only about 2/3 of the questions in the pre-2008 Unit 4 VCAA exams are relevant to the current SS; a lot of it is on periodic trends, electronic configurations, the history of atomic theory, etc.

[HasibA]

what do we have to know about calorimeter efficiency ? had a prac for a sac, and its asking how calorimeter are efficient/why they're efficient and how to determine if a specific calorimeter is in fact efficient by looking at the temperatures inside within a 10 minute period, using electricity to heat up the content inside for the first 3 minutes, and recording temps at intervals after that for 7 mins.

hope i explained my question properly lol

edit: more questions. When a reaction's temp inside a calorimeter increase, is it always a endothermic reaction? as it absorbs energy and so the temp increases? vice versa for exothermic too, correct?

[jyce]

what do we have to know about calorimeter efficiency ? had a prac for a sac, and its asking how calorimeter are efficient/why they're efficient and how to determine if a specific calorimeter is in fact efficient by looking at the temperatures inside within a 10 minute period, using electricity to heat up the content inside for the first 3 minutes, and recording temps at intervals after that for 7 mins.

hope i explained my question properly lol

edit: more questions. When a reaction's temp inside a calorimeter increase, is it always a endothermic reaction? as it absorbs energy and so the temp increases? vice versa for exothermic too, correct?

I'm guessing in regards to your first question, you needed to refer to whether or not the calorimeter was losing energy over time - i.e., did the temperature within the calorimeter remain relatively constant, or did it decrease significantly? You'd hope it remained fairly constant.

As for your second question, are you saying that a temperature increase inside of a calorimeter would be indicative of a endothermic reaction? If so, you'd be incorrect - endothermic reactions involve an overall absorbance of energy from the environment, therefore decreasing the temperature within the calorimeter. The thermal energy is transformed into chemical energy stored within the bonds of the products.

[HasibA]

-snip-

miswrote exo and endo. so temp increase=exo, decrease=endo - same scenario, correct?

[jyce]

miswrote exo and endo. so temp increase=exo, decrease=endo - same scenario, correct?

yep