VCE Chemistry Question Thread

[friedchromosome]

Could someone please explain the difference between STP and SLC and how to convert em ? For example in this question:
"SO2 in the atmosphere contributes to acid rain. The equation for the formation is represented by 2SO2+O2+2H2O--> H2SO4. What mass of sulphuric acid will form 50.0 L of sulphuric dioxide at SLC?"
Please answer that along with explanation
thanks in advance   

[knightrider]

A 1.50 g sample of lawn fertiliser was boiled with 25.00 mL of 0.9987 M sodium hydroxide solution. When no further ammonia gas was evolved from the mixture, it was cooled and titrated with 0.2132 M hydrochloric acid, using phenolphthalein as an indicator. A titre of 19.78 mL was required. Calculate the percentage of ammonium ions in the fertiliser.

In the solution attached,

when they find the number of mole of ammonium ions as highlighted, how do they work out what ratio the NaOH is in as the equation doesnt have  NaOH in it ?

[Biology24123]

Could someone please explain the difference between STP and SLC and how to convert em ? For example in this question:
"SO2 in the atmosphere contributes to acid rain. The equation for the formation is represented by 2SO2+O2+2H2O--> H2SO4. What mass of sulphuric acid will form 50.0 L of sulphuric dioxide at SLC?"
Please answer that along with explanation
thanks in advance   

The molar mass of STP is 22.4 while 24.5 for SLC. Use the equation, n=V/Vm
Vm is equal to 22.4 or 24.5 (24.5 in the question)

[wobblywobbly]

A 1.50 g sample of lawn fertiliser was boiled with 25.00 mL of 0.9987 M sodium hydroxide solution. When no further ammonia gas was evolved from the mixture, it was cooled and titrated with 0.2132 M hydrochloric acid, using phenolphthalein as an indicator. A titre of 19.78 mL was required. Calculate the percentage of ammonium ions in the fertiliser.

In the solution attached,

when they find the number of mole of ammonium ions as highlighted, how do they work out what ratio the NaOH is in as the equation doesnt have  NaOH in it ?

NaOH is the same as OH in the equation. They've just left out the Na+ since it is a spectator ion

[HasibA]

Hey guys, can anyone help me with part (A) - first time seeing a '‘standard addition’ calibration curve', and i am not sure how to read it. gather data from it :/ thank you guys

Link to questions: http://imgur.com/a/o4QAn

[keltingmeith]

Hey guys, can anyone help me with part (A) - first time seeing a '‘standard addition’ calibration curve', and i am not sure how to read it. gather data from it :/ thank you guys

Link to questions: http://imgur.com/a/o4QAn

lolwut
I'd honestly ignore this question - standard additions are waaaay off the study design, you won't get asked about them. Like, I first learned about them near the end of second year uni.

If you still want to give it a shot, plot like anything else - when the horizontal axis is at "x+10" (it has that bit labelled), put a dot at 0.25. When the horizontal is at "x+20", put a dot at 0.34, etc. until you've plotted all 5 points. Then, just draw your line of best fit.

[HasibA]

lolwut
I'd honestly ignore this question - standard additions are waaaay off the study design, you won't get asked about them. Like, I first learned about them near the end of second year uni.

If you still want to give it a shot, plot like anything else - when the horizontal axis is at "x+10" (it has that bit labelled), put a dot at 0.25. When the horizontal is at "x+20", put a dot at 0.34, etc. until you've plotted all 5 points. Then, just draw your line of best fit.

thank you!!!! My teacher insisted i do it, but i said to him that 'i'm pretty sure these aren't on the study design' ahh smh haha thanks Euler!!

[melons]

Are the answers to these correct? Thanks

For the first one I got delta H = -78.6 kJ/mol. The second one is right, I believe. Did you get the same as me?

[Maz]

Hi humans
i did an experiment in class the other day- and i have a validation on it in the next couple of days...i was wandering if someone could please help me out?
The experiment was on equilibrium and Le CHatelier's principle
first we got K₂CrO₄ and added HCl solution to it...then to the same solution we added the base NaOH. originally the K₂CrO₄ was orange, with added base it went to yellow, then when we added acid to the solution it went back to orange..
the question is: By referring to the collision theory account for the observed colour change that occurred when HCl solution is added to the K₂CrO₄ solution
I'd appreciate any help 
Thankyou so much in advance

[Gogo14]

Can someone tell me what it means for a reaction to be a redox reaction?i know it has to have a reduction / addition in electrons.
My teacher gave us a test MC where we had to circle the redox reaction. The answer was an acid base neutralisation reaction, but there were deplacement reactions and combination reactions. Wouldnt they also be redox reactions since they are transferring electrons?

[lzxnl]

Can someone tell me what it means for a reaction to be a redox reaction?i know it has to have a reduction / addition in electrons.
My teacher gave us a test MC where we had to circle the redox reaction. The answer was an acid base neutralisation reaction, but there were deplacement reactions and combination reactions. Wouldnt they also be redox reactions since they are transferring electrons?

Redox = change in oxidation number.

Classifying reactions as 'displacement' or 'combination' reactions really isn't that informative sometimes. Generally, those aren't redox reactions, but there are exceptions.

What do we mean by electron transfer? We really mean, changing 'ownership' of the electrons. Consider the reaction between hydrogen gas and chlorine gas to form hydrogen chlorine. Originally, all four atoms own zero 'net' electrons; they all donate one electron to bond with another atom. As these are covalent bonds between identical atoms, each atom 'owns' half of the two electrons in the bond, i.e. they're considered to keep their electron for the sake of calculating oxidation numbers.
Now consider HCl. Here, the H-Cl bond is polarised and the chlorine atom is taken to 'own' the two electrons in the bond because it's more electronegative. Hence, chlorine has gained an electron and hydrogen has lost an electron. Oxidation numbers can be considered as the charge on an atom if you assume all bonds are ionic and assign electrons in covalent bonds to the more electronegative atom.

Hi humans
i did an experiment in class the other day- and i have a validation on it in the next couple of days...i was wandering if someone could please help me out?
The experiment was on equilibrium and Le CHatelier's principle
first we got K₂CrO₄ and added HCl solution to it...then to the same solution we added the base NaOH. originally the K₂CrO₄ was orange, with added base it went to yellow, then when we added acid to the solution it went back to orange..
the question is: By referring to the collision theory account for the observed colour change that occurred when HCl solution is added to the K₂CrO₄ solution
I'd appreciate any help 
Thankyou so much in advance

The equilibrium here is 2CrO₄^2- + 2H⁺ <-> Cr₂O₇^2- + H₂O
Add HCl to it -> more reactants -> more collisions between reactants -> faster forward reaction rate -> more products formed than reactants -> see more products

[Maz]

.

The equilibrium here is 2CrO₄^2- + 2H⁺ <-> Cr₂O₇^2- + H₂O
Add HCl to it -> more reactants -> more collisions between reactants -> faster forward reaction rate -> more products formed than reactants -> see more products

thankyou...how do you know that that is the equilibrium reaction?

[Gogo14]

Redox = change in oxidation number.

Classifying reactions as 'displacement' or 'combination' reactions really isn't that informative sometimes. Generally, those aren't redox reactions, but there are exceptions.

What do we mean by electron transfer? We really mean, changing 'ownership' of the electrons. Consider the reaction between hydrogen gas and chlorine gas to form hydrogen chlorine. Originally, all four atoms own zero 'net' electrons; they all donate one electron to bond with another atom. As these are covalent bonds between identical atoms, each atom 'owns' half of the two electrons in the bond, i.e. they're considered to keep their electron for the sake of calculating oxidation numbers.
Now consider HCl. Here, the H-Cl bond is polarised and the chlorine atom is taken to 'own' the two electrons in the bond because it's more electronegative. Hence, chlorine has gained an electron and hydrogen has lost an electron. Oxidation numbers can be considered as the charge on an atom if you assume all bonds are ionic and assign electrons in covalent bonds to the more electronegative atom.

The equilibrium here is 2CrO₄^2- + 2H⁺ <-> Cr₂O₇^2- + H₂O
Add HCl to it -> more reactants -> more collisions between reactants -> faster forward reaction rate -> more products formed than reactants -> see more products

Ok I understand a bit. So how would you determine if the reaction would be a redox or not? How would you work it out. So basically, every reaction with products containing elements with high eletronegativity would be a redox?

[melons]

Ok I understand a bit. So how would you determine if the reaction would be a redox or not? How would you work it out. So basically, every reaction with products containing elements with high eletronegativity would be a redox?

Some elements can have a varying oxidation number. So if given an equation determine the oxidation of every element on both sides of the reaction. If the oxidation number changes its a redox reaction. An increase in oxidation number shows an oxidation reaction and a decrease shows a reduction reaction.

[lzxnl]

thankyou...how do you know that that is the equilibrium reaction?

I just do

In all seriousness, knowing stuff like this happens is the result of a few years of university chemistry.

Ok I understand a bit. So how would you determine if the reaction would be a redox or not? How would you work it out. So basically, every reaction with products containing elements with high eletronegativity would be a redox?

You're meant to, in theory, check all of the oxidation numbers. However, some reactions are obviously redox:
Reactions involving elemental metals as reactants, combustion, reactions involving reactive non-metals like halogens or hydrogen etc
You really need experience to tell.