HSC Chemistry Question Thread

[anotherworld2b]

I was also wondering if I can have help with electrolysis. I am not sure how you identify the possible reactions. Also how do know water is involved and which equation it is? ( there are numerous water equations)

[winstondarmawan]

Would appreciate help with the following (MC 2011 HSC)
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22264927_1342875159171337_923228709_n.jpg?oh=e208756ef1b1436ff5f53d49a6168d71&oe=59D82712
Thanks in advance.

[bsdfjnlkasn]

Would appreciate help with the following (MC 2011 HSC)
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22264927_1342875159171337_923228709_n.jpg?oh=e208756ef1b1436ff5f53d49a6168d71&oe=59D82712
Thanks in advance.

Hey there!

Remember when you did the bromine water experiment? This question is very similar, only this time it is for propane instead of (cyclo)hexane which you would have used for the experiment in class. The question is asking you to recall that when an alkene and bromine water are reacted, the alkene breaks down into a single bonded alkane with the two bromines from the bromine water (Br₂) adding to the two new free electrons (which were before tied up in the double bond).

In the case of propene, it becomes propane allowing the bromine to add to any of the carbon atoms. So that leaves us with 1, 1 - dibromopropane or 1,2 - dibromopropane. The "di" indicates how many bromines there are. The answer is the latter one as the first one doesn't follow correct naming conventions. If the Br's were on the same carbon for instance, then you would have 1, - dibromopropane as that 1 specifies the number of the carbon atom to which the bromines are attached. If the bromines are attached to the first and third carbon, you would have to name the compound: 1, 3 - dibromopropane not 1, 1 - dibromopropane as you need to make sure you count from the carbon that you assign number one (you can't have two number ones). You always number the carbons so that you will have the lowest numbers out the front of the name.

Hopefully this is clear

[MisterNeo]

I was also wondering if I can have help with electrolysis. I am not sure how you identify the possible reactions. Also how do know water is involved and which equation it is? ( there are numerous water equations)

The first one is dilute NaCl, which is the same as electrolysis of water because the salt just acts as an electrolyte to conduct electricity.


The second one has chlorine gas, hydrogen gas, and sodium hydroxide products because the higher concentration of salt allows the chloride to be oxidised over water.

Would appreciate help with the following (MC 2011 HSC)
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22264927_1342875159171337_923228709_n.jpg?oh=e208756ef1b1436ff5f53d49a6168d71&oe=59D82712
Thanks in advance.

Bromine water has bromine as Br₂. When the bonds open up, there are two possible additions on the separate carbons. Refer to the attachment below.

[arton123]

I just have a question for when combustion. When they ask you why combustion is monitored, are we allowed to use ethanol as the fuel in our equations? Most use octane, but ethanol is easier for me as I remember the molar ratio of the reactants and products, while with octane it is a bit harder. So will I be penalised if I use ethanol in the complete and incomplete combustion questions?

[mary123987]

Hey does anyone know like how to know the states in general for an equation ? or is it all dependent on memory?

[MisterNeo]

I just have a question for when combustion. When they ask you why combustion is monitored, are we allowed to use ethanol as the fuel in our equations? Most use octane, but ethanol is easier for me as I remember the molar ratio of the reactants and products, while with octane it is a bit harder. So will I be penalised if I use ethanol in the complete and incomplete combustion questions?

I’m pretty sure you can use either, as long as you show why complete combustion is preferred over incomplete combustion. I used octane for both of mine.

Hey does anyone know like how to know the states in general for an equation ? or is it all dependent on memory?

The states of an equation depends on the conditions in which the reaction takes place.
Hydration of ethylene occurs at 300’C, so the water and ethanol will be gases. Combustion occurs at room temperature, so the water will be liquid. (The heat is a product of combustion, not a condition)
Fermentation occurs in an aqueous solution of glucose, so the ethanol produced will also be aqueous. You can tell what state it is by the temperature or aqueous/non-aqueous condition of the reaction

[winstondarmawan]

Hey does anyone know like how to know the states in general for an equation ? or is it all dependent on memory?

Adding onto this, how would you know the states of various hydrocarbons?

And also would appreciate help with the following:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22291667_1343257242466462_1759583141_n.jpg?oh=3696c9107bddde65553435bbdcbb3c25&oe=59D90D9F
Thanks in advance!

[MisterNeo]

Adding onto this, how would you know the states of various hydrocarbons?

And also would appreciate help with the following:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22291667_1343257242466462_1759583141_n.jpg?oh=3696c9107bddde65553435bbdcbb3c25&oe=59D90D9F
Thanks in advance!

For alkanes and alkenes, the 1-4 carbons are gases whereas the rest are liquids.
For the galvanic cell question, reaction with acid is being oxidised by the H⁺ ion.
You can tell by the table that Metal Q reacts with everything whereas Metal R doesn't. This means that Metal Q must be higher than the H₂O + e^- on the Data Sheet (around potassium) and wants to oxidise. Metal R would be below the H⁺ 0.00V value on the Data Sheet (around copper/silver) and wants to reduce. Since Q is on the top of the Data Sheet and R is on the bottom, there would be a big E^o value when they are set up in a galvanic cell. Hence, the answer would be C.

[winstondarmawan]

For bromine water test equations, would it be better to use the reaction with Br2 or HOBr?

[MisterNeo]

For bromine water test equations, would it be better to use the reaction with Br2 or HOBr?

HOBr is technically the correct method, but the HSC accepts the Br₂ method as well. Personally, I’d use the Br₂ because it’s easier. That previous HSC question from the bromine water + propene uses the Br₂ method.

[angelahchan]

Hi, could someone please help me with the question "explain why sodium hydroxide produced by the mercury cell is not contaminated with salt"
thanks

[MisterNeo]

Hi, could someone please help me with the question "explain why sodium hydroxide produced by the mercury cell is not contaminated with salt"
thanks

The mercury cell features negatively charged liquid mercury as the cathode to reduce sodium ions to form an amalgam of Na/Hg. An amalgam is an alloy mixture of a metal and mercury, and NaCl is insoluble in mercury, which prevents contamination.

The liquid amalgam of sodium and mercury flows into a separate chamber in the cell where it is reacted with water to form sodium hydroxide where no NaCl is present.

Whereas, the other methods of NaOH production rely on a porous diaphragm/membrane to stop chloride from migrating but allow sodium ions to cross, which does not always stop contamination.

[f_tan]

If sulphuric acid is a strong diprotic acid and citric is weak but triprotic, which one is the stronger acid?

[MisterNeo]

If sulphuric acid is a strong diprotic acid and citric is weak but triprotic, which one is the stronger acid?

Sulfuric acid.
A strong acid is an acid that has its protonations (giving hydrogen ions) lying well to the right, whereas a weak acid lies well to the left and is denoted by an equilibrium arrow.
For example, you have 100 molecules of sulfuric acid and 100 molecules of citric acid in water.
Sulfuric acid will give out 99 of their first proton, whereas citric acid will only give out 3 because it is weak.