HSC Chemistry Question Thread

[Aaron12038488]

so for the molar heat equation:
can i just plug in the numbers without the -?

[winstondarmawan]

Is that a carbon in the top right?

No, sorry that's meant to be a Chlorine.

[Natasha.97]

so for the molar heat equation:
can i just plug in the numbers without the -?

Hi!
Yes, you can:
Q is representative of the enthalpy change (chemical energy of products - chemical energy of reactants)
         - Heat of combustion reactions involve the release of heat
         - Products store less chemical energy than the reactants
         - Enthalpy change is negative, therefore there is a minus sign in the front

Hope this helps

[MisterNeo]

No, sorry that's meant to be a Chlorine.

It would be 2-bromo-2-chloro-1,1,1-trifluoroethane.
You find the most electronegative element there and give it numbering priorities, then list alphabetically.

[Aaron12038488]

i have a prac test coming up. how should i remember all the colours of the preciptates.

[Natasha.97]

i have a prac test coming up. how should i remember all the colours of the preciptates.

Hi!
I've attached my notes for reference

Anions

Cations

Tip: Create flashcards with the name of the ion, the colour, and equation on one side, and put the test used to determine the ion on the other for each ion
Hope this helps 

EDIT: For Step 3 in cation testing, the ion that precipitates out is Ba²⁺, not SO₄^2-

[Aaron12038488]

so in my textbook theres equations for calculating molar heat:
delta h = mc delta t
OR
delta h = -mc delta t

so for my prac test coming up, i will be answering calculation questions. Which formula would i use? thx.
Does it have to do with endo and exothermic reactions?
so if the temperature increases does this mean its going to have the - sign.

[MisterNeo]

so in my textbook theres equations for calculating molar heat:
delta h = mc delta t
OR
delta h = -mc delta t

so for my prac test coming up, i will be answering calculation questions. Which formula would i use? thx.
Does it have to do with endo and exothermic reactions?
so if the temperature increases does this mean its going to have the - sign.

I would always use it with the negative since that's the actual formula.
Exothermic reactions have a negative value, whereas endothermic have a positive value.

[Aaron12038488]

like for a calculation question, how do i do know if they are asking for an endothermic reaction or exothermic?

[anotherworld2b]

I am not quite sure how to do this

[MisterNeo]

I am not quite sure how to do this

This reaction is actually very similar to the reaction of sodium in water to produce NaOH.

2HOH + 2Na
2NaOH + H₂

2C₂H₅OH + 2Na
2C₂H₅ONa + H₂
You see, ethanol is just water with one of its hydrogens being an ethyl group, so it undergoes similar redox.

Oxidation

Reduction

Net Ionic

Hope this helps

like for a calculation question, how do i do know if they are asking for an endothermic reaction or exothermic?

Do you have an example question? The calculation questions with -mCΔT always involve exothermic reaction of a fuel, and you calculate variable of the equation.

[anotherworld2b]

Thank you for your help
Could I get some help with this question please?

This reaction is actually very similar to the reaction of sodium in water to produce NaOH.

2HOH + 2Na
2NaOH + H₂

2C₂H₅OH + 2Na
2C₂H₅ONa + H₂
You see, ethanol is just water with one of its hydrogens being an ethyl group, so it undergoes similar redox.

Oxidation

Reduction

Net Ionic

Hope this helps ;DDo you have an example question? The calculation questions with -mCΔT always involve exothermic reaction of a fuel, and you calculate variable of the equation.

[Aaron12038488]

heres an example question mister neo.
how much energy will be required to raise the temperature of 1L of water in a kettle from 17 degrees ->100 degrees.
The answer is using the equation without the negative.
but later on the textbook.
When 1 g of NaOH is dissolved in 100 g in water, in an insulated cup, the temperature of the water rises from 20 degress --> 22.66 degrees. Calculate the heat of solution for sodium hydroxide.
For this question^^, its using the negative sign.

[MisterNeo]

heres an example question mister neo.
how much energy will be required to raise the temperature of 1L of water in a kettle from 17 degrees ->100 degrees.
The answer is using the equation without the negative.
but later on the textbook.
When 1 g of NaOH is dissolved in 100 g in water, in an insulated cup, the temperature of the water rises from 20 degress --> 22.66 degrees. Calculate the heat of solution for sodium hydroxide.
For this question^^, its using the negative sign.

When it asks for "how much energy", it asks for the amount of joules needed, which is a positive value because it requires an absolute value.
Kinda like me giving someone $5, instead of giving -$5.

Heat of solution refers to the change in enthalpy when something dissolves. This will either be exothermic or endothermic so you must use the negative sign for NaOH to represent the heat released.

[anotherworld2b]

I found three isomers so far
1-butanol
2-butanol
2-methyl-propan-2ol
i'm not sure what the fourth isomer is

Thank you for your help
Could I get some help with this question please?