HSC Chemistry Question Thread

[jakesilove]

Thanks so much Jake!

Also, what would be the main points that we need to know about AAS? Is knowing how to draw a diagram of it important? My notes include stuff on hollow-cathode lamp selection and preparing the standard solution - are things like this necessary? Thank you 

That's already more information than you need! You need to understand the purpose of AAS, the method to perform AAS (ie. aspirate liquid in a flame, shine light through, compare with calibration curve) and the benefits of AAS, but that's basically it! Having those extra few details is nice, but definitely not necessary

[jakesilove]

wait for glavanic cells, if it is the case that a the voltage is independent of the concentration in the cells - that is the molar ratio doesnt matter as it will simply use one faster than the other - then should we use 1 molar solutions? - may eat up prac time.

Always use 1 molar solutions. Voltage is not independent of molar concentration. There is just a complicated relationship between concentration and voltage, which you do not need to know.

[chelseam]

Hi! Could someone please explain how a susbtance can be polar? Thank you so much

[jakesilove]

Hi! Could someone please explain how a susbtance can be polar? Thank you so much

Hey! A polar substance is just any molecule with a positive end and a negative end (we call these dipoles). Alternatively, a non-polar substance is uniform in change everything, and thus does not have a positive and negative end.

Below is an example;


Water is polar because it has a positive end (around the Hydrogen atoms) and a negative end (towards the oxygen atom). This arises because of the bent shape of Oxygen.

Hope this helps!

 

[beau77bro]

heres a couple of the major pracs in the first too mods:

any comments or helpful tips are greatly appreciated. thankyou and wish me luck ahhaha

[beau77bro]

heres some more - note none are completed fully. but again any advice on whats there.

[chelseam]

Hope this helps!

When hydrogen bonding occurs in water, the positive hydrogen attracts to the negative oxygen and its lone pairs.

Thank you! This is so helpful

[chelseam]

Hi! How can you identify when a reaction is exothermic or endothermic? Thank you

[legorgo18]

Hi! How can you identify when a reaction is exothermic or endothermic? Thank you

If delta H <0, then exo >0 is endo

or do you mean like specific reactions like decomposition?

[chelseam]

If delta H <0, then exo >0 is endo

or do you mean like specific reactions like decomposition?

I mean in specific reactions! Thanks

[MisterNeo]

I mean in specific reactions! Thanks

You can identify if a reaction is exothermic or endothermic, if no ΔH is given, by looking at the type of atom rearrangement that is taking place in the reaction.
Forming new bonds releases energy (exothermic), whereas braking bonds requires energy (endothermic).
Most reactions are a combination of both breaking and forming bonds BUT one will always outweigh the other, hence exothermic and endothermic.

Some examples:
COMBUSTION

You should be able to identify a combustion reaction as exothermic. This is because new bonds are being formed. Also because the reaction is spontaneous and doesn't require further energy input since it supplies its own energy.

NEUTRALISATION

Also an exothermic reaction because new bonds are being formed.

DECOMPOSITION

Most decomposition is endothermic because heat causes instability in molecules and breaks the bonds at certain temperatures.

SYNTHESIS

Usually the opposite of decomposition, hence it being exothermic as new bonds are formed.

Basically, bonds formed is exothermic; bonds broken is endothermic.
But I would recommend just memorising certain types of reactions being this and that, instead of trying to spot it through an equation.
Hope this helps

[Kekemato_BAP]

Hey guys!!

I need help with this syllabus dot point for Industrial chem..

-discuss the issues associated with shrinking world resources with regard to one identified natural product that is not a fossil fuel, identifying the replacement materials used and/or current research in place to find a replacement for the named material

Other than fossil fuels, which are the best/easiest materials to do? Our teacher suggested rubber or soap, but don't know how to relate the advantages to its shrinking supply.
Thank you!! 

[chelseam]

Basically, bonds formed is exothermic; bonds broken is endothermic.
But I would recommend just memorising certain types of reactions being this and that, instead of trying to spot it through an equation.
Hipe this helps

This clears up so much of my confusion! Thank you so much!!

[chelseam]

Hi! Could someone please explain Le Chatelier's principle in terms of pressure and what happens on an equilibrium graph? Thank you

[MisterNeo]

Hi! Could someone please explain Le Chatelier's principle in terms of pressure and what happens on an equilibrium graph? Thank you 

According to LCP, an increase in pressure will shift the equilibrium system to the side with fewer moles of gas, whereas a decrease in pressure will shift to the side with more moles of gas.
Changes in volume are inverse to the above changes in pressure because increasing volume decreases pressure, and decreasing volume increases pressure.

For example, assume the following reaction:

In this hypothetical equilibrium reaction, the products have no moles of gas; but the reactants have 2 moles of gas.
This means that if you wanted to maximise the yield of the products, the pressure would be increased to push the system to the right; or the volume was decreased to increase pressure to push the system to the right.

On an equilibrium graph, it would look like this if pressure were increased:

That sharp change in concentration represents an increase in pressure as the system acts to minimise the change.

Hope this helps!!