prelim stuff (1 Viewer)

[velox]

cool now i understand Yay! thanks

 

[Xayma]

Originally posted by wrx
Also how do u know when to add the Mono, Di, Tri etc to a formula?

I think it is you add it to the covalent substances inside the material (I think you can also leave the mono off if that is the only form it comes in, eg Carbon and Oxygen can make Carbon Monoxide and Carbon Dioxide, where as Hydrogen and Flouride wont make Dihydrogen Flouride (excluding possible ions of Hydrogen/Flouride))

 

[velox]

with chemical equations do all the ions always have to equal 0? or is this just the case i some questions where they specify. Also how would u find the formula to nitric acid, Hyrdoiodic acid, nitrous acid, and lead hydrogen carbonate. I did them but got wrong answers...not sure how to do them. thanks!

 

[Xayma]

Unless the chemical itself is ion, the total sum of all charges=0

Hence for Nitric Acid, it comprises Hydrogen and Nitrate ions, (+1 and -1 respectively) hence it is HNO₃etc

 

[velox]

ah ic so carbonic acid would consist of carbonate and hydrogen ions?

 

[Xayma]

Hmm I dont know if they are actually ions (CM_Tutor should know) but I used it above cause it makes it easier.

But yes Carbonic Acid consists of carbonate and hydrogen "ions".

Similarly Hydrochloric acid consists of Hydrogen and Chlorine "ions",

With things like Sulfuric acid and Sulfours acid it gets a little more complex.

 

[velox]

ah ic thanks

 

[velox]

Hyrdoiodic acid - whats the formula for this? and how do we get that....

 

[CM_Tutor]

Hydroiodic acid is HI. It is one of the six strong acids, so it completely ionises in solution to hydronium ions and iodide ions.

PS: Re discussion of carbonic acid above. Carbonic acid is a weak acid, and thus should be thought of as having net uncharged, covalently bonded, H₂CO₃ molecules. When it does ionise, it does so in a step-wise manner, producing first hydrogencarbonate ions and then carbonate ions, in addition to hydronium ions. Since it is weak, these ionisation equilibria lie to the left.

 

[velox]

thanks CM_Tutor, clears things up for me Xayma as regarding this "Hmm I dont know if they are actually ions (CM_Tutor should know) but I used it above cause it makes it easier." My teacher said you should do that in all compunds, but not molecules as molecules dont have an overall charge...i think im right.

 

[Xayma]

Well they do have a small difference in change due to the difference in electronegativity of the molecules.

But regarding each one as an ion just makes it alot easier to see how many Hydrogen atoms are needed in the molecule, although they covalently bond.

 

[CM_Tutor]

Originally posted by Xayma
But regarding each one as an ion just makes it alot easier to see how many Hydrogen atoms are needed in the molecule, although they covalently bond.

It may make it 'easier' but it can also lead to mistakes. For example, in the US they tend to write acetic acid as C₂H₄O₂, and the acetate ion as C₂H₃O₂^-. Obviously, we know that acetic acid is monoprotic, as is easily seen from the form CH₃COOH, but this 'method' could cause a problem when you represent in other ways. Simialrly, boric acid, H₃BO₃ is only capable of producing the equivalent of one proton, despite having three hydrogens.

IMO, it is better to understand the actual situation, and apply it, than to try and use a method that you know has problems.

 

[velox]

what does this symbol mean? its like this d with a superscript - then a super script +, i think its something do do with ionic lattice and hydrogen ions....i cant read the writing from my friends book

 

[Xayma]

is it a curvey d? If so it is actually the small greek letter delta, it effectively means small so delta⁺ would be a small positive charge. You may also see it in maths with points of inflexion where it indicates a small region left or right.

 

[xiao1985]

delta positive would be mostly used (in hsc syllabus) in terms of describing a dipole interaction between 2 covalently bonded atoms... because they have different electron negativity, they exert different pull on the electrons that are being shared... hence electrons will be around the atom - which has a stronger pull on electrons - more than the other... hence creating a slightly positive end and a slightly negative end in a covalently bonded molecule...

one good example of this is water, which has a molecular bent shape... the huge difference (relatively) between hydrogen atom and oxygen atom creates a strong dipole interaction... so strong that water has one of the highest boiling pts among the covalent substances...

 

[untying_average]

Yeah i am having trouble with a year 11 dotpoint too... It's actually a really easy one and im pretty sure i know what it means, but it's not in the textbook so just to be sure -

- What is the difference between a chemical and physical property of a substance?

Thanks for any help.

 

[Xayma]

um search for "chemical physical" by CM_tutor he did a pretty good one.

 

[velox]

its in this thread look for it

Originally posted by untying_average
Yeah i am having trouble with a year 11 dotpoint too... It's actually a really easy one and im pretty sure i know what it means, but it's not in the textbook so just to be sure -

- What is the difference between a chemical and physical property of a substance?

Thanks for any help.

What would u do for this one?
choose resources and process information from secondary sources to construct and discuss the limitations of models of ionic lattices, covalent molecules and covalent and metallic lattices

 

[untying_average]

oh ok sorry thanks i missed it

 

[CM_Tutor]

Originally posted by wrx
What would u do for this one?
choose resources and process information from secondary sources to construct and discuss the limitations of models of ionic lattices, covalent molecules and covalent and metallic lattices

This dot point is in the Chemical Earth module, so it's not meant to be especially deep. I think the important aspects of this are:

* be able to classify substances into the appropriate category of structure out of ionic lattice, covalent molecular structure, network covalent structure and metallic lattice.

* be able to describe each of the structures and the bonding that is present

* be able to discuss the relationship between properties such as MP, BP, solubility, solid, liquid and aqueous conductivity and bonding

* be able to discuss the limitations of each model, and cases where the lines between them blur. Such cases include:
- HCl is a covalent molecular substance, but becomes ionic in aqueous solution.
- Polymers are technically covalent molecular, but macromolecules do not tend to be rigidly categorised.
- Graphite has a network covalent structure, but conducts in solid state via a mechanism similar to that of metals.
- Due to relatively small differences in electronegativity, some 'ionic' substances have distinctly covalent molecular properties. Copper(II) bromide would be a typical example.

BEAR IN MIND that I am lifting examples from throughout the syllabus, and thus some of this would not be relevant for a Year 11 student in early second term, but this is what is relevant for a Year 12 preparing for the HSC.