MBBS' Chemistry help thread

[MBBS]

Okay guys, I have a lot of stupid easy questions that devolop over time. So I'll ask them here.

Firstly, when the question is worded, how do I know the states of the reactants and products?

What's combustion?

[iNerd]

Combustion is a type of reaction that always has O₂ as a reactant [ oxygen gas ].
It produces heat and/or light.

[Greggler]

If something is a solution it is aqueous
if something is molten it is a liquid, otherwise the only other liquid i come across is water
Gases should be pretty obvious, water is sometime a gas when they are talking about steam being produced or used
and solids, well basically thats your precipitates, sometimes they might be things you combust.

States are sort of things you need to be pedantic about, but they are definately something you shouldnt really stress about.
At 3/4 level, equations for most questions are given, and if you have to write an equaition it will usually be a relatively familiar or straightforward one where the states will be quite obvious.

[jasoN-]

yes, its reacting with O2 producing CO2 and H2O, a typical combustion reaction (not to mention release of heat energy)

[MBBS]

Ok, It this is a neutralization reaction. Once we find the concetration of H₂SO₄, do we have to half if because each H₂SO₄ donates two protons?

H₂SO₄(aq) + 2NaOH (aq)

[Studyinghard]

Ok, It this is a neutralization reaction. Once we find the concetration of H₂SO₄, do we have to half if because each H₂SO₄ donates two protons?

H₂SO₄(aq) + 2NaOH (aq)

I am 95.67% sure you dont have to double it. However, I hate pH and concentration.

[MBBS]

^^ Can you explain?

Surely if each of the H₂SO₄ molecules release two H⁺ ions and each of the NaOH release one OH^- ion for each molecule. Given we have the same amount of Volume of each, to neutralize wouldn't we need half the amount of H₂SO₄ molecules ?

Is ion the correct term in this context?

[MBBS]

Ok more stupid questions.

In terms of states, can an ionic compound ever be gaseous? *All the gasses I can think of are covalent.

Do I need to know how to draw molecules? I can't remember any of the shapes/drawing techniques we learn in year 11!

[Studyinghard]

^^ Can you explain?

Surely if each of the H₂SO₄ molecules release two H⁺ ions and each of the NaOH release one OH^- ion for each molecule. Given we have the same amount of Volume of each, to neutralize wouldn't we need half the amount of H₂SO₄ molecules ?

Is ion the correct term in this context?

Sorry, not 100% sure so I dont want to give out wrong answers. I'l let some of the pro's handle that one.

Ok more stupid questions.

In terms of states, can an ionic compound ever be gaseous? *All the gasses I can think of are covalent.

Do I need to know how to draw molecules? I can't remember any of the shapes/drawing techniques we learn in year 11!

An ionic bond is between a metal and non metal element. For example NaCl - Na (metal) and Cl (non metal)
A gas is Cl (non metal) and Cl ( non metal). As such an ionic compound cannot be gaseous.

For year 12, you will need to know how to draw acids especially. You will need to brush up on your isomers aswell. But dont worry too much, they are fairly basic but if you want to be on top of everything I would advise to practice during holidays as it will DEFINITELY be there. 

[stonecold]

^^ Can you explain?

Surely if each of the H₂SO₄ molecules release two H⁺ ions and each of the NaOH release one OH^- ion for each molecule. Given we have the same amount of Volume of each, to neutralize wouldn't we need half the amount of H₂SO₄ molecules ?

Is ion the correct term in this context?

You are correct.  You do double it.  Common exam question, because so many people forget to double it.

This was in our exam this year on an MCQ.  They told us to find the pH of 0.005M Ba(OH)2

But in the context of the question you don't double it because the mole ratio 2 NaOH to 1 H2SO4 is obviously balanced.

But if your are working with the ionic equation, H+ +  OH- ---> H2O, then you double

[MBBS]

Phew!

We got the Concentration+Volume of NaOH and Volume for H2SO4.

Then I used C1V1=C2V2,
Then I halved the concentration needed.

This is OK yeah? Because C1V1=C2V2 works for a 1:1 ratio and after having established that I halved for a 1:2 ratio?