VCE Chemistry Question Thread

[TheMereCat]

Thanks lzxnl 

if you were given the formula for something how would you tell if its a Covalent molecular or Covalent network?

Say for example ethanol C2H6O

Something like C2H6O will be covalent molecular because it's structure is that of a simple molecule, whereas something such as diamond has a continuous network of carbon atoms extending throughout it's structure.

[knightrider]

Something like C2H6O will be covalent molecular because it's structure is that of a simple molecule, whereas something such as diamond has a continuous network of carbon atoms extending throughout it's structure.

Thanks so much TheMereCat 

[warya]

I'm so confused about moles lol
in an equations, such as H2O+H2O-->H3O+ + OH-, how come we say that there are two moles of h2o, 1 mole of OH etc, why do we calculate moles then if we can just see it in the equation (v. dumb question don't laugh)

[cosine]

I'm so confused about moles lol
in an equations, such as H2O+H2O-->H3O+ + OH-, how come we say that there are two moles of h2o, 1 mole of OH etc, why do we calculate moles then if we can just see it in the equation (v. dumb question don't laugh)

I was in the same boat ages ago, then the great EulerFan101 enlightened me;
"You must understand the basics, in order to understand the complexities".

Think of moles as a large number of atoms, or multiple atoms if we are referring to molecules. Instead of scientists saying there is 8473745839 atoms in 300 grams of water, they use moles.

1 mole of an element = =  atoms

And so when you say there is 2 moles of H2O, you are saying that there is molecules of water in the sample.

I hope this helped even the slightest bit, if not, I'll leave it for the pros. xD

[warya]

Haha thanks Cosine, I understand what mole is I'm just confused about how they're represented in an equation. Like for every 1 mole of x, 2 moles of y are produced etc, but then we calculate mole to be 0.57643 or something 

[paper-back]

Haha thanks Cosine, I understand what mole is I'm just confused about how they're represented in an equation. Like for every 1 mole of x, 2 moles of y are produced etc, but then we calculate mole to be 0.57643 or something 

Are you referring to molar ratios?
If you are then the reason we say it's like that is because we are referring to the ratio of one reactant/product to another
E.g.
For the reaction:
2NaOH +H2SO4 -> Na2SO4 + 2H2O
From looking at it, one can say for every 2mol of NaOH reacted 1 mol of H2SO4 reacts. This is the simplified ratio.
Doing the calculations you might find that you have  0.2mol of NaoH reacting with 0.1mol of H2SO4.
However, notice that this is still in the 2:1 molar ratio (so for every 0.2mol of NaoH, 0.1mol of H2SO4 which is equivalent to saying for every 2[mol] of NaoH reacted 1[mol] of H2SO4 reacts)

Does this make sense?

[grannysmith]

Haha thanks Cosine, I understand what mole is I'm just confused about how they're represented in an equation. Like for every 1 mole of x, 2 moles of y are produced etc, but then we calculate mole to be 0.57643 or something 

The equation informs us of the ratio of reactants and products, among other things. We can then use this as a basis for working out, for e.g., the required amount of reactant A given we have x moles of reactant B, to form y moles of product C.

Edit: Paper-back gave a nicer explanation.

[warya]

Are you referring to molar ratios?
If you are then the reason we say it's like that is because we are referring to the ratio of one reactant/product to another
E.g.
For the reaction:
2NaOH +H2SO4 -> Na2SO4 + 2H2O
From looking at it, one can say for every 2mol of NaOH reacted 1 mol of H2SO4 reacts. This is the simplified ratio.
Doing the calculations you might find that you have  0.2mol of NaoH reacting with 0.1mol of H2SO4.
However, notice that this is still in the 2:1 molar ratio (so for every 0.2mol of NaoH, 0.1mol of H2SO4 which is equivalent to saying for every 2[mol] of NaoH reacted 1[mol] of H2SO4 reacts)

Does this make sense?

Yes that makes sense finally! thanks

[TheAspiringDoc]

I'm so confused about moles lol
in an equations, such as H2O+H2O-->H3O+ + OH-, how come we say that there are two moles of h2o, 1 mole of OH etc, why do we calculate moles then if we can just see it in the equation (v. dumb question don't laugh)

Don't even know if what I'm about to say is relevant to your question, but why not
A mole is just another number.
A couple = 2
A few = 3
A dozen = 12
A mole = 6.022 x 10^23

So if we have one dozen h2o molecules, we'll have two dozen hydrogen atoms and one dozen oxygen atoms.

In the same way we have 2 moles of hydrogen atoms in 1 mole of hydrogen molecules.

At least  think that's right.

[warya]

Why is temperature the only change that can affect K? Adding/removing reactants etc also favours a particular reaction (forward or backward) rate to increase, likewise with an increase in temperature, which favours which ever reaction is endothermic. So why is the value of K only defined in terms of temperature, is it just convention?

[qwerty101]

why does stirring the mixture increase rate of reaction?

[IndefatigableLover]

why does stirring the mixture increase rate of reaction?

Stirring keeps reactant particles in motion increasing the chances of collision and increasing the rate of reaction

[lzxnl]

Why is temperature the only change that can affect K? Adding/removing reactants etc also favours a particular reaction (forward or backward) rate to increase, likewise with an increase in temperature, which favours which ever reaction is endothermic. So why is the value of K only defined in terms of temperature, is it just convention?

If you add/remove reactants, you change the reaction quotient, the concentration fraction, and THAT is what causes the position of equilibrium to shift. If you change the temperature, the reaction quotient does not change but the equilibrium constant does, explaining the shift in equilibrium. The value of K is only a function of temperature because...that's all that changes it. The point of an equilibrium constant is so that we have a function of equilibrium concentrations that doesn't change with reactant/product concentrations except with temperature.

[warya]

Why does the pH increase when an acid solution in water is diluted? Does the same thing hold for strong acids?

[lzxnl]

If you're adding pure water, you're adding a solution of pH 7 to a solution of pH less than 7 (if it's acidic at 298 K). It makes sense then that the pH of the original solution will increase.