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Equlibrium constant (1 Viewer)
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kawaiipotato
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What do you have trouble with specifically?
rand_althor
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By "these types of questions" do you mean calculations involving equilibrium constants? If so, have a read through this: http://chemwiki.ucdavis.edu/Physical_Chemistry/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant. You don't need to know about the equilibrium constant of pressure.
sharoooooo
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Draw an ICE table to solve for the equilibrium constant.
Magnoliatient
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Make sure that you only include gaseous or aqueous species (Liquids and solids do not count)
Make sure that when you use the ICE table, you are working with concentrations, so if the question gives you the number of moles, you first convert it to concentration C=n/v
Make sure that when you use the ICE table, you are working with concentrations, so if the question gives you the number of moles, you first convert it to concentration C=n/v
I thought liquids counted and it was just solids that don't count.
MaccaFacta
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Calculating an equilibrium constant (K) is a tricky thing, as there are a bunch of different equilibria which are covered in HSC Chemistry and they need to be looked at in slightly different ways.
The usual K question in a HSC Chemistry exam involves a mixture of gases at a high temperature (low temperature gases can condense and that really makes things complicated). The ionisation constant for water (Kw = 1 x 10^-14) is a possible source of questions but I can't recall seeing any on it in recent years. "K" questions might also be asked about slightly soluble substances e.g. calcium hydroxide (limewater), but this also (potentially) involves some complicated chemistry. Solubility equilibria is the main way that the "solids that don't count" idea applies.
Do liquids count? Tough one. Suppose you had a mixture of four liquids: an alcohol, a carboxylic acid, their ester and water then you might possibly be able to calculate an equilibrium constant. But I would view this type of question as being well beyond HSC level unless some really big hints were dropped. The place where a liquid definitely does NOT count is in reactions in aqueous solution where water is the solvent and also a reactant or a product. In this case, the change in the concentration of the water is effectively zero, which means that water can be left out of the K expression. It is a bit more complicated than that, involving things called "activities", but that is the guts of the reason why.
If you want to read some more about activities Wiki isn't bad... https://en.wikipedia.org/wiki/Thermodynamic_activity
Hope this helps...
The usual K question in a HSC Chemistry exam involves a mixture of gases at a high temperature (low temperature gases can condense and that really makes things complicated). The ionisation constant for water (Kw = 1 x 10^-14) is a possible source of questions but I can't recall seeing any on it in recent years. "K" questions might also be asked about slightly soluble substances e.g. calcium hydroxide (limewater), but this also (potentially) involves some complicated chemistry. Solubility equilibria is the main way that the "solids that don't count" idea applies.
Do liquids count? Tough one. Suppose you had a mixture of four liquids: an alcohol, a carboxylic acid, their ester and water then you might possibly be able to calculate an equilibrium constant. But I would view this type of question as being well beyond HSC level unless some really big hints were dropped. The place where a liquid definitely does NOT count is in reactions in aqueous solution where water is the solvent and also a reactant or a product. In this case, the change in the concentration of the water is effectively zero, which means that water can be left out of the K expression. It is a bit more complicated than that, involving things called "activities", but that is the guts of the reason why.
If you want to read some more about activities Wiki isn't bad... https://en.wikipedia.org/wiki/Thermodynamic_activity
Hope this helps...