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https://www.nobraintoosmall.co.nz/html/senior_chemistry/NCEA3_chemistry_aqueous.html
https://www.nzqa.govt.nz/ncea/assessment/view-detailed.do?standardNumber=91392
External Exam
Friday 18th November, 9:30am
5 literacy and numeracy Credits
The following topic will be covered in the last 6 weeks of term 3. A reminder that you are only learning this external if you are intending to specifically do a chemistry course at university next year. If you are unsure if this applies to you then please talk to me.
Learn
Learn
Level 2 recap
Level 2 recap
Le Chateliers Principle
Le Chateliers Principle
States that whenever a change is imposed on a chemical system. The system do react in a way to minimize the change. For example if you increase the concentration of the reactants, then the forward reaction will be favoured so to use the reactant up.
Equilibrium constants (K)
Equilibrium constants (K)
This value will tell you where the equilibrium lies. It is calculated by [products] / [reactants]. And a reminder to never include solids or water in equilibrium constants. If Kc is large then the products are favoured. If it's small then the reactants are favoured.
Formulae from Yr12
pH= - log [H3O+]
[H3O+]= 10^ -pH
Can use pOH and [OH-]
Kw= [OH-] x [H3O+]= 1x10^-14
The Solubility Product, Ks
(Pg 242- 248)The Solubility Product, Ks
Just like with any equilibrium this is calculated by the concentration of products divided by reactants. However because we are talking about a solid dissolving into the ions its made of we will never include the reactant in the equation as it is a solid.
If Ks is large then we know the compound is very soluble as the concentration of dissociated ions is very high. Reverse for a low Ks.
Manipulation of Ks
Manipulation of Ks
The solubility of the ions will be in equal proportions. This means if we know the solubility (s) of one ion that is equal to the other.
For AB type equations (not coefficients) This can be simplified down to Ks= s^2. Or s= square root(Ks).
Things get a little bit more complicated for AB2 equations.
They explain it in the SciPad and I'll explain it in class but the main thing to remember is that for these equations, Ks= 4s^3. Or reverse that equation to work out s.
Dilution factor
Dilution factor
When combining two solutions the concentrations of the ions involved will change. The way to calculate the new concentration is via the following formula:
Common Ion
(pg 250-253)Common Ion
Following on from the idea of dilution factor we need to consider the effect common ions will have on the system (Le Chateliers). For instance if the the common ion being analyzed is a product then the equilibrium will shift to the left and favour the reactants.
The Ionic Product, Q
(pg254-257)The Ionic Product, Q
This is calculating the exact same thing as the solubility product. However,- where Ks only applies to saturated solutions, Q can be applied to solutions of any concentrations. Which allows us to make the following predictions with regard to whether we think a precipitate will form or not.
Effect of pH on solubility
(pg 258- 261)Effect of pH on solubility
Dissolving a solid under acidic or basic conditions can greatly affect the solubility. If the dissociated ions present might react with the acid or base then the equilibrium will shift to the right to compensate for this and try to make more of it- hence dissolving more of the solid.
Mainly metals which may form complexes with basic substance or hydroxides/ carbonates which react with acid to form water.
A3 revision sheets
A3 revision sheets
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