May 07, Friday

I wasn't late today, so I volunteered to do the blog. :) :)


First, we did the questions 3 & 4 in the Solubility Equilibria Assignment #1. Obtain the answers from Ms. K. Note: in letters b and d you have to change the unit to moles/liter before you can solve for the Ksp.


We also went over the pages 31-35 on the Chemical Equilibrium booklet. In that, we learned a new lesson, determining ion concentration from Ksp. For all of you who hates ICE, you'll life will be easier. It's no different from the ICE method we've been doing, except that:

• the initial concentration of the ions (products), before dissolving, is zero.

• reaction is consists of a saturated solution as reactants and ions as products so you get to IGNORE the saturated solution, the solid one, in your ice table.

Example: The Ksp of magnesium hydroxide is 8.9 x 10 ⁻12. Determine the solubility of Mg(OH)₂.

• Mg(OH)₂(s) <=> Mg2+ (aq) + 2OH⁻(aq)

I...............___.................0................0
C.............___................+x................+2x
E..............___...............+x................+2x


To solve for x, use the formula, Ksp = [products] and substitute the values. We'll find that the solubility of Mg(OH)₂ is 1.3 x 10-4.


That's determining ion concentration using Ksp in PURE WATER. Another thing we learned is determining ion concentration with an ION IN COMMON WITH THE COMPOUND.

Example: The Ksp of magnesium hydroxide is 8.9 x 10 ⁻12. What is the solubility of magnesium hydroxide in a 0.10 M solution of NaOH.

Step 1: Determine the concentration of the common ion, OH ⁻.

NaOH(s) <=> Na+(aq) + OH -(aq)
[OH⁻] = [NaOH] = 0.10 M

Step 2: Set up an ICE table for Mg(OH )₂.

• Mg(OH)₂(s) <=> Mg2+ (aq) + 2OH⁻(aq)

I...............___.................0................0
C.............___................+x................+2x
E..............___...............+x................0.10 +2x

***You have to include the concentration of OH - from NaOH(s) in the table because when we dissolve Mg(OH)₂ in the solution, OH ions are present. According to Le Chatelier's Principle, adding more OH ions to a saturated solution would increase the overall concentration of the products shifting the equilibrium to the left. This would result in more solid formed and a decrease solubility.***


Step 3: Substitute values into the solubility product expression.

Ksp = [Mg2+][OH-]2

8.9 x 10⁻12= (x)(0.10 + 2x)2

**at this point, you can see that you'll have a quadratic equation if you solve further, so, ignore the x of the ion in common, OH. Ms. K said that it's too tiny to matter.**

8.9 x 10⁻12= (x)(0.10)2

8.9 x 10⁻12= 0.01x

8.9 x 10⁻10M = x

Let's check if the value makes sense. The solubility of Mg(OH)₂ in pure water is 1.3 x 10-4 M and in a 0.10 solution of NaOH is 8.9 x 10⁻10M. The solubility decreased, just as we predicted.



I dont know if I have to pick for someone to do the next scribe... if so, i'll pick Ericka! :) :)

April 05 and April 06 2010

Hey there! I am scribing for two days:


One is the day after the Spring Break, Monday, April 05, 2010.

In class, we continued our studies on Chemical Kinetics. We answered the assignment assigned to us before Spring Break which was to do the questions on page 10 of our booklet in Chemical Kinetics. You have to read the previous pages to understand the answers. Here are they: (note: the answers are my own words, Ms. K was too lazy to write hers on the board...)(oh yeah, it said that you have to show diagrams.. but I don't know how to show mine here so figure that one yourself. :)

1.) The increase of concentration of the reactants the faster the reaction to occur therefore, increasing the reaction rate.

2.) a.) Increase in temperature causes the molecules to move faster, therefore more collision. More collisions/reactions --> increase in reaction rate. Decrease in temperature does the opposite --> slower reaction.
b.) The larger or more exposed the particle means more surface area. More surface area means greater chance of collisions --> increase in reaction rate. Less surface area, you know, slower reaction.
c.) More pressure means less travelling space. Less travelling space means faster collisions among the molecules --> increase in reaction rate. Less pressure means more travelling space and the collisions won't occur as fast.

3.) Catalysts provides an easier path for the reaction to proceed. It lowers the activation energy of a reaction, so, more particles are able to surpass the activation energy required. These means more collisions and an increase in the reaction rate.

4.) a. ) i. is faster because in a solution, reaction will occur more quickly to the ions because they are free to move.
b.) ii is faster because Ag is hungrier for elections than Cu. This brings us back to our previous unit, Atomic Structure. Ag has a higher Electronegativity value than Cu, so it can attract more electrons than Cu... I think.
c.) i is faster because all elements in the solution are ions.
d.) ii is faster because all of the elements are in the same state, gaseous form.

By the way, we only spent half of the class; we were called down to the gym for a meeting with the principal.


The second day is today, April 06, 2010.

We did a lab... and that's pretty much it. It was about how elements involved, concentration of the elements, temperature, a catalyst and particle size affect the reaction rate. We only did two parts of the lab sheet, so expect that we will do more of it tomorrow.


Yay! Jeck, it's your turn. :)