Question of Chemical Equilibria

Pure hydrogen iodide (HI) is a gas, which at high temperatures, partially dissociates into hydrogen and iodine according to the equation:

2HI (g) < —- > H2(g) + I2(g)

At 500K, the equilibrium constant, Kc, for the dissociation reaction is 6.25 x 10^-3.  Some pure HI is placed into an evacuated 2.0 dm3 glass tube and heated to 500K. In the equilibrium sample, the concentration of I2 is 3.10 x 10^-5 moldm^-3.

ai) Write an expression for the Kc for the dissociation of HI.
ii) What are the concentrations of H2(g) and HI(g) in this equilibrium mixture at 500K?
iii) How many moles of HI  must have been placed into the 2.0dm3 glass tube originally?

b) At 600K, Kc for the dissociation of HI is 1.56×10^-2. Describe and explain the appearance of the contents in this glass as it is heated to 600K.

c) Hydrogen chloride at 500K undergoes no significant dissociation into its elements. Any dissociation of hydrogen chloride can be completely ignored. What is the pressure in a 750cm3 flask containing 8.20 x 10^-2 mol of pure HCl at 500K?

Once you have the answers, you can post it here or email me at cambridgechemistry(NOSPAM)@yahoo.com.sg

Enjoy!

Question on Thermochemistry

a) Using zinc chloride, ZnCl2, as an example, define lattice energy.

b) Using the data given below as well as relevant data from the Data Booklet, construct a Born-Haber energy level diagram for zinc chloride.
Electron affinity of chlorine = -346 kJmol-1
Enthalpy change of atomization of zinc = +131 kJmol-1
Enthalpy change of formation of ZnCl2(s) = -451 kJmol-1

Use the energy level diagram to calculate the lattice energy of zinc chloride

c) How would you expect the melting point of zinc chloride to compare with that of zinc bromide?
Explain your reasoning.

If you have the answer, you can post or email me at cambridgechemistry(NOSPAM)@yahoo.com.sg

Mole concept – for the Year Ones

Today I started a JC 1 class revising secondary 4 work on mole concept and calculations.

For the stronger Chemistry students, this chapter might seem easy to handle, which is a good thing. However, do not be complacent, and always be alert with calculations.

It is easy to make careless mistakes and overlook the number of  zeroes and decimal place.

Also, the mole reacting ratio can be rather challenging at times.

If you need further help ,please feel free to post a comment here for all to learn or email me at cambridgechemistry[at]yahoo[dot]com[dot]sg

Merry Xmas and a…..

‘Tis the season to be jolly, falalalalalala..
MERRY CHRISTMAS BOYS AND GIRLS

To those who have graduated, congratulations! You have made it!

For those embarking the journey in Chemistry or in the midst, hope you are enjoying the journey thus far, or at least have a better appreciation of Chemistry.

It is always good to start early, so remember to START DOING YOUR SCHOOL HOLIDAY HOMEWORK! hahaha.

Let me know if you need any clarification or assistance.

Titration between acids / bases

Equivalence point or end-point (when equivalence volume is added such that no more acids nor bases are left, only left with salt)

SALT can be classified as
1) NEUTRAL (usually between strong acids and strong bases. pH = 7, due to hydration only)
2) ACIDIC (usually between strong acids and weak bases. pH < 7 due to conjugate acid of weak base)
3) BASIC  (usually between weak acids and strong bases. pH > 7 due to conjugate base of weak acid) 

If a base is weak, i.e. undergoes partial ionisation; then its conjugate acid also undergoes partial ionisation. Therefore, a weak base has its own Kb value, and thus its conjugate acid will have its own Ka value ( where Ka = Kw/Kb)

Reverse is true for weak acids and its conjugate bases.

In general, for any substance that behaves as a weak acid (recall, Lowry-Brosted theory states that acid donates H+), to calculate pH, 
- [H+] = sq. root (Ka.[acid])
-pH = -lg [H+]

In general, for any substance that behaves as a weak base (recall, Lowry-Brosted theory states that base accepts H+), to calculate pH, 
- [OH-] = sq. root (Kb.[base])
-pOH = -lg [OH-]
-pH = 14 – pOH

Thus a salt can behave as a weak acid or a weak base depending on whether it can accepts or donates H+

Hope this helps !

Organic tips

Just a quick tip: 

This is for the electrophilic substitution/ bromination for Phenols and Phenylamines.

1) Br2 (g) in CCl4 —-> mono-substituted phenol/phenylamine
2) Br2 (l)              —–> di-substituted phenol/phenylamine
3) Br2 (ag)           ——> tri-substituted phenol/phenylamine

Another quick revision tip:

Metal-acid reaction is also a reduction/oxidation process

Please remember that in Nucleophilic addition, a racemic mixture is most likely to be formed, unless the reaction is performed naturally by enzymes in living organisms

Quick Revision Test for Ionic Equilibria

Question: Methylamine, CH₃NH₂, is a weak base with pK_b value of 3.36 in water.
15.00cm³ of 0.500moldm^-3 of methylamine solution was titrated against 0.30moldm^-3 of aqueous hydrochloric acid. Sketch the acid-base titration graph.

All the ‘miracles’ in the world to help you during exams

Sorry peeps,

I have not been writing new posts, as I am busy with the exam preparation for the students. Lots to catch up, follow up and to mark.

But here’s a tip for the examination:-

Say you have done all the preparation you needed, ( don’t worry, I believe you have if you have been spending at 6 hours each week for Chemistry), before you enter the examination hall, give a big smile to yourself, breathe out hard all the worries and butterflies you have, walk towards your table, sit down and relaxxx….

I cannot tell you how important the state of mind is for any examination or tests in this world.

Once you learn how to control that emotion and that mind of yours, you will see the wonders it can do to you in life!!

Some will take Essence of Chicken, others might take Cod Liver’s Oil or supplements like Gingko pills to boost the brain. AND it works!

In my school days, I take Essence of Chicken, now I am giving my students Gingko pills. Bear in mind though, Gingko is not a ‘miracle’ pill — one still needs to put in hard work for results. Gingko is just meant to keep the mind alert and not have the ‘mental block’.

(Contact me if you need more information about the supplements)

Gingko

And Yes! Proper regular sleeping hours/pattern and wholesome nutrition are all part of the gameplan as well. Eat well, and not MacDonald’s all the time will help boost the memory and brain activity too.

If you need to, go do a 20-30min run every other day. It improves blood circulation and so more blood goes to the brain.

In this period, the brain is working so hard, it is under oxidative stress, many free radicals are formed- you may not realize it but it is happening. Take fruits with high antioxidant levels like blueberry and grapes. Google and you will know more.

So good luck everyone and share with me what are some of your exam techniques.

As always, if you have any questions, post them here or email me.

Adieus. Yuyan

Kinetics – graphical work

After teaching for more than 5 years, I always feel that kinetics is one of the easiest chapters to score, provided one does not make careless mistakes!

Remember fundamentals are important!!

The basics of Kinetics: remember…
1) Rate = change of concentration / time
2) Rate = k[A]^n (where A is reactants, n is order of reaction)
3) units of rate = concentration units /time (eg, moldm-3s-1 , molm-3min-1 etc)
4) orders of reaction is an integer with values 0,1 and 2 only

Graph work:

ZERO ORDER REACTIONS
- Concentration vs time graph = downwards straight line. Gradient gives you rate. (from the equation Rate = change of concentration / time)
- Rate vs Concentration = flat straight line. Gradient gives you k. (from the equation, Rate = k[A]^n)
- Essentially, rate is INDEPENDENT of concentration, rate = k

FIRST ORDER REACTIONS
- Concentration vs time graph = downwards parabolic curve. Gradient gives you rate. (from the equation Rate = change of concentration / time)
- Rate vs Concentration = increasing constant straight line. Gradient gives you k. (from the equation, Rate = k[A]^n)
- Essentially, rate is DIRECTLY PROPORTIONAL to concentration
- Rate constant, k, is INDEPENDENT of concentration
- Concentration/time graph has a half-life that is constant, hence, we have the equation, t 1/2 = ln2 /k

SECOND ORDER REACTIONS
- Concentration vs time graph = downwards curve. Gradient gives you rate. (from the equation Rate = change of concentration / time)
- Rate vs Concentration = upwards curve. Gradient gives you k. (from the equation, Rate = k[A]^n)
- Essentially, rate is NOT PROPORTIONAL to concentration
- Rate constant, k, is DEPENDENT of concentration
- Concentration/time graph has a half-life that is NOT constant, dependent on initial [A]
- Half life t 1/2 = 1/k[A]

TIP:
When finding half-life, note the y-axis.

If it is concentration of REACTANTS against time,
determine half-life at 1/2 [A], 1/4[A] and 1/8[A], where [A] refers to the INITIAL concentration of reactant.

If it is the concentration of PRODUCTS against time,
determine half-life at 7/8 [B], 3/4 [B] and 1/2[B], where [B] refers to the MAXIMUM concentration of products formed (if it can be possibly found from the question)

The trick with Chemical Energetics

Chemical Energetics is the study of energy changes in chemical reactions. Energy cannot be created nor destroyed; it can only be converted from one form to another.

Change : You can have either negative energy change ( net heat given off = exothermic), no change ( no net heat given off or taken in = equilibrium) or positive energy change (net heat taken in = endothermic)

Enthalpy Change ∆H is the net heat change of a given reaction.

The trick with Chemical Energetics is to be able to define exactly the respective enthalpy changes AND be able to convert the definition into equations with STATE SYMBOLS. (state symbols are EXTREMELY IMPORTANT and essential to include in questions and answers).

What are the definitions of ∆H that you have to know:

1) ∆H of formation
2) ∆H of reaction
3) ∆H of combustion
4) ∆H of hydration
5) ∆H of atomisation
6) ∆H of vaporization
7) ∆H of fusion
∆H of sublimation
9) ∆H of solution
10) Lattice Energy
11) Ionization energy
12) Electron Affinity

13) Define Hess’ Law

Born-Haber cycle is just a method that relates to lattice energy of ionic compounds.

So the above 12 definitions, are you able to write them into equations?