This question was asked for a Secondary 4 prelim paper, which I find it interesting to share with all Chemistry students.

The making of a mini-volcano.

Did you know that ammonium dichromate, (NH4)2Cr2O7 when exposed to heat, will decompose to form Nitrogen gas, Water Vapor and a green compound Chromium III oxide, Cr2O3.

This reaction is highly exothermic, and gives off orange sparks, throws the green Cr2O3 upwards, mimicking the volcanic-effect.

!!!  Hazards  !!! DO NOT TRY THIS AT HOME. Ammonium Dichromate causes cancer!

Chromium salts are irritating to the skin and respiratory tracts, and are also carcinogenic.    The best way to do this demonstration is on a large piece of aluminum foil, which can then be used to wrap up the chromium salts produced.

Videos to watch:

A very good video (1min 30 sec) to explain how crude oil is separated into its separate fractions by fractional distillation.

Many students wonder what “heat under reflux” is like in the laboratory.

It is something like distillation but it is performed for long hours, sometimes overnight to ensure all the reactants are thoroughly boiled.

Watch this short video to have an idea of what reflux conditions are like — and you will understand why reflux are often considered drastic conditions.

A detailed demonstration (2min):

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!

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

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

‘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.

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 !

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

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.