Organic Chemistry: Household Plastics

a-guide-to-common-plastics

All plastics we use or encounter are substances called polymers. These polymers are themselves formed from chemical entities called monomers. Monomers can be a range of differing compounds, but specific polymers will generally contain monomers of only one or two types. The polymers are formed by joining together many monomers, like a long chain of paperclips, to form one long molecule.

Let’s look at a simple example to make this clearer. Polyethene is a plastic, or polymer, that is widely used in plastic shopping bags, plastic films, and to make some toys. It’s formed from many monomers of a a small molecule, ethene. At high temperature and pressure, and in the presence of oxygen as a catalyst, one of the two bonds between the two carbons in the molecule can be made to break, and allow them to form linking bonds to other ethene molecules. In the diagram below, n is a large number – each resultant polyethene chain from this process can be formed from as many as 20,000 individual ethene molecules.

addition-polymerisation

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How paper chromatography works

How paper chromatography works

Paper chromatography depends on how the substances in a mixture interact with the chromatography paper and the solvent.

Capture

The more soluble a substance is in the solvent, the further it will travel up the chromatography paper.

 

Sec 3 Chemistry Workshop is back this June Holidays!

This is an intensive course where pupils will learn and understand the MAIN concepts of Chemistry, in an easy and interesting way 🙂 

Option A: 12 & 13 Jun (Mon & Tue) 3 pm to 6.30 pm

Option B: 15 & 16 Jun (Thu & Fri) 3 pm to 6.30 pm

Option C: 19 & 20 Jun (Mon & Tue) 10 am to 1.30 pm

Option D: 22 & 23 Jun (Thu & Fri) 10 am to 1.30 pm

Fee per option: $280

Get 10% E.B.D. for a 2-session payment made by 20 May 2017 (Sat)

 

There will be a review of past topics to ensure pupils have a strong foundation before moving forward. They will also have a headstart on new topics (very important to get this concept — Stoichiometry and The Mole Concept, right as early as possible) for the new school term, with extra useful information being taught for better understanding of the Formulation. Pupils will also be coached to work on some O Level problems. This program is also suitable for Sec. 4 pupils who want to have a thorough revision of their Sec. 3 Chemistry topics before the start of the hectic Semester 2, moving on quickly towards the O Level exams.

List of Modules to be covered in this workshop:

  1. Atomic structure (1 session)
  2. Chemical Bonding & Structures of Substances (2 sessions)
  3. Kinetic Particle Theory (1 session)
  4. Compounds, Mixtures & Separation Techniques (2 sessions)
  5. Chemical Formula, Stoichiometry & the Mole Concept (2 sessions)

Pupils who join this program will be entitled to these privileges:

One $30 Discount Voucher for Mentoring Sessions in Semester 2 of 2017

WhatsApp us at 8121 6628 or email us at admin@matharena.com.sg to to register your interest.

Sec 4 Chemistry Workshop: Prepare for Prelims this June Holidays!

This is an intensive course where pupils will review and apply all concepts learned onto exam questions to prepare themselves for preliminary and O level examinations. 

Option A: 12 & 13 Jun (Mon & Tue) 10 am to 1.30 pm

Option B: 15 & 16 Jun (Thu & Fri) 10 am to 1.30 pm

Option C: 19 & 20 Jun (Mon & Tue) 3 pm to 6.30 pm

Option D: 22 & 23 Jun (Thu & Fri) 3 pm to 6.30 pm

Fee per option: $280

Get 10% E.B.D. for a 2-session payment made by 20 May 2017 (Sat)

 

There will be a review of core topics learned in schools at Sec 4 level to ensure pupils have a strong foundation before applying concepts onto past year preliminary exam papers.

List of Modules to be covered in this workshop:

  1. Stoichiometry and The Mole Concept (1 session)
  2. Metals (1 session)
  3. Organic Chemistry (2 sessions)
  4. Oxidation and Reduction (1 session)
  5. Electrolysis (1 session)
  6. Acids, Bases and Salts (1 session)
  7. Energy Changes & Rate of Reactions (1 session)

Pupils who join this program will be entitled to these privileges:

One $30 Discount Voucher for Mentoring Sessions in Semester 2 of 2017

WhatsApp us at 8121 6628 or email us at admin@matharena.com.sg to to register your interest.

Limiting Reactants – A Worked Example

Zinc reacts with dilute hydrochloric acid according to the equation:

Zn (s) + 2HCl (aq)      ->       ZnCl2 (aq) + H2 (g)

If 0.05 mole of zinc was added to 0.075 mole of hydrochloric acid,

(a)        identify the limiting reactant;

(b)       calculate the amount (in moles) of the excess reactant which remained unreacted;

(b)       calculate the amount (in moles) of zinc chloride formed.

Solution:

Continue reading “Limiting Reactants – A Worked Example”

Misconception : Confusing the Strength of Acids with the Concentration of Acids.

Recall this : Acid solutions contain hydrogen ions. The higher the concentration of hydrogen ions, the lower the pH. Hydrochloric acid is a strong acid and ethanoic acid is a weak acid. Strong acids are fully ionised but weak acids are only partly ionised in solution.


Dilute Acid  is not the same as Weak Acid

Concentrated Acid is not the same as Strong Acid


Strength of Acids refers to the extent of dissociation (or ionisation) of an acid. We will see words like “STRONG” and “WEAK.”

Concentration of Acids refers to how much of an acid (the solute) is dissolved in the solution. We will see words like “CONCENTRATED” and “DILUTE”

Concentration is a concept covered when you study Mole Concepts or Mole Calculations.

The concentration of a solution is given by the amount of a solute dissolved in a unit volume of the solutionConcentration of an acid can be changed. You can decrease the concentration of an acid solution by adding more solvent to it. You can also increase the concentration of an acid solution by adding more solute to it.

How can a weak acid solution have the same pH as a strong acid solution?

It is possible. The weak acid would have to be more concentrated. pH is determined by the concentration of hydrogen ions in solution.

By the way,  you must know this fundamental question :

How is pH related to a solution’s acidity?

If the pH of a solution is less than 7, the solution is called acidic; if the pH is about 7, the solution is neutral; if the pH is greater than 7, the solution is is called basic. In an acidic solution, then, the concentration of hydrogen ions is greater than the concentration of hydroxide ions (commonly tested in MCQ)