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

Strong and Weak Acids have Different Reaction Rates

Magnesium will react with an acid and make hydrogen gas. The more concentrated the acid, the faster the magnesium will react and you will see bubbles of hydrogen being produced more quickly.

A strong acid and a weak acid of the same concentration will react at different rates with the same metal.

You can see the difference between a strong acid and a weak acid of the same concentration by looking at the reaction with magnesium. The strong acid reacts faster and you see more bubbles of hydrogen. This is because the strong acid has more hydrogen ions in the solution even though it is at the same concentration as the weak acid. A piece of magnesium the same size should be used in both acids for the comparison to be fair.

The weak acid will produce the same amount of hydrogen as the strong acid
from the same amount of magnesium but the weak acid will take longer to do it.

strong-acid-weak-acid

Examiner’s Tip

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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”