Friday, February 20, 2009

Mock Exam Paper 1 and Paper 2 2008 (Aug)

Mid-course exam (Trial exam for AS) is coming soon. You can start practising some of the trial exam papers. Answers for the papers will only be published one week later.

Paper 1 2008

Paper 2 2008

Wednesday, February 11, 2009

Acidosis and alkalosis: good to know more

Remember, I mentioned two pH related diseases acidosis and alkalosis in our lesson when I taught buffer solutions and explained why it’s important for our body to maintain a fairly constant pH. Here’s more information about these diseases. This information is just for your general knowledge.

A healthy person has a blood pH close to 7.4.
Acidosis is a state of excess acidity in the blood and occurs when blood pH falls below 7.35 Alkalosis is a state of excess alkalinity when blood pH is over 7.45.
When the blood pH falls below 7.0, it is diagnosed as severe acidosis and in severe alkalosis it may rise above 7.5. Acidosis is more common than alkalosis.

pH in our blood or fluids is controlled by three following mechanisms
1. Blood's protein that's acts as a buffer that limits changes in acidity.
2. Lung, which give off carbon dioxide to prevent accumulation of carbonic acid.
3. Kidneys which manufacture hydrogen carbonate ions and retain acids or bases.

Causes of Acidosis:
1. Kidney failure reduces the efficiency to regulate acidity.
2. Starvation produces excessive acid due to metabolism of large amounts of body fat which produce lactic acid
3. Increased protein intake – eat too much meat
4. Ingestion of large amounts of acidic substances eg pickled fruits
5. Diabetes

Causes of Alkalosis:
1. Severe diarrhea may cause excessive loss of basic fluids
2. Loss of carbonic acid because of fast or rapid breathing.
3. Excessive vomiting causing loss of hydrochloric acid (stomach content)
4. Excessive intake of antacids to relieve acid in the stomach.

Friday, February 6, 2009

Chemistry resources

Recently, some students asked me to recommend them some good chemistry websites. For me, I usually look at these two websites.

LearnNet

Chemguide

Please take a look at these sites .
Do you find them useful? Please comment.

Monday, February 2, 2009

Dare to take this challenge

Take this challenge and download the past year questions for the Chemistry Olympiad UK. Give yourself a Chemistry test. Reminder: Look at the answers only after you have tried the questions.
Chemistry Olympiad 2008 Examination

Answer

Important AS Definitions Part II

Activation energy is the minimum energy required by colliding particles to start off a reaction.


A catalyst is a substance which changes the rate of a chemical reaction without itself undergoing any chemical change at the end of the reaction.

Homogenous catalyst is a catalyst in which the physical state is in the same phase as the reactants.

Heterogenous catalyst is a catalyst in which the physical state is in the different phase as the reactants.

Increase the concentration of reactants will increase the rate since there will be more molecules per unit volume resulting in an increase in the collision frequency between the reactants. This leads to higher number of effective collisions.

Increase in the temperature will increase the rate since at higher temperature the particles gain more kinetic energy resulting an increase in collision frequency at higher temperature. In addition, more molecules have kinetic energy greater than or equal to the activation energy. Consequently, the number of effective collisions increases.

A catalyst operates by providing an alternative route of lower activation energy for the reaction.

Geometric isomers are compounds with different spatial arrangement due to the restriction in the rotation about the double bond in an alkene. In geometric isomers, two different groups at each end of the double bond can either be on the same side (cis) or on opposite sides (trans)

Optical isomers are mirror images that have the same geometric and structural formula, but cannot be superimposed onto one another.

Cracking is a process to break long alkane into a shorter alkane and an alkene. Hydrogen may also be produced. Thermal cracking requires temperature between 400°C to 900°C and high pressure whereas catalytic cracking requires a zeolite catalyst and a temperature of about 450°C. The purpose of cracking is to form extra gasoline and to produce alkenes with higher commercial value

Homolytic fission is the breaking a covalent bond in which one electron goes to each atom to form free radicals.

Heterolytic fission is the breaking a covalent bond in which one atom takes both electrons forming negative and positive ions.

A free radical is a highly reactive atom or molecule with an unpaired electron.

An electrophile is an electron-deficient species that can form a new covalent bond by accepting an electron pair provided by the carbon compound. Example : positive ion such as hydrogen ion or nitronium ion and a polar molecule with a d+ charge

A nucleophile is an electron-rich species with a lone pair of electrons that can form a new covalent bond by donating a pair of electrons to the electron deficient carbon atom. Example : negative ions with at least a lone pair.