My 3 posts for grading

Mdm Riza, my posts are 24 January, 1 February and 1 March

1st March 2011

Today we started on a new topic, chemical changes.

Chemical changes:
- permanent
- new substances formed

- irreversable

- involves chemical change
- heat/light energy may be taken in or given out

- properties of starting reactants differ from products
- for example, cooking, burning of charcoal and steel rusting

Physical change:

- temporary
- no new substances formed
- reversable

- does not involve chemical change

- no/little light or heat energy is given in or taken out

- properties of substances do not change
- for example, changes in state and dissolving salt

Chemical reactions:
- changing reactants into products
- provides identies of reactants and products
- provides possible reaction possibilities
- represented using chemical equations

- can be written using word or chemical equation

Balanced chemical equation:

1) write down chemical formulae of reactants and products

2) check atoms of each elements on both sides of the reaction

3) balance the chemical equation by placing numbers in front of the element in the question

- for example, N3+H2=NH3
N2+3H2=2NH3

Chemical equations involving heat

- combustion

- combination

- thermal decomposition

Combustion:

- burning

- substance combine with oxygen in the air
- products are heat and light

- octane+oxygen+heat=carbon dioxide+water

Combnation:
- two or more reactants combine chemically to form a new product

- sodium+chlorine=sodium chloride

Thermal decompositon:

- a substance breaks down into two or more simpler substances(decomposition)
- a substance breaks down by the effect of heat(thermal decomposition

-sugar+heat=carbon+water vapour

chemical reactions involving light:

- photosynthesis

- photography

Photosynthesis:

- green plants in the presence of chlorophyll and light energy manufacture carbohydrates(glucose) from carbon dioxide and water

- carbon dioxide+water+light energy+chlorophyll=glucose+water

Photography:

- photographic films are coated with silver bromide

- when films are exposed to light, silver bromide decomposes to form silver particles

- silver bromide+light energy=silver+bromine

Electrolysis;

- passing electric currents may change the propeties of the substance temporarily or permanently

- some substances decomposes into simpler ones when electricity is passed through

- electroplatimg

Chemical reactions involving matter:
- digestion

- neutralisation
- rusting

Digestion:

- involves chemical reactions caused by mixing

- when saliva and starch are mixed, the enzymes in the saliva break down the food

Neutralisation

- when an acid and alkali is mixed, salt and water is formed

- hydrochloric acid+sodium hydroxide=sodium chloride+water

Rusting
- iron comes in contact with oxygen in the air and water

- combination

- iron+oxygen+water=hydrated iron(III)oxide

1st February 2011

Today, we started on the topic atoms and molecules and finished the chapter on the next subsequent lessons.

Atom:

An atom is a basic building block of life. an element is in its simplest form and an atom is the smallest particle of an element. Atoms belonging to the same element would be the same while atoms of other elements would be different. Atoms are made up of three sub-atomic particles called the electrons, the protons and the neutrons.

Sub atomic particles:
protons:
- positively charged(+1)

- has the mass of 1 unit

- is found in the nucleus.

neutron:
- electrically neutral

- has the mass of 1 unit

- is found in the nucleus

electrons:
- negatively charged(-1)
- has the mass of 1/2000 unit
- it orbits around the nucleus in electron shells.

Mass number and proton/atomic number:
The proton/atomic number can be found on the bottom of a chemical symbol in a periodic table while the mass number is the one at the top of the symbol. The chemical symbol is represented by the letters.

The atomic/proton number is the number of protons in a atom. Since all atoms are neutral, the number of protons in an atom is equal to the number of electrons.

The mass number is the the total number of protons and neutrons in an atom. It can also be called the nucleon number. So to find out the number of neutrons there are in an atom, simply subtract the proton number from the mass number.

Electronic configuration:
The electrons in an atom orbit around the nucleus in regions known as the electron shell. The first electron shell can only hold 2 electrons while the subsequent shells can hold eight electrons each. Arrangement of the electrons is called an atoms electronic configuration or electronic structure. The arrangement of electrons determine the chemical properties of the atom.

For example, Lithium has so its electronuc configuration is 2.1. Magnesium has 12 electrons so its electronic configuration is 2.8.2.

ions:
- An ion is a changed particle formed from an atom or a group of atoms by the loss or gain of electrons.

- An ion is formed when an atom loses or gains electrons.

- The number of protons will be different from the number electrons.

- If the atom loses electrons, there are more protons than electrons so the atom will be positively charged.

- If the atom gains electrons, there are less protons than electrons so the atom will be negatively charged.
- Ions can only be formed through chemical reactions.

- Only electrons are able to move in or out of the atom as the protons are packed in the nucleus while the electrons are orbiting around the nucleus.

- Metals loses electrons while non-metals gains electrons.
- Example 1: a megnesium atom has 12 protons and 12 electrons and its electronic configuration is 2.8.2, so it will lose 2 electrons to become a positively-charged ion. So a magnesium ion has 12 protons and 10 electrons.

- Example 2: a chlorine atom has 17 protons and 17 electrons and its electronic configuration is 2.8.7, so it will gain 1 electron to become a negatively-charged ion. So a chloride ion has 17 protons and 18 electrons.

- Positively-charged ions are known as cations while negatively-charged ions are known as anions.

Noble gases:
- Noble gases are elements like helium, neon, argon, krypton, xenon and radon.

- They are found at the most right column.

- The atoms of noble gases are unreactive or chemically stable.

- They do not cmbine witn other atoms or form compounds.

- Exist as an individual(monatonic)

- The electronic configuration of a noble gas can exist as an octet or duplet.

- A duplet contains 2 electrons while a octet has electrons 8 electrons on the outer ring.

Molecules:

- Some atoms combine with others to form molecules
- These atoms often exist as a molecule rather than a single atom.

- A molecule is made up of 2 or more atoms chemically combined together.

Molecules of elements:

- Usually consist of a fixed number of one type atoms chemically combined together.

- Many non-metals exist as molecules.

- Example: oxygen, hydrogen and nitrogen.

- Molecules of elements are held by single, double or triple bonds.
- For example, hydrogen is held by single bond, oxygen is held by double bond and nitrogen is held by triple bond.
- Hydrogen, oxygen and nitrogen have 2 atoms of the same type, this is known as a diatomic molecules as it contains 2 atoms.

- All elements from group 7 like chlorine, bromine and iodine will form diatomic molecules.

Molecules of compounds:

- It is formed when different types of atoms combine together.

- They contain a fixed number of different types of molecules chemically combined together.
- The number of atoms in each type of molecule does not change.

- Examples: water (H2O), carbon dioxide (CO2), ammonia (NH3) and hydrogen chloride (HCL).
- A compound can be represented by a compound formula.

- A chemical formula tells us the types of atoms present and the ratio of the different atoms present.

- For example: Glucose: contains 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. (C6H12O6)