Secondary 3 Chemistry Notes & Resources
*Click on a topic to expand the notes and videos
Mole concept & Chemical calculations
What is mole? It has the same function as “dozen” and “pair”, which are counting units. While dozen represents twelve items, and pair represents two, mole represents 6 x10²³ particles.
Now, what are particles then? It depends on the substance that we are looking at
1 mol of Na metal will have 6 x10²³Na atoms.
1 mol of CH4 will have 6 x10²³ CH₄ molecules.
1 mol of Na₂CO₃ will have 6 x10²³ Na₂CO₃ ionic units.
Take Na₂CO₃ as an example. In 1 ionic unit, it is
made up of 2 Na⁺ ions and 1 CO₃²⁻ ions.
Since 1 mol of Na₂CO₃ will have 6 x10²³ Na₂CO₃
ionic units, this means that 1 mol of Na₂CO₃ it
• 2 x 6 x10²³ Na⁺ ions
• 6 x10²³ CO₃²⁻ ions.
Thus, the total number of ions are 3 x 6 x 10²³
Take dichloromethane as an example, CH₂Cl₂. 1 molecule is made up of 1 Carbon atom, 2 Hydrogen atoms and 2 Chlorine atoms.
Since 1 mol of CH₂Cl₂ will have 6 x 10²³ CH₂Cl₂ molecules, this means that 1 mol of CH₂Cl₂ will have
• 2 x 6 x 10²³ H atoms,
• 2 x 6 x 10²³ Cl atoms,
• 6 x 10²³ C atoms.
Now that we know what mol represents, let’s look at the important formulas to determine number of mol.
1. Molar mass (g/mol)
Molar mass is defined as the mass of 1 mol of substance. This means, if a student wants to weigh one mol of Carbon, they need to weigh out 12 g of Carbon. If a student wants to weigh one mol of sodium hydroxide, they need to weigh out 40 g of NaOH. Where do we get these values?
The molar mass of a substance will have the same value as the relative atomic mass of an element, or relative molecular mass of a compound. The only difference between relative molecular mass and molar mass is that molar mass has unit (which is g/mol) while relative molecular mass does not.
What is the formula to find number of mol related to mass?
2. Molar volume (24 dm³ ) of gases
Molar volume is the volume occupied by 1 mole of gas, and at room temperature and pressure (r.t.p.), this volume is 24 dm³.
When the volume of gas is given, always check that the units is the same as the onein molar volume, which is in dm³.
Under the topic of molar volume, we need to look at Avogadro’s law, which states that equal volumes of gas under the same conditions of temperature and pressure, have equal number of particles.
To illustrate the example, we have two balloons containing 100cm3 of methane and carbon dioxide respectively. Although the balloons contain different gases, both balloons contain the SAME number of mole and molecules of gases as the VOLUME is the same. However, mass of the balloons will be different as their MOLAR MASSES are different.
3. Molar concentration of solutions (mol/dm³ )
An aqueous solution consists of solute(s) dissolved in water. Concentration of solution is defined as amount of solute dissolved per unit volume.
We can qualitatively describe a solution as diluted or concentrated based on the amount of solute dissolved per unit volume.
To make a solution more diluted, add more water. To make a solution more concentrated, add more solute or remove water.
Calculations using Chemical Equations
Stoichiometry is the method which allows scientists to study the relative quantities of reactants and products in chemical reactions.
These are the important steps in stoichiometric calculations:
- Construct a balanced chemical equation
- Convert the mass or volume of a gas or concentration and volume of a solution to the number of moles.
- Compare mol ratio between what was found in step 2 to the substance we need to find to determine the no. of mol of unknown substance.
- Convert number of moles of unknown substance to mass or volume or concentration, demanded by the question.
To prepare a chicken burger, we will need two buns and a chicken patty. However, what happens if I have 8 buns and 5 patties? How many chicken burgers of the same recipe can I create?
We can only create 4 burgers, with 1 chicken patty in excess. The amount of product, the chicken burger in this case, depends on the number of buns. The buns are what we call the limiting reagent. In chemistry, the limiting reactant is the one that is completely consumed in the reaction, and it determines the amount of product formed.
When do we need to find the limiting reagent?
• When there are 2 information of at least 2 reactants provided in question.
The steps to solving questions involving limiting reagents are similar to that for stoichiometric calculations.
- Construct a balanced chemical equation
- Find the number of mol of both reactants.
- Compare mol ratio between the reactants to determine limiting reagent.
- Once limiting reagent is identified, compare mol ratio of limiting reagent to product of interest.
- Convert number of moles of product of interest to mass or volume or concentration, demanded by the question.
Secondary Chemistry Revision: Atomic Structure
Atomic Structure is the most fundamental topic to master if you want to master Chemistry!
Let’s dive straight in!
The table below shows the properties and location where the subatomic particles are found. It is important to know the properties of the subatomic particles and their location.
|Proton||+ 1||1||Nucleus of Atom|
|Electron||– 1||1/1840||Electron Shell / Orbital|
What you see in the periodic table for an element will be denoted as:where a is the proton (atomic) number, b is the mass (nucleon) number and X is the chemical symbol of the element. Proton (Atomic) Number is the number of protons the element has in the nucleus of an atom. Mass (Nucleon) Number is defined as the total number of protons and neutrons in the nucleus of an atom. As an atom is electrically neutral (having an overall charge of zero), the number of electrons must be equal to the number of protons in an atom. An element only has one proton number. The number of protons gives the identity of the element. Therefore, to identify the elements of a particle (atom or ion), look at the number of protons the particle has. A common mistake is mixing up between proton number and mass number. Therefore, when referring to the periodic table, check the legend too to see you are looking at the right number. The periodic table is your best friend in chemistry examinations apart from your calculator.
The protons and neutrons are found in the nucleus of the atom, which is surrounded by electron shell(s) containing electrons. An example of a lithium atom is shown below:
- Atoms of an element can have different nucleon (mass) number.
- If a proton is added to the nucleus of the atoms of an element, the element changes into a different element.
- The nuclei of all atoms contain both protons and neutrons.
- The proton number is always less than or equal to the nucleon (mass) number.
- True. Some elements have isotopes (atoms of the same element with the same proton number but different mass number).
- True. An element only has one proton number. If the number of protons in the atom changes, the identity of the atom changes as well, according to the proton number.
- False. Refer to the periodic table that hydrogen has a mass number of 1, and a proton number of 1. Thus, its mass number is due only that 1 proton, and 0 neutrons.
- True, as shown in the periodic table. Except for hydrogen where the proton number is equal to its mass number, the rest of the elements has their proton number lower than their respective mass number.
IsotopesIsotopes are atoms of the same element carrying the same proton number but a different mass number. Thus, they have a different number of neutrons in the nucleus of the isotopes. For example, carbon is made up of isotopes of carbon-12, carbon-13 and carbon-14. The relative atomic mass of carbon in the periodic table is the average mass of all the carbon isotopes according to their relative abundance in nature.
Question: Do isotopes of the same element have different reactions?The answer is no, they have the same reactions. This is because reactivity is dependent on the number of valence electrons. For isotopes, they have the same proton number, thus they have the same number of electrons, and hence, the same chemical reactivity. Question: Deuterium, , is an isotope of hydrogen. Which statement about deuterium is false?
- Its diatomic molecules diffuse at a faster rate than that of hydrogen.
- It has a higher density than hydrogen.
- It has the same number of protons but a different number of neutrons as hydrogen.
- It undergoes the same chemical reactions as hydrogen.
- False. Deuterium is heavier than hydrogen. As the speed of diffusion decreases with the increasing mass of gas, deuterium molecules diffuse slower than hydrogen.
- True. As density equals mass/volume, for the same volume of deuterium and hydrogen, the density of deuterium is higher than hydrogen as it is heavier.
- True. The definition of isotopes are atoms of the same element with the same proton number but a different mass number, and thus a different number of neutrons.
- True, isotopes undergo the same chemical reactions.
Secondary Chemistry Revision: Drawing of Dot and Cross Diagram
Tips & Tricks to Draw Dot and Cross Diagram:
- Covalent bonding is the sharing of electrons between non-metal atoms. In the overlap of the valence shells, there must at least be a dot from one non-metal atom and a cross from another non-metal atom.
- Both non-metal atoms must achieve stable noble gas electronic configuration (duplet or octet - full valence shell)
- Hydrogen and Group VII atoms: 1 bond.
- Group VI atoms (oxygen, sulfur etc): 2 bonds
- Group V atoms (nitrogen, phosphorus): 3 bonds
- Group IV atoms (carbon): 4 bonds.
Take note that students are to draw the full electronic configuration of the atoms unless specified in the question to only draw valence electrons.
Some Examples for Dot and Cross Diagrams
Common error #1 that students do is to miss out on the charges on the ions. Since ions are formed due to the transfer of electrons from metal atoms to non-metal atoms, there should be both positive and negative charges in the diagram.
Another common error #2 is forgetting that the electron transfer to the non-metal atom has to follow the legend for the metal atom.
For example, in the diagram above, the dot electron represents the valence electron from the sodium atom that got transferred to the chlorine atom so that both atoms can obtain stable noble gas electronic configuration to form sodium ion and chloride anion.
Popular Exam Question #1: Hydrogen Chloride (Both Ionic & Covalent)
Hydrogen chloride is a covalent molecule in a gaseous state (a). However, it can dissociate in water to form H+ and Cl- ions (b - Ionic).
|Hydrogen chloride - (a) Simple covalent molecule (In gaseous state)||-||Both hydrogen and chlorine atoms should attain a full valence shell of electrons - stable noble gas electronic configuration (duplet of octet).||Hydrogen chloride|
-> Covalent compound
-> sharing of electrons
If you enjoy such learning content and would like to have more, go here → chemistry tuition specialist website to learn more!
Secondary Chemistry Topic Revision: Chemical Bonding
In Chemical Bonding, there are 3 main parts - Melting Points, Electrical Conductivity and Dot and Cross Diagrams. In this summary, we will be covering the first 2 parts 🙂
Chemical Bonding Part 1: Melting Point of Ionic, Covalent & Metallic Elements/Compounds
|Substance||Ionic compound||Simple covalent molecules||Giant covalent substance||Metals|
|Structure: Structure the substance adopts.||Giant ionic lattice||Simple covalent structure||Extensive/ Giant network of covalent bonds||Giant metallic lattice|
|Attractive forces: Strong or weak attractive forces between atoms or ions etc.||Strong electrostatic forces of attraction between oppositely charged ions||Weak intermolecular forces of attraction (Van der Waals) between molecules||Strong covalent bonds between atoms||Strong electrostatic forces of attraction between positive ions and sea of delocalised electrons|
|Energy: The energy required to overcome the attractive forces.||A lot of energy is required to overcome these strong electrostatic forces of attraction.||Less energy is required to overcome these weak Van der Waals forces between molecules.||A large amount of energy is required to break these strong covalent bonds between atoms.||A large amount of energy is required to overcome these strong electrostatic forces of attraction.|
|Conclusion: Linking it back to the question.||Thus, (name of substance) has a high/low melting/boiling point.|
The answer is no. A common misconception that students have is that the covalent bonds are weak since covalent compounds have low melting point, which is false. Covalent bonds are very strong due to the sharing of electrons between atoms. The reason for the low melting point of covalent compounds is due to the weak intermolecular forces of attraction between molecules, known as Van der Waals forces.
When a question requires an explanation relating to melting or boiling points of substances, answer using SAEC (Structure - Attraction - Energy - Conclusion) format. Below is a table showing a summary on how to tackle a question asking to explain the high melting point of different compounds.
Sometimes, the question may ask to compare the melting points of two different substances. Similarly, SAEC format needs to be used to explain the difference in melting points.
Question: Explain why MgO have a higher melting point than H2O.
A common error #2 that students make would be just stating that MgO is an ionic compound while H2O is a simple covalent molecule, without explaining further.
MgO is an ionic compound with strong electrostatic forces of attraction between Mg2+ and O2- ions in its ionic lattice structure, while H2O is a simple covalent molecule with a simple covalent structure and have weak intermolecular forces of attraction between molecules. As a larger amount of energy is required to overcome the strong electrostatic forces of attraction in MgO than the weak intermolecular forces in H2O, MgO thus has a higher melting point than H2O.
Chemical Bonding Part 2: Electrical Conductivity
|State||Can the substance conduct electricity?|
|Ionic compound||Simple covalent molecules||Giant covalent substance||Metals|
|Liquid or Molten||Yes||No||No||Yes||Yes|
(Dissolved in water)
|Yes||Most covalent compounds are insoluble except for *hydrogen halides. Therefore, hydrogen halides dissolved in water (aqueous hydrogen halide) can conduct electricity.|
*In the table above, most simple covalent molecules are insoluble in water - except for hydrogen halides. Aqueous solutions of hydrogen halides are known as hydrohalic acids (hydrochloric acid, hydrofluoric acid etc).
Note: Pure water does not conduct electricity. However, presence of impurities may affect its electrical conductivity. For example, if the impurity is an ionic salt, then the impure water can conduct electricity. On the other hand, if the impurity is a simple covalent substance that is not a hydrogen halide, then the water is still not able to conduct electricity.
When a question tests the concept of electrical conductivity of substances, the keywords examiners are looking out for are shown below:
mobile charge carriers
mobile charge carriers
Silicon carbide has a tetrahedral structure where all carbon and silicon atoms use all of its valence electrons for covalent bonding. As there are no free electrons that can act as mobile charge carriers, silicon carbide is not able to conduct electricity.
Continue reading on Energy for Chemical here
There are 3 methods to prepare salts:
1. Precipitation method
– to produce INSOLUBLE salts
– Reagents: (aqueous) + (aqueous)
– Examples: BaSO4, CaSO4, AgCl, PbCl2 etc.
2. Excess/ICA method
– to produce SOLUBLE salts
– Reagents: (aqueous; usually an acid) + (solid; excess METAL/ METAL OXIDES/ METAL HYDROXIDES/ METAL CARBONATES)
– Note: some metals such as Cu/Ag cannot be used as they are unreactive & Group I metals such as Na/K as they are too reactive**
– Examples: CuSO4, CuCl2, Ba(NO3)2 etc.
3. TITRATION method
– to produce SOLUBLE A1 (Ammonium + Group I) SALTS
– Reagents: (aq; acid) + (aq; base)
– Examples: NaCl, KCl, NH4NO3 etc.
– you have to be very familiar with the solubility table; to produce insoluble salts there is only one method – PRECIPITATION.
– However, if the salt you want to produce is soluble, you have to determine if it is an ‘A1’ salt. If it is, use the TITRATION method. If it is not ‘A1’ salt, use the EXCESS method.
– Remember A1 (Ammonium + Grp I) to get your A1 in Chem!
Would you like to take up O level Chemistry tuition in AD and get access to more information like these? Click on the link before this sentence!