Chapter 6: Chemical formulae and equations. Flashcards
What is a chemical formula?
- A chemical formula of a substance shows the elements that makes up the substance and the number of atoms of each constituent element.
What is a chemical formula consisted?
- A chemical formula consists of the chemical symbols of the elements which can be found in the periodic table.
- A chemical formula also consists of subscripts which indicate the number of atoms of the elements.
- E.g. of chemical symbols: K, Mg, Fe, Cu
What are the elements that affects the chemical formulae of elements?
The chemical formulae of elements depend on whether the elements exist as:
1. Monatomic elements: are made up of 1 atom.
- E.g. Ne, and Ar.
2. Diatomic molecules: are made up of 2 atoms.
- E.g. H2, O2, N2, Cl2, and Br
3. Polyatomic molecules: are made up of / more atoms
- E.g. O3.
What are the chemical formulae(s) of some compounds?
- Potassium iodine
Chemical formula:
- Kl
Particles that make up the chemical formula:
- 1 potassium ion
- 1 iodine ion - Magnesium chloride
Chemical formula:
- Mgcl2
Particles that make up the chemical formula:
- 1 magnesium ion
- 2 chlorine ions - Aluminium oxide
Chemical formula:
- Al2O3
Particles that make up the chemical formula:
- 2 Aluminium ions
- 3 oxide ion - Hydrogen bromide
Chemical formula:
- HBr
Particles that make up the chemical formula:
- 1 hydrogen atom
- 1 bromine atom - Methane
Chemical formula:
- CH4
Particles that make up the chemical formula:
- 1 carbon atom
- 4 hydrogen atoms - Sulfur dioxide
Chemical formula:
- SO3
Particles that make up the chemical formula:
- 1 sulfur atom
- 3 oxygen atoms
How do we write the chemical formula of an ionic compound?
- To write the chemical formula of an ionic compound, we need to know the valency of the elements, and ions that make up the compound.
How do we find the valency of an element?
- The valency of an element can be found from the number of electrons gained/lost/shared by the element.
- E.g. 1: Magnesium has 2 valence electrons, and a magnesium atom loses its 2 valence electrons to form a Mg2+ ion.
- Thus, magnesium has a valency of two.
- E.g. 2: Chlorine (Cl) has 7 valence electrons, and a chlorine atom gains 1 electron to form a Cl- ion.
- Thus, chlorine has a valency of one.
- E.g. 3: Hydrogen (H) has 1 valence electron, a chlorine atom can lose its valence electron to form H+ ion, and it can also share the electron with another atom to achieve a stable electronic configuration.
- Thus, hydrogen has a valency of one.
What are the examples of elements and their valency which is derived from the periodic table?
- Lithium, sodium, potassium
Group number: 1
Valency: 1 - Magnesium, calcium, barium
Group number: 2
Valency: 2 - Boron, aluminium
Group number: 13
Valency: 3 - Carbon, silicon
Group number: 14
Valency: 4 - Nitrogen, phosphorus
Group number: 15
Valency: 3 - Fluorine, chlorine, bromine, iodine
Group number: 17
Valency: 1
How can we derive the valency of an ion?
- The valency of an ion can be derived from the charge of the ion.
What are the examples of ion(s) that have different charge(s)?
Some transition elements (elements groups 3-11 of the periodic table) form ions with different charge:
1. Copper (I)
Chemical formula:
- Cu+
Valency:
- 1
- Copper (II)
Chemical formula:
- Cu2+
Valency:
- 2 - Iron (II)
Chemical formula:
- Fe2+
Valency:
- 2 - Iron (III)
Chemical formula:
- Fe3+
Valency:
- 3
What are the examples of polyatomic ions that have more than 1 atom covalently bonded together?
- Ammonium
Chemical formula:
- NH4+
Valency:
- 1 - Hydroxide
Chemical formula:
- OH-
Valency:
- 1 - Nitrate
Chemical formula:
- NO3-
Valency:
- 1 - Carbonate
Chemical formula:
- CO3 2-
Valency:
- 2 - Sulfate
Chemical formula:
- SO4 2-
Valency:
- 2 - Phosphate
Chemical formula:
- PO4 3-
Valency:
- 3
Is the charge of the cation and anion in an ionic compound balanced or UNbalanced?
- The charge of the cation and anion in an ionic compound is balanced.
- E.g. 1: Magnesium has a valency of 2, while chlorine has a valency of 1, one Mg2+ ion needs 2Cl- ions to balance its charge.
- Thus, the chemical formula of magnesium fluorides is Mgcl2.
- E.g. 2: The iron (II) ion has a valency of 2, while the phosphate ion has a valency of 3, so the total charge of 3 Fe2+ ions is balanced by the total charge of 2 PO4 3- ions.
- Thus, the chemical formula of iron (II) phosphate is Fe3 (PO4)2.
How can we write a chemical formula?
- We can write a chemical formula of a covalent compound based on the number of atoms of the elements, which share electrons with each other to achieve the stable electronic configuration of a noble gas.
- E.g. 1: Nitrogen has 5 valence electrons, so a nitrogen atom shares 3 valence electrons with another atom to form 3 covalent bonds and achieve a stable electronic configuration.
- E.g. 2: Hydrogen has 1 valence electron, so a hydrogen atom shares its valence electron with another atom to form 1 covalent bond and achieve a stable electronic configuration.
- E.g. 3: Since 1 nitrogen atom forms 3 covalent bonds with 3 hydrogen atoms, the chemical formula of the covalent compound formed is NH3.
How is the chemical formula of covalent compounds that involve hydrogen expressed?
For the chemical formulae of covalent compounds that include hydrogen:
- The chemical symbol of hydrogen is written after the chemical symbols of elements in groups 14 and 15 of the periodic table.
- E.g. CH4 and NH3. - The chemical symbol of hydrogen is written before the chemical symbols of elements in groups 16 and 17 of the periodic table.
- E.g. H2O and HCl.
What is another way to express/write the chemical formula of a covalent compound?
- The chemical formula of a covalent compound can also be expressed based on its chemical name.
- A covalent compound may have a prefix in its name if it contains 1/more atoms of the same element.
1. Mono-
No. represented: 1
2. Di-
No. represented: 2
3. Tri-
No. represented: 3
4. Tetra-
No. represented: 4
What is the chemical formulae of some covalent compounds with prefixes in their names?
- Carbon monoxide
Chemical formula: CO - Carbon dioxide: CO2
Chemical formula: CO - Sulfur dioxide:
Chemical formula: SO2 - Sulfur trioxide:
Chemical formula: SO3 - Carbon tetrachloride:
Chemical formula: CCl4
What are the informations that chemical equations provide?
A chemical equation provides information about a chemical reaction:
- Identities of the reactant(s) and product(s)
- Physical states of the reactant(s) and product(s)
- Relative amounts of the reactant(s)
How is a chemical equation expressed?
- In a chemical equation, the chemical formulae of the reactant(s) is/are written on the left-hand side (LHS), while the chemical formulae of products is/are written on the right-hand side (RHS).
- An arrow separates the chemical formulae of the reactant(s) and the product(s).
What are the functions of a state symbol?
- State symbols are used to show the physical states of the substances in a chemical equation.
1. Physical state: Solid
State symbol: S
2. Physical state: Liquid
State symbol: l
3. Physical state: G
State symbol: g
4. Physical state: Aqueous solution
State symbol: aq
What happens when there is a balanced chemical equation?
- In a balanced equation, the number of atoms of each element involved in the chemical reaction is the same on both sides of the equation.
- E.g. (balanced equation with state symbols)
2H2 (g) + O2 (g) –> 2H2O (l)
*** The numbers are used to balance the number of atoms on both sides of the equation.
*** The brackets; “()” are the state symbols.
What is the first scenario’s steps for writing a balanced chemical equation?
1:
Nitrogen gas and oxygen gas react to form nitrogen dioxide gas.
Step 1:
- Write the chemical formula of the reactants of the left-hand side (LHS) of the chemical equation, and the chemical formulae of the product on the right-hand side (RHS).
- Also remember to include the state symbols after the chemical formulae.
- Nitrogen and oxygen are the reactant while nitrogen dioxide is the product.
- N2 (g) + O2 (g) –> 2NO2 (g)
Step 2:
- Count the number of atoms of each element on both sides of the chemical equation.
- N2 (g) + O2 (g) –> 2NO2 (g)
Step 3:
- Balance the number of atoms of each element by adding a number before the chemical formula of one/more substances until there is an equal number of atoms of the element on both sides of the chemical equation.
- The number of N atoms on each side of the chemical equation can be balanced by adding “2” before chemical formula NO2.
- N2 (g) + O2 (g) –> 2NO2 (g)
- There are now 2 more O atoms of the right-hand side than the left-hand side (LHS), and the number of O atoms ca be balanced by adding “2” before the chemical formula O2.
- N2 (g) + 2O2 (g) –> 2NO2 (g)
What is the second scenario’s steps for writing a balanced chemical equation?
2:
Aqueous sodium hydroxide and aqueous copper (II) chloride react to form aqueous sodium chloride and solid copper (II) hydroxide:
Step 1:
- Write the chemical formulae of the reactants on the left-hand side (LHS) of the chemical equation, and the chemical formulae of the products on the right-hand side (RHS) .
- Also remember to include the state symbols after the chemical formulae.
- NaOH (aq) + Cucl2 (aq) –> NaCl (aq) + Cu (OH) 2 (s)
Step 2:
- Count the number of each type of ion on both sides of the chemical equation.
- NaOH (aq) + CuCl2 (aq) –> NaCl (aq) + Cu (OH)2 (s)
Step 3:
- Balance the number of each type of ion by adding a number before the chemical formula of one/more substances until there is an equal number of the type of ion on both sides of the chemical equation.
- The number of Cl- ions can be balanced by adding “2” before the chemical formula, NaCl.
NaOH (aq) + CuCl2 (aq) –> 2NaCl (aq) + Cu (OH)2 (s)
- There is now 1 more Na+ ion and 1 more OH- ion on the right-hand side (RHS) than the left-hand side (LHS) , and the number of these ions can be balanced by adding “2” before the chemical formula NaOH.
- 2NaOH (aq) + CuCl2 (aq) –> 2NaCl (aq) + Cu (OH)2 (s)
What is an ionic equation?
- An ionic equation is a simplified version of a chemical equation.
- It also shows the ions involved in a reaction, and it contains at least 1 substance that is not in the aqueous state.
What are the steps of writing an ionic equation?
- Sodium hydroxide and copper (II) chloride react to form sodium chloride and copper (II) hydroxide.
Step 1:
- Write a balanced chemical equation with state symbols.
- 2NaOH (aq) + CuCl2 (aq) –> 2NaCl (aq) + Cu (OH)2 (s)
Step 2:
- Write the chemical formulae of the compounds that are in an aqueous state in the form of ions.
- 2Na+ (aq) + 2OH- (aq) + Cu2+ (aq) + 2Cl- (aq) –> 2Na+ (aq) + 2Cl- (aq) + Cu (OH)2 (s)
Step 3: Cancel out the chemical formulae of the ions that are present on both sides of the chemical equation and these ions are known as SPECTATOR IONS.
- *2Na+ (aq) + 2OH- (aq) + Cu2+ (aq) + *2Cl- (aq) –> *2Na+ (aq) + *2Cl- (aq) + Cu (OH)2 (s)
*: The substances that are cancelled/removed (?).
Step 4:
- The ionic equation for the reaction is obtained.
- Cu2+ (aq) + 2OH- (aq) –> Cu (OH)2 (s)
