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Pure chemistry 🧫⚗️💉 > Chapter 6: Chemical formulae and equations. > Flashcards

Chapter 6: Chemical formulae and equations. Flashcards

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1
Q

What is a chemical formula?

A
  • A chemical formula of a substance shows the elements that makes up the substance and the number of atoms of each constituent element.
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2
Q

What is a chemical formula consisted?

A
  • 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
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3
Q

What are the elements that affects the chemical formulae of elements?

A

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.

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4
Q

What are the chemical formulae(s) of some compounds?

A
  1. Potassium iodine
    Chemical formula:
    - Kl
    Particles that make up the chemical formula:
    - 1 potassium ion
    - 1 iodine ion
  2. Magnesium chloride
    Chemical formula:
    - Mgcl2
    Particles that make up the chemical formula:
    - 1 magnesium ion
    - 2 chlorine ions
  3. Aluminium oxide
    Chemical formula:
    - Al2O3
    Particles that make up the chemical formula:
    - 2 Aluminium ions
    - 3 oxide ion
  4. Hydrogen bromide
    Chemical formula:
    - HBr
    Particles that make up the chemical formula:
    - 1 hydrogen atom
    - 1 bromine atom
  5. Methane
    Chemical formula:
    - CH4
    Particles that make up the chemical formula:
    - 1 carbon atom
    - 4 hydrogen atoms
  6. Sulfur dioxide
    Chemical formula:
    - SO3
    Particles that make up the chemical formula:
    - 1 sulfur atom
    - 3 oxygen atoms
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5
Q

How do we write the chemical formula of an ionic compound?

A
  • 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.
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6
Q

How do we find the valency of an element?

A
  • 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.
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7
Q

What are the examples of elements and their valency which is derived from the periodic table?

A
  1. Lithium, sodium, potassium
    Group number: 1
    Valency: 1
  2. Magnesium, calcium, barium
    Group number: 2
    Valency: 2
  3. Boron, aluminium
    Group number: 13
    Valency: 3
  4. Carbon, silicon
    Group number: 14
    Valency: 4
  5. Nitrogen, phosphorus
    Group number: 15
    Valency: 3
  6. Fluorine, chlorine, bromine, iodine
    Group number: 17
    Valency: 1
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8
Q

How can we derive the valency of an ion?

A
  • The valency of an ion can be derived from the charge of the ion.
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9
Q

What are the examples of ion(s) that have different charge(s)?

A

Some transition elements (elements groups 3-11 of the periodic table) form ions with different charge:
1. Copper (I)
Chemical formula:
- Cu+
Valency:
- 1

  1. Copper (II)
    Chemical formula:
    - Cu2+
    Valency:
    - 2
  2. Iron (II)
    Chemical formula:
    - Fe2+
    Valency:
    - 2
  3. Iron (III)
    Chemical formula:
    - Fe3+
    Valency:
    - 3
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10
Q

What are the examples of polyatomic ions that have more than 1 atom covalently bonded together?

A
  1. Ammonium
    Chemical formula:
    - NH4+
    Valency:
    - 1
  2. Hydroxide
    Chemical formula:
    - OH-
    Valency:
    - 1
  3. Nitrate
    Chemical formula:
    - NO3-
    Valency:
    - 1
  4. Carbonate
    Chemical formula:
    - CO3 2-
    Valency:
    - 2
  5. Sulfate
    Chemical formula:
    - SO4 2-
    Valency:
    - 2
  6. Phosphate
    Chemical formula:
    - PO4 3-
    Valency:
    - 3
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11
Q

Is the charge of the cation and anion in an ionic compound balanced or UNbalanced?

A
  • 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.
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12
Q

How can we write a chemical formula?

A
  • 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.
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13
Q

How is the chemical formula of covalent compounds that involve hydrogen expressed?

A

For the chemical formulae of covalent compounds that include hydrogen:

  1. 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.
  2. 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.
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14
Q

What is another way to express/write the chemical formula of a covalent compound?

A
  • 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
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15
Q

What is the chemical formulae of some covalent compounds with prefixes in their names?

A
  1. Carbon monoxide
    Chemical formula: CO
  2. Carbon dioxide: CO2
    Chemical formula: CO
  3. Sulfur dioxide:
    Chemical formula: SO2
  4. Sulfur trioxide:
    Chemical formula: SO3
  5. Carbon tetrachloride:
    Chemical formula: CCl4
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16
Q

What are the informations that chemical equations provide?

A

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)

17
Q

How is a chemical equation expressed?

A
  • 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).
18
Q

What are the functions of a state symbol?

A
  • 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
19
Q

What happens when there is a balanced chemical equation?

A
  • 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.

20
Q

What is the first scenario’s steps for writing a balanced chemical equation?

A

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)

21
Q

What is the second scenario’s steps for writing a balanced chemical equation?

A

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)

22
Q

What is an ionic equation?

A
  • 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.
23
Q

What are the steps of writing an ionic equation?

A
  • 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)

Pure chemistry 🧫⚗️💉 (12 decks)

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