Week 2 (Claire)

Question 1

What are the basic principles of a chemical equation?

Answer 1

1) Reactants
2) Products
3) Physical states (e.g. gas, liquid, solid, aqueous)

Question 2

What law do balanced equations obey?

Answer 2

Balanced equations obey the law of Conservation of Mass (“Mass is neither created nor destroyed in chemical reactions”)

Question 3

What is Stoichiometry?

Answer 3

Stoichiometry is the relationship between the number of reactant and product molecules in a chemical equation.

E.g. CaCO3(s) + 2HNO3(aq) —> Ca(NO3)2(aq) + CO2(g) + H2O(l)
[2HNO3 is the stoichiometry coefficient]

Question 4

What are the steps to balancing a chemical equation?

Answer 4

1) Write an unbalanced equation with correct formulas for all substances.

2) Balance atoms of one of the elements;
i. start with the most complex molecule.
ii. change the stoichiometric coefficients.
iii. do NOT alter the chemical formulas.

3) Balance the remaining elements.

Question 5

What types of chemical reactions are there?

Answer 5

1) Combination reactions.

2) Decomposition reactions.

3) Exchange reactions.

4) Precipitation reactions.

Question 6

Explain Combination Reactions:

Answer 6

X + Z —> XZ

Element + Halogen/O2;
e.g. 2Mg(s) +O2(g) —> 2MgO(s)

Other types;
2SO2(g) + O2(g) —> 2SO3(g)

Question 7

Explain Decomposition Reactions:

Answer 7

XZ —> X + Z

Often initiated by heat;
CaCO3(s) —[800-1000 degrees]—> CaO(s) + CO2(g)

Occasionally by shock;
4C3H5(NO3)3(l) —> 12CO2(g) + 10H2O(l) + 6N2(g) + O2(g)

Question 8

Explain Exchange Reactions:

Answer 8

AD + XZ —> AZ + XD

AgNO3(aq) + HCl(aq) —> AgCl(s) + HNO3(aq)

Question 9

What chemical species are soluble?

Answer 9

• All ammonium & Group 1A (Na+, K+, NH4+ salts)
• All nitrates (NO3-)
• Most chlorides, bromides, & iodides (exceptions; AgX, Hg2X2, PbX2).
• Most sulfates (SO42-) (exceptions; CaSO4, SrO4, BaSO4, Ag2SO4, PbSO4, & Hg2SO4).
• All chlorates (ClO3-)
• All percolates (ClO4-)
• All acetates (CH3COO-)

Question 10

What chemical species are insoluble?

Answer 10

• All phosphates (PO43-) [exception; Group 1A, NH4+]
• All carbonates (CO32-) [exception; Group 1A, NH4+]
• All hydroxides (OH-) [exception; Group 1A, NH4+]
  [Sr(OH)2, Ba(OH)2, & Ca(OH)2 are slightly soluble]
• All oxalates (C2O42-) [exception; Group 1A, NH4+]
• All oxides (O2-) [exception; Group 1A]
• All sulfides (S2-) [exception; Group 1A, NH4+]
  [Mgs, CaS, & BaS are slightly soluble]

Question 11

Explain Precipitation Reactions:

Answer 11

• When mixing ionic solutions, a precipitate may form.
• However, not all ions react.
• A reaction occurs if a product is insoluble.

** Solubility rules help predict reactions **

Question 12

What are the general writing rules for precipitation reactions?

Answer 12

1) Write a balanced equation.

2) Solubility?

3) Dissociate the soluble components.

4) Write the complete ionic equation.

5) Cancel the ions appearing on both sides (spectators).

6) Check the charges are balanced.

** this process gives you the net ionic equation **

Question 13

Briefly explain REDOX reactions…

Answer 13

Oxidation is the process of adding oxygen.
Reduction is the process of reducing an ore to a metal (reverse of oxidation).

If something is oxidised something must be reduced.

Oxidation is a loss of electrons (+e-).
Reduction is a gain of electrons (-e-).

Question 14

Name at least 2 general rules for oxidation numbers…

Answer 14

• Pure element =0.
• Monatomic ion = charge of ion.
• Some elements have the same oxidation number in almost all their compounds.
• The sum of the oxidation numbers of all atoms in any species = the charge of the species.

Question 15

Give one example of a biological redo reaction…

Answer 15

• Hämoglobin transporting oxygen in the body.
• Iron containing protein (Fe2+ / Fe3+)

Question 16

Find the oxidation number for all elements in SO3^2-…

Answer 16

O = -2 (not a peroxide, not bonded to hydrogen)

S - sum of oxidation numbers = charge = -2
(oxidation number for S) + 3(oxidation number for O) = -2
(ox. no. for S) + 3(-2) = -2
(ox. no. S) -6 = -2
Therefore S = +4

Question 17

True or False?
Oxidation numbers always change during redox reactions?

Answer 17

True.
If an element reacts to form a compound, its a redox reaction.

Question 18

What is an example of a biological redox reaction?

Answer 18

Haemoglobin transports oxugen in the body.

Fe containing protein (Fe^2+ / Fe^3+)

Question 19

What is atomic mass (amu)?

Answer 19

Different chemicals have different weights.

amu = atomic mass units (same as gmol-1)

Question 20

What is Avogadro’s number?

Answer 20

Avogadro’s number is one mole (1 mol) contains 6.022x10^23 entities.

Question 21

What is molar mass? How do you calculate this?

Answer 21

The molar mass is 1 mole of an element, expressed in grams (g).

How to calculate:
1) Obtain the molar mass of each element.
2) Multiply each molar mass by the number of moles in the formula.
3) Calculate the molar mass by adding the masses of the elements together.

Question 22

Explain empirical formula…

Answer 22

The empirical formula is the simplest formula for a compound that agrees with the elemental analysis and gives rise to the smallest set of whole numbers of atoms.

Question 23

Explain molecular formula….

Answer 23

Molecular formula is the formula of the compound as it exists, it may be a multiple of the empirical formula.

Question 24

What are some examples showing the empirical formula and molecular formulas…

Answer 24

Hydrogen Peroxide:
EF; HO
MF; H2O2

Borane:
EF; BH3
MF; BH3

Diborane:
EF; BH3
MF; B2H6

Octene:
EF;CH2
MF; C8H16

Question 25

How do you determine both empirical and molecular formula?

Answer 25

First assume a 100.0 g sample (% becomes mass in grams).

Then divide each mass by its atomic mass (gives the number of moles of each in 100.0g).

Divide each by the smallest answer found (smallest whole number ratio is the empirical formula).

The molecular formula can be determined if the molecular mass is known.