Topic 1: Stoichiometric Relationships

Question 1

Element

Answer 1

A substance that cannot be decomposed into simpler substances by chemical changes. It can exist as single atoms or molecules.

Question 2

Compound

Answer 2

A substance made up of two or more elements, chemically combined.

Question 3

Mixtures

Answer 3

Combinations of two or more pure substances in which each substance retains its own composition and properties. They do not have to be in the same state.

Question 4

Homogenous Mixture

Answer 4

A homogenous mixture is made up of substances in the same phase. It appears uniform throughout.

Question 5

Heterogenous Mixture

Answer 5

A heterogenous mixture is made up of substances in different phases. Visually distinctive components.

Question 6

Solute

Answer 6

The substance being dissolved.

Question 7

Solvent

Answer 7

The substance in which the solute dissolves.

Question 8

The Kinetic Theory of Matter

Answer 8

The average kinetic energy of the particles is directly related to the temperature of the substance. (In simpler terminology, when we increase the temperature, we increase the kinetic energy of the substance.)

Question 9

Give the definition of a State Change and explain why the temperature does not increase during this stage.

Answer 9

During a state change the energy added is used to break the intermolecular forces between the particles to cause the substance to change state, not to raise the kinetic energy, so the temperature remains constant.

Question 10

Evaporation

Answer 10

The change of liquid to a gas. Evaporation occurs at the surface and takes place at temperatures below the boiling point i.e puddles on the street.

Question 11

Boiling

Answer 11

Boiling occurs at a specific temperature (when the vapour pressure is greater than or equal to the atmospheric, external, pressure). It is characterized by bubbles, particles leaving the body of liquid.

Question 12

Liquid

Answer 12

• Variable shape, fixed volume
• Particles more spaced apart.
• Weaker inter-particle forces, particles can slide over each other.

Question 13

Solid

Answer 13

• Fixed shape and volume.
• Closely packed particles.
• Strong inter-particles forces, particles vibrate in lattice structure.

Question 14

Gas

Answer 14

• Variable shape and volume.
• Inter-particle forces are negligible, particles move freely.
• Particles are spread out.

Question 15

Define a mol

Answer 15

Amount of a substance that contains the same amount of atoms, molecules or particles in 12 grams of Carbon-12.

Question 16

Relative Atomic Mass formula

Answer 16

relative atomic mass (Ar) = weighted average of one atom of the element/one twelfth mass of one atom of carbon-12

Question 17

Define the relative atomic mass

Answer 17

The relative atomic mass (Ar) of an element is the weighted average of the masses of its isotopes relative to 1/12 of the mass of a carbon-12 atom.

Question 18

Define the relative molecular mass

Answer 18

The relative molecular mass (Mr) of a substance is the weighted average of the masses of the molecules relative to the 1/12 of the mass of a carbon-12 atom.

Question 19

Molecular Formula

Answer 19

The total number of atoms of each type in a molecule.

Question 20

Empirical Formula

Answer 20

The lowest whole number ratio of atoms in a molecule.

Question 21

Theoretical Yield

Answer 21

The maximum amount of product obtainable, assuming 100% of the limiting reactant is converted to product. Usually expressed in grams or moles.

Question 22

Why may the experimental yield be different to the theoretical yield?

Answer 22

Reasons for low yield:
- Loss during transfer.
- Equilibrium/reaction did not go to completion.
- Side reactions.

Reasons for high yield:
- Insufficient drying
- Oxidation (gain of oxygen)
- Side reactions

Question 23

Molar concentration

Answer 23

Moles of solute per decimetre of solution. mol dm⁻³

Question 24

Gases - Avogadro’s Law

Answer 24

Equal volumes of all gases, when measured at the same temperature and pressure, contain an equal number of particles.

Question 25

Kinetic Molecular Theory

Answer 25

1) Particles are in constant motion
2) That motion depends upon the absolute temperature measured in Kelvin

Question 26

STP for gases

Answer 26

Standard Temperature and Pressure
0°C = 273K
1atm = 100kPa = 100,000Pa

Question 27

Under STP 1 mol of gas occupies..?

Answer 27

22.7dm³ (This is called the molar gas volume Vm).
Contains 6.02x10²³ molecules.

Question 28

How do you write a net ionic equation?

Answer 28

Write balanced molecular equation
Write the state symbols (s, l, g, aq) for each substance
Split strong electrolytes into ions (the complete ionic equation)
Cross out the spectator ions that appear on both sides of the complete equation
Write the remaining substance as the net ionic equation

Question 29

What are common spectator ions?

Answer 29

NO₃²⁻, Na⁺, K⁺