Thermochemistry

Question 1

Ionisation energy
(e.g. Cl (g) —> Cl+ (g) + e)

Answer 1

• energy required to remove one mole of electrons from one mole of atoms in the gaseous state

Question 2

First ionisation energy

Answer 2

• energy required to remove the 1st valence electron

Question 3

Standard enthalpy of formation

Answer 3

enthalpy change when 1 mole of a compound is formed in its standard state from elements in their standard states

Question 4

Standard enthalpy of combustion

Answer 4

enthalpy change when 1 mole of a substance is combusted in oxygen with all reactants and products in their standard states (products usually CO2 and H2O)

Question 5

Standard enthalpy of vaporisation

Answer 5

enthalpy change when 1 mole of a substance is vaporised at its boiling point (l) to (g)

Question 6

Standard enthalpy of fusion

Answer 6

enthalpy change when 1 mole of a substance is melted at its melting point (s) to (l)

Question 7

Standard enthalpy of sublimation

Answer 7

enthalpy change when 1 mole of a substance sublimes at its sublimation point (s) to (g)

Question 8

Exothermic

Answer 8

transfers heat energy from the system into the surroundings (warm to touch)

Question 9

Endothermic

Answer 9

takes heat from the surroundings into the system (cool to touch)

Question 10

Atomic radius

Answer 10

• decreases as you go across the period
• increases as you go down a group

Question 11

Amount of shielding?

Answer 11

the number of electron shells shielding the attractive force of the nucleus from the valence shell

Question 12

How does nuclear charge affect atomic radius?

Answer 12

e.g. “although they have the same amount of shielding, __ has a greater nuclear charge because more protons are attracting the valence shell. This means the valence shell is pulled in closer = smaller radius”

Question 13

Ionic radius (between Na and Na+)

Answer 13

• Na+ has lost an entire energy shell, making it physically smaller
• this means it experiences less shielding, so the valence shell is attracted more strongly to the nucleus

Question 14

Electronegativity

Answer 14

• a measure of attraction an atom has for electrons in a bond
• weaker electrostatic attraction = lower electronegativity
• top right in the periodic table is most electronegative

Question 15

More protons in the nucleus (electronegativity) =

Answer 15

greater electrostatic attraction for electrons in a bond

Question 16

More protons in the nucleus (ionisation energy) =

Answer 16

attracts the electrons in its valence shell more strongly, thus taking more energy to remove an electron

Question 17

Temporary dipole forces

Answer 17

• very weak bond between neighboring molecules
• caused by uneven distribution of electrons in one end of a molecule
• this causes electrons in neighboring molecules to be repelled, creating a temporary force of attraction

Question 18

Permanant dipole forces

Answer 18

• stronger type of intermolecular bond (but still weak)
• caused by polar molecules with permanent dipoles being attracted to one another
• permanent dipoles = attraction always maintained

Question 19

Hydrogen bond

Answer 19

• the strongest of the intermolecular forces
• only occurs in specific molecules (H-F, H-O, H-N bond)
• F, O, N are especially electronegative; thus forming especially strong dipoles
• high boiling point (high heat energy to break the intermolecular forces)

Question 20

Heating curve of water

Answer 20

• shows how the temperature of water changes as it is heated over time
• it involves the standard enthalpy of fusion and vaporisation (solid to liquid, liquid to gas)

Question 21

Slope on the graph (heating curve)

Answer 21

• increasing temp, increasing kinetic energy (particles vibrate more quickly)
• NO change in state occurs (e.g. solid stays solid)

Question 22

Flat line on the graph (heating curve)

Answer 22

• change in state (solid turns to liquid)
• NO temperature change
• heat energy is used to break the intermolecular forces (IMF) between water molecules and the potential energy of the molecule increases
• (s) to (l) = some IMF break, (l) to (g) = ALL IMF break

Question 23

Entropy (symbol s)

Answer 23

a measurement of the disorder of particles in a system

Question 24

Low entropy

Answer 24

particles are ordered in a SOLID state

Question 25

High entropy

Answer 25

• particles become more disordered during a reaction, either DISSOLVED (aq) or in a GAS STATE (g)

Question 26

What causes a higher entropy?

Answer 26

• more moles = more chances for particles to become disordered
• faster moving particles = more likely to be disordered

Question 27

What does spontaneous mean (entropy)

Answer 27

the reaction/process just happens (or carries on once it starts) e.g. salt readily dissolves, alcohol burns

Question 28

Why is a reaction spontaneous (in terms of entropy)

Answer 28

∆S(total) must be posi�ve
∆S(total) = ∆S(system) + ∆S(surroundings)
- talk about the system and the surroundings and whether entropy increases or not
- since total entropy change is positive, the increase in entropy of the system must be greater than the decrease in entropy of the surroundings (and vice versa)

Question 29

Transition metals

Answer 29

• lie from Sc to Zn
• form ions by losing their 4s electrons before their 3d
• 4s electrons are lower energy level than 3d so they are easier to remove

Question 30

What does the abbreviation VSEPRT stand for in relation to identifying shapes from lewis diagrams?

Answer 30

Valence Shell Electron Pair Repulsion Theory
(repel for maximum separation)

Question 31

Why is Fluorine the most electronegative (reference the factors affecting periodic trends)

Answer 31

• Fluorine is at the top of Group 17
• going up a group, the atomic radius decreases, as does shielding from the inner energy levels
• this means there is a strong electrostatic attraction between the positive nucleus and any bonding electrons