4.3 Enthalpy and Hess's law

Question 1

2 things Hess’s law states

Answer 1

(1) Overall enthalpy change is independent of the pathway between the initial and final states

(2) the sum of the enthalpy changes around a complete circle is zero

Question 2

what is enthalpy

Answer 2

The total heat content of system, indicating the stored chemical energy in a system at a given pressure and temperature

Question 3

heat capacity

Answer 3

The ability to store heat (J g^-1 K^-1)

Question 4

enthalpy change

Answer 4

ΔH = enthalpy of products (energy needed to break bonds) - enthalpy of reactants

Question 5

dissociation

Answer 5

physical process, separation of existing ions

Question 6

dissolution

Answer 6

physical process, a solid or a liquid breaks up to form a homogenous mixture (solution)

Question 7

standard enthalpy of formation

Answer 7

When one mole of a compound in its standard state is formed from IRS constituent elements in their standard states.

Question 8

breaking bonds:

Answer 8

Requires energy, therefore is endothermic

Question 9

forming bonds:

Answer 9

Creates energy, therefore is exothermic

Question 10

heat of solution equation

Answer 10

∆Hsoln = -mCΔT

Question 11

what is latent heat

Answer 11

heat or energy that is absorbed or released during a phase change of a substance (without changing temperature)

Question 12

latent heat of evaporation

Answer 12

enthalpy of vaporisation aka ∆Hvap
- at 1atm
- from 1 mol liquid to gas

Question 13

latent heat of fusion

Answer 13

enthalpy of fusion aka ∆Hfus
- at 1atm
- from 1 mol solid to liquid

Question 14

what is enthalpy

Answer 14

total heat content of system;

sum of the kinetic and potential energies of 1 mole of all particles

Question 15

what is bond energy

Answer 15

amount of energy required to break 1 mol of a bond into its constituent gaseous atoms under standard conditions (25C, 1 bar)

Question 16

using bond energy to calculate enthalpy

Answer 16

∆H = Σ(reactant bonds) - Σ(product bonds

Question 17

standard state of solid and liquid

Answer 17

100 kPa pressure

Question 18

standard state of gas

Answer 18

100 kPa pressure (under ideal gas law)

Question 19

standard state of solutions

Answer 19

1M concentration

Question 20

standard temperature

Answer 20

298.15K or 25C

Question 21

standard enthalpy of formation

Answer 21

ΔfHθ or ∆Hot

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

Question 22

standard enthalpy of combustion

Answer 22

ΔcHθ

enthalpy change when one mole of a substance combusts completely with oxygen

Question 23

using standard enthalpy of formation to calculate enthalpy

Answer 23

∆Hot = Σ∆Hot(products) - Σ∆Hot(reactants)

Question 24

standard enthalpy of formation for DIATOMIC MOLECULES is…

Answer 24

0 kJ/mol