energetics

Question 1

energy

Answer 1

a measure of the ability to do work

Question 2

heat

Answer 2

the TOTAL kinetic energy of particles in a substance

Question 3

temperature

Answer 3

the AVERAGE kinetic energy of particles in a substance

Question 4

enthalpy (H)
- what is the system? surroundings?

Answer 4

• stored in bonds as potential energy

Question 5

exothermic rxns

Answer 5

• ΔH is NEGATIVE
• products are more stable than reactants (favourable) like rocket fuel
• bond formation, removing heat brings atoms closer
• ex) combustion (during combustion, a substance burns as it combines with oxygen, releasing energy in the form of heat and light.)

Question 6

endothermic rxns

Answer 6

• ΔH is POSITIVE
• products are less favourable than reactants
• breakage of bonds, adding heat moves atoms further apart
• ex) photosynthesis - absorb sunlight/heat energy

Question 7

Representing enthalpy change

Answer 7

1. writing the energy term in eq’n
2. writing ΔH value w/ eq’n
3. Molar enthalpy - ΔHrxn (kJ/mol)
4. Potential energy diagrams

Question 8

molar enthalpy (ΔHrxn)

Answer 8

enthalpy change per mole of a reactant or product (specified)

Question 9

Formula for ΔHrxn:

NOTE:
ΔH (kJ) / n = mol / ΔHrxn = kJ/mol

Answer 9

ΔH = nΔHrxn

Question 10

NOTE: ΔHrxn can also be…

• ΔHcomb, ΔHneu, ΔHsol, ΔHlat

Question 11

Potential Energy Diagrams
- what is activation energy?

Answer 11

• the minimum amount of energy required for reactants to react to produce a product

Question 12

NOTE: Calorimeter - when rxns occurs, heat is absorbed or released; a calorimeter measures change in temp of H2O (surroundings)

Question 13

REMEMBER:
- the amount of energy gained/lost by the system must be exactly equal to the NEGATIVE of the amount of energy lost/gained by the surroundings.

• SO, ΔH = -Q (surr.)

Question 14

5 calorimetry assumptions

Question 15

hint: “determine final temp” questions, set
Q (system) = - Q (surr)

Question 16

Hess’ Law

Answer 16

if a rxn is carried out in a series of steps, the overall change in enthalpy is the
SUM of the enthalpy changes of each individual step