Unit 4- Energetics

Question 1

Formation

Answer 1

• enthalpy change of formation (KJmol^-1) under standard conditions
• energy for a mole of a molecule to be formed from its components

Question 2

Combustion

Answer 2

• enthalpy change of combustion (KJmol-1) under standard conditions
• energy for a a mole of molecule to fully combust with oxygen

Question 3

Hess’s law

Answer 3

• any two routes in a chemical reaction have the same overall energy change no matter which route is taken
• eg formation and combustion can both be found for any reaction, forming a triangle of reactions
• A=-B+C
• Can be used to calculate enthalpy changes of individual reactants or products, or calculate overall enthalpy change

Question 4

Maxwell-Boltzman distribution

Answer 4

• bell curve graph showing energy of molecules
• Ea is activation energy, molecules to it’s right can react
• if temperature increases curve shifts more to the right
• if concentration increases curve increases in height but peaks stay where they are
• catalysts move Ea to the left

Question 5

Standard conditions

Answer 5

• 100kPa

- 298K

Question 6

Calorimetry

Answer 6

• e=mct, energy transferred = mass of substance being heatedspecific heat capacitychange in temperature
• calculate moles of fuel used in heating the substance, calculate the energy transferred and find the enthalpy change of the heat source

Question 7

Bond enthalpy

Answer 7

• mean bond enthalpies will not always be accurate to calculated values as they are averages of many different results
• this is the mean energy needed to break a certain type of bond, can be used in calculations to find enthalpy change