Topic 4: Energies Flashcards
Enthalpy change def
Heat or energy change under constant pressure. ΔH
Standard conditions (enthalpy)
Temp. 298K
Pressure 100KPa
Exothermic
Gives out heat, negative enthalpy change
Endothermic
Takes in heat, positive enthalpy change
Enthalpy change unit
KJmol^-1
Energy Level Diagram for Exothermic
Reactants at higher energy level than products. Reactants higher, products lower on diagram.
Energy Level Diagram for Endothermic
Reactants are at lower energy than products. Reactants lower, products higheron diagram.
Activation Energy on energy level diagram
Difference between highest peak and the reactants, much larger for endothermic
Activation Energy
The minimum amount of energy required in order for a successful collision to occur which will cause the reaction to happen.
Enthalpy change calc using bond dissociation energies
enthalpy to break bonds - enthalpy to make bonds
break - make
Calorimetry
DO THIS LATER ABI https://www.youtube.com/watch?v=ErdEAJgUHME
q in calorimetry and equation and units
heat change (J) careful with this as not in kj
m in calorimetry and equation and units
Mass (Mass of water usually) (g)
c in calorimetry and equation and units
Specific heat capacity (JK^-1g^-1)
ΔT in calorimetry and equation and units
Temperature change (K) (As its change in temp, don’t have to convert to K)
Enthalpy change calc steps
q=mcΔT
convert J to KJ
work out moles from mass and mr
will be in heat per moles
divide by itself (mole value) to get heat per one mole
kJmol^-1
Add sign for endo or exothermic
Calorimetry Equation
q= mcΔT
Why is calculated enthalpy change an inaccurate value?
- heat loss
- incomplete combustion can occur
Hess’ Law
The overall enthalpy change for a reaction is the same regardless of the route taken.
Enthalpy of formation
The enthalpy change when one mole of a substance is formed from its elements in their standard states and under standard conditions.
Why doesn’t hydrogen have enthalpy of formation?
Already in standard states so no change would occur.
Arrow direction in Hess’ cycle
Think about which way the arrow would go with enthalpy of formation and what that reaction actually is.
Enthalpy of combustion
The enthalpy change when one mole of a substance is burned completely in oxygen in standard states under standard conditions.
