AS.7 Enthalpy Changes Flashcards
Enthalpy
Measure of heat of a system
H
Enthalpy change
Heat change of a chemical reaction associated with an equation
Triangle H
Exothermic
When heat is given from the system to the surroundings
“Release heat”
Chemical system loses energy so enthalpy change is negative
Endothermic
When heat is absorbed from the surroundings to the system
“Absorb heat”
Chemical system gains energy so enthalpy change is positive
Exothermic graph
Reactants are higher than products
Endothermic
Reactants are lower than products
Exothermic bonds
Energy released when making bonds is greater than energy required when breaking bonds
Endothermic bonds
Energy required when breaking bonds is greater than energy released when making bonds
Activation energy
The minimum amount of energy required for a reaction to occur
Drawing Ea on the graph
From reactants to activation energy
Arrow pointing up always
Drawing Ec on a graph
From reactants to lower activations energy
Arrow pointing up always
Drawing enthalpy change on a graph
Reactants to products
If up - endothermic
If down - Exothermic
Standard conditions for enthalpy
100kPa
298K
Enthalpy of combustion
The enthalpy change when one mole of a compound is burned completely in excess oxygen under standard conditions
Enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
Enthalpy of neutralisation
The enthalpy change for the reaction of acid with base to produce one mole of water under standard conditions
Measuring enthalpy changes
Calorimeter
Calculation for enthalpy change
1) Q= mc ^T
2) -Q / n = ^H
Average bond enthalpies
The energy required to break one mole of bonds in gaseous molecules
Calculating bond enthalpies
^rH = bonds broken - bonds made
kJmol-1
Hess law
The enthalpy change for a reaction is the same whichever route is taken from reactants to products
Using enthalpy of combustion data for Hess law
If given combustion data, arrows point down
Using enthalpy of formation data for Hess law
If given formation data, arrows point up
Why are standard enthalpy changes of combustion less exothermic than expected when experimented
Energy lost as heat to surroundings
Non standard conditions
Incomplete combustion
