Chemical Energetics Flashcards
What is ΔH
Enthalpy change
overall energy absorbed or released for one mole of a substance/ molar quantities specified in a chemical reaction
sum of energy absorbed in bond breaking (reactants) + sum of energy released in bond forming (products)
expressed in kJ/mol
What is q
Heat change
q= ΔHr x moles of limiting reagent
What is H
Enthalpy
Energy content in a substance
Bond breaking and Bond forming
BBAE
BFRE
Exothermic
sum of energy absorbed in bond breaking lesser than sum of energy released in bond forming
ΔH -ve
Products are more energetically stable than reactants
Examples: Neutralisation, Condensation, Combustion
Endothermic
sum of energy absorbed in bond breaking greater than sum of energy released in bond forming
ΔH +ve
Reactants are more energetically stable than products
Examples: Photosynthesis, dissolving NH4NH3 (aq) , melting
Standard state
298K
1 bar
Thermochemical equation
Balanced chemical equation
Shows state symbols of substances
Is associated with enthalpy change
Factors affecting the magnitude and sign of ΔH
- Amount of substances
when multiplying n to both sides of the equation , ΔH must be multiplied by n as well - Physical states of reactants and products
3.ΔH of forward and backward reactions
the magnitude of ΔH is the same for forward and backward reactions however
if forward reaction is ΔH -ve, backward reaction will be ΔH +ve
if forward reaction is ΔH +ve backward reaction will be ΔH -ve
Standard enthalpy change of formation ΔHf
The heat change when one mole of a substance is formed from its constituent elements in their standard state 298K, 1 bar
Standard enthalpy change of reaction ΔHr
The heat change when molar quantities of substances react together based on their stochiometric ratio at the standard state 298K and 1 bar
Standard enthalpy change of combustion ΔHc
The heat released when one mole of a substance is completely burnt in excess oxygen at 298K and 1 bar
Standard enthalpy change of neutralisation ΔHn
The heat energy released when an acid reacts with an alkali to give one mole of water at 298K and 1 bar
Standard enthalpy change of hydration ΔHhyd
The energy released when one mole of gaseous ions are hydrated to form an indefinitely dilute solution at 298K and 1 bar
Standard enthalpy change of solution ΔHsol
The energy change when one mole of substance is dissolved in a solvent to form an indefinitely dilute solution at 298K and 1 bar
Standard enthalpy change of atomisation ΔHat
The energy absorbed when one mole of gaseous atoms is formed from its constituent elements at 298K and 1 bar