Chemical Energetics I Flashcards
Enthalpy and enthalpy change
Enthalpy of a system is measure of energy content of the system, the higher the enthalpy, the more unstable it is
Enthalpy change is difference between quantity of heat absorbed to break bonds in reactants and that released during formation of new bonds in products at constant pressure
Difference between exothermic and endothermic reaction
Definition:
Difference between exothermic and endothermic reaction (Definition, temperature and enthalpy change)
Definition:
EXOTHERMIC is a reaction where energy is released to surroundings
ENDOTHERMIC is reaction where energy is absorbed from surroundings
Temperature Change:
EXOTHERMIC: Temperature of surroundings increase
ENDOTHERMIC: Temperature of surroundings decreases
Enthalpy Change:
EXOTHERMIC: Enthalpy change is negative, products energetically more stable than reactants
ENDOTHERMIC: Enthalpy change ie positive, products energetically less stable than reactants
Which reaction is more energetically feasible, does that mean they will occur?
Exothermic reactions are more energetically feasible and more likely to occur
Enthalpy change is indication of energetic stability and not its kinetic stability of products with respect to reactants
Standard conditions
Pressure at one bar
Temp usually at 298K
Substances involved at standard state (its most stable form at above conditions)
Definitions of different types of enthalpy change (TEMPLATE)
(Name of process) is the energy (change/ released/ required) when one mole of (substances in specified physical state) is (description of process) under standard conditions.
Standard enthalpy change of reaction
What does kJ mol-1
Energy change in a chemical reaction when molar quantities of reactants stated in the chemical equation react under standard conditions.
Related to amounts of all substances given in balanced equation
Standard enthalpy change of formation of a substance
How can this predict stability of compound?
Energy change when 1 mole of pure substance in specified state is formed from its constituent elements in their standard states under standard conditions.
The more negative the enthalpy change of a compound, the more stable the compound is relative to its constituent elements and less likely decomposition of compound back into its constituent elements
Standard enthalpy change of formation of an element
Standard enthalpy change of formation of an element in its standard state under standard conditions is zero
Standard enthalpy change of combustion
How can this indicate energy values of fuel?
Energy released when 1 mole of the substance is completely burnt in excess oxygen under standard conditions
More heat being liberated upon complete combustion, the better the fuel
Standard enthalpy change of neutralisation
Why is enthalpy change the same for all strong acids and strong bases?
Energy released when acid and base react to form 1 mole of water under standard conditions
Strong acid undergoes complete dissociation in aqueous solution, while strong base undergoes complete ionisation in aqueous solution. Reaction between acid and base in aqueous solution is the reaction between H+ and OH- to produce one mole of H2O.
Why is neutralisation between weak acid/ base less exothermic?
Weak acid and base molecules have to undergo further dissociation/ ionisation, which is endothermic. Since some energy is consumed to bring about further dissociation/ ionisation of molecules during neutralisation reaction, energy released from reaction will be less.
Standard enthalpy change of atomisation of element
Standard enthalpy change of atomisation
Why are they always positive?
Energy absorbed when 1 mole of gaseous atoms (need to specify) is formed from element (at standard state) under standard conditions
Energy absorbed when 1 mole of compound (specify) is converted to gaseous atoms under standard conditions
Always positive because energy must be absorbed to break all bonds between atoms in element/ compound during atomisation reaction
Bond dissociation energy
Always positive? What about reverse reaction?
Why different BDEs for each different bond?
Energy required to break 1 mole of that particular bond (X-Y) in a particular compound (specify) in gaseous state
Always positive because bond breaking requires energy while bond forming releases energy
Chemical environment when each bond is broken is different
Bond energy
Enthalpy change of atomisation of diatomic gases
Average energy required to break 1 mole of bond in gaseous state
For diatomic gas molecules, bond energy= 2 times of standard enthalpy change of atomisation of the gas molecule
