Topic E: Chemical Energetics Flashcards
What is enthalpy
Enthalpy is a measure of the heat energy contained in a substance. It is stored in the chemical bonds and intermolecular forces as potential energy
How is enthalpy related to stability
The higher the enthalpy of a substance, the less stable it is
What is the formula for enthalpy change in terms of enthalpy of formation
ΔH = (Sum of enthalpy of formation of products) - (Sum of enthalpy of formation of reactants)
What are the key factors affecting enthalpy change
Temperature
Pressure
Concentration of solutions
State of reactants and products
What is standard enthalpy change of formation
Enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions of 298K and 100 kPa
How does the standard enthalpy change of formation relate to stability
In general, the more negative the standard enthalpy change of formation for a compound, the more energetically stable it is
What is standard enthalpy change of combustion
Enthalpy change when one mole of substance in its standard state is completely burned in excess oxygen under standard condition
Why is standard enthalpy change of combustion always negative
Heat is always evolved during combustion
What is standard enthalpy change of neutralisation
Enthalpy change when one mole of water is formed between the neutralisation of an acid and base under standard condition
Why is standard enthalpy change of neutralisation always negative
Heat is always evolved during neutralisation
What is the equation for the enthalpy change of an exothermic and endothermic reaction
Exothermic: ΔH = - (Q/n)
Endothermic: ΔH = + (Q/n)
n is the number of moles of the limiting reagant
What is bond enthalpy
Energy needed to break one mole of bonds in the gaseous molecule, under standard conditions
What is the formula for enthalpy change in terms of enthalpy of combustion
ΔH = (Sum of enthalpy of combustion of reactants) - (Sum of enthalpy of combustion of products)
What is the formula for enthalpy change in terms of bond energy
ΔH = (Sum of bonds broken in reactants) - (Sum of bonds broken in products)
*Only valid for gases
What is lattice energy
Standard enthalpy change when one mole of a solid ionic compound is separated into gaseous ions under standard conditions
What is the relationship between enthalpy change in bond energy and enthalpy change of atomisation
Enthalpy change in bond energy = 2 * (enthalpy change of atomisation)
What is first electron affinity
Standard enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms to form one mole of singly negatively charged gaseous ions
e.g O (g) + e → O- (g)
Why is first electron affinity exothermic
Energy is released due to the attraction formed between the gaseous atom and the electron
What is second electron affinity
Standard enthalpy change when one mole of gaseous electrons is added to one mole of negatively charged gaseous ions to form one mole of doubly negatively charged gaseous ions
e.g O- (g) + e → O2- (g)
Why is second electron affinity endothermic
Energy is absorbed to overcome the repulsion between the singly negatively charged ion and the electron
What is first ionisation energy
The minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly positively charged gaseous ion
e.g Na (g) → Na+ (g) + e
Why is the first ionisation energy endothermic
Energy is absorbed to overcome the effective nuclear charge between positively charged nuclear and the outgoing electron
What is the standard enthalpy change of atomisation
Standard enthalpy change when one mole of gaseous atoms is formed from the element in its standard states
What does change in Gibbs free energy tell us about a reaction
If the change in Gibbs free energy (ΔG) is negative, the reaction is spontaneous
If ΔG is positive, the reaction is non-spontaneous
What is the formula for ΔG
ΔG = ΔH - TΔS
ΔG = (Sum of gibbs free energy of products) - (Sum of gibbs free energy of reactants)
What is the formula for change in entropy
ΔS = (Sum of entropy of products) - (Sum of entropy of reactants)
What does a positive and negative value of ΔS mean?
Positive ΔS:
- The reaction is more feasible
- Final state is more disordered than the initial state
Negative ΔS:
- The reaction is not feasible
- Initial state is more disordered than the final state
What are the factors affecting entropy of a chemical system
Change in temperature
Change in phase
Change in number of particles (especially gas molecules)
Mixing of particles
