Chemical Energetics: Thermochemistry & Thermodynamics Flashcards
standard enthalpy change of reaction ΔH°r definition
the enthalpy change when molar quantities of reactants as specified by the chemical equation react to form products at standard conditions, 1 bar and 298 K.
standard enthalpy change of formation ΔH°f definition
the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 298K and 1 bar
standard enthalpy change of combustion ΔH°c definition
the heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298K and 1 bar
standard enthalpy change neutralisation ΔH°neut definition
the heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base at 298K and 1 bar
formula of heat change of solution
q = m c ΔT
q = C ΔT
ΔH of an endothermic reaction and an exothermic reaction equals…
ΔH = +q / n
ΔH = -q / n
Hess’ law of constant heat summation
The enthalpy change (ΔH) of a reaction is determined only by the initial and final states and is independent of the reaction pathway taken
bond energy definition
bond energy is the energy required to break 1 mole of a covalent bond in the gaseous state
ΔHr = ?
ΔHr = BE bonds broken - BE bonds formed
standard enthalpy change of atomisation ΔH°atom definition
the energy required when 1 mole of gaseous atoms is formed from the element at 298K and 1 bar (element)
the energy required to convert 1 mole of the compound into gaseous atoms at 298K and 1 bar (compound)
Lattice energy definition
lattice energy is the heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions.
ionisation affinity definition
the first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous atoms
why is the first EA always a negative
effective nuclear charge of an atom leads to an attraction of the incoming electron. Stronger attraction, more energy given off, EA more negative
why is the second EA positive
energy is required to overcome the electrostatic repulsion between the incoming electron and anion
what does a difference between the theoretical and experimental LE show?
theoretical (assumes that it is completely ionic) and experimental (uses born-haber cycle) LE shows that there is covalent character in the ionic compound. This is most apparent when a cation with a high charge density distorts an anion with a large electron cloud
standard enthalpy change of hydration ΔH°hyd definition
the heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar.
standard enthalpy change of solution ΔH°sol definition
the enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at 298K and 1 bar
what does ΔH°sol > or < 0 mean?
ΔH°sol > 0: salt is insoluble
ΔH°sol <0: salt is soluble
entropy definition
entropy (S) is a measure of the randomness or disorder in a system, reflected in the number of ways that the energy of a system can be distributed through the motion of its particles
when is ΔS > or < 0?
ΔS > 0 if a reaction results in more ways to disperse or distribute the energy
ΔS < 0 if a reaction results in less ways to disperse or distribute the energy
what are some factors that affects ΔS
change in temperature
change in phase
change in the number of gas particles
mixing of particles
dissolution of an ionic solid
change in temperature
as temperature increases, the average kinetic energy of the particles and the range of energies increases. There are more ways to disperse the energy among the particles. Hence, entropy increases.
change in phase
the particles in the solid state vibrate about their fixed positions. The energy is thus the least dispersed and the solid has the lowest entropy. when the solid melts, the particles move more freely in the liquid state and become more disordered. Hence there is an abrupt increase in entropy as there are more ways to distribute the particles and their energy in the liquid state.
during vaporisation, in gaseous state, particles are able to move even more freely. Hence there is a large increase in entropy as there are more ways to distribute the particles and their energy in the gaseous state.
change in the number of particles
when there is an increase in the number of gas particles, the particles in gas are the most disordered and the number of ways that the particles and the energy can be distributed increase greatly.
mixing of particles
expansion of gases
as the volume available for each gas increase (or distribution of the particles for expansion of gas), there are more ways to distribute their particles and hence their energy. entropy increased.
dissolution of an ionic solid
2 cases
entropy increases because the ions in the solid are free to move in solution
entropy decreases because water molecules that were originally free to move become restricted in motion as they arrange themselves around the ions
depends on which is more significant
formula for gibbs free energy change
ΔG = ΔH - TΔS
what does ΔG < or > 0 mean
ΔG > 0: reaction is not spontaneous
ΔG = 0: reaction is at equilibrium, there is no net reaction in the forward or backward direction
ΔG < 0: reaction is spontaneous
(u want a reaction to be spontaneous)
what are the limitations of ΔG°
ΔG° can only be used to predict spontaneity under standard conditions
while spontaneity can be determined, it does not take into account the kinetics of the reaction, which is the rate at which the reaction takes place
