Topic 5 &15 Energetics Flashcards
Enthalpy of Solution
enthalpy change when one mole of solid is dissolved in sufficient solvent to give an infinitely dilute solution of aqueous ions
e.g. NaCl(s) + aq —> Na+(aq) + Cl- (aq)
enthalpy of hydration is always? (endo or exo)
exothermic because attractions are being formed between cations and partially negative oxygen in water and partially positive H+ in water (bond forming will always be exothermic!)
enthalpy of hydration
enthalpy change when one mole of GASEOUS ions is dissolved in sufficient solvent to give an infinitely dilute solution of aqueous ions
e.g. Na+ (g) + aq —> Na+ (aq)
why is the enthalpy of hydration of Cl- less exothermic than Na+
Cl- has a larger atomic radius than Na+ and therefore weaker attractions with water, so it is less exothermic
lattices with ions of (a) charge and (b) radii have more endothermic lattice energy values than those with ions of __(c)__ charge and _(d)__ radii
a) higher
b) smaller
c) lower
d) larger
stronger ionic bonding means a more _(endo or exotheric)__ lattice energy
endothermic,because it requires more energy to break the lattice
standard enthalpy of formation
enthalpy change that occurs when one mole of substance is formed from its elements in their standard states under standard conditions of 100kPa and 298K
standard enthalpy change of formation of an element in its most stable form is equal to:
zero
enthalpy of combustion formula
= sum of combustion of reactants - sum of combustion of products
Hess’s law
enthalpy change for any reaction is independent of the route by which the rxn takes place, provided the starting and final conditions, and rxns and products, are the same
enthalpy of formation formula
sum of formation of products - sum of formation of reactants
entropy
measure of disorder or randomness
molecules with more atoms have ___ standard entropies than those with fewer atoms
higher
as temperature increases, the entropy of the chemical
increases
entropy change of a system formula
= sum of entropy of products - sum of entropy of reactants
why is breaking bonds endothermic
energy is required to overcome electrostatic attraction between shared pair of electrons and positive nuclei of bonded atoms
average bond enthalpy
energy needed to break one mole of bonds of gaseous molecules under standard conditions, averaged from values of bond enthalpy in a range of similar molecules
first electron affinity
energy change associated when 1 mole of electrons is added/gained by 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions
how does first electron affinity change down a group
gets less exothermic because incoming e- and nucleus is weaker as the atomic radius increases
standard enthalpy of atomization
enthalpy change when one mole of gaseous atoms is formed from element in its standard state under standard conditions
e.g. Na(s) -> Na(g)
or for a diatomic molecule:
e.g. chlorine: 1/2Cl2(g) -> 1 Cl (g)
lattice energy
energy change when one mole of an ionic solid is broken down into its constituent gaseous ions under standard conditions
first ionization energy
amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly charged positive gaseous ions
which two pieces of information do you need to calculate the enthalpy of solution
- lattice enthalpy i.e. NaCl (s) -> Na+ (g) + Cl-(g)
- enthalpy of hydration i.e. (g) -> (aq)
how would a larger ionic radius affect the enthalpy of hydration
larger ionic radius = weaker attractions with water therefore less exothermic
which reactions are exothermic
neutralization, combustion, condensation
is first electron affinity generally endo or exothermic and why
exothermic, because attractions are forming between the positive charge of the atom’s nucleus and the electron
why is the second electron affinity always endothermic
an electron is being added to an ion which is already negative resulting in repulsion, which requires energy to overcome
which equation corresponds to the bond enthalpy of H-I bond
HI(g) -> H(g) + I(g)
if Kc > Q what does this mean for the equilibrium and the value of delta G
reaction proceeds in the forward direction, so delta G is < 0
if Kc < Q what does it mean for the eqm and value of delta G
reaction proceeds in the reverse direction, so delta G is > 0
if Kc = Q what does it mean for eqm and value of delta G
rxn @ eqm; delta G = 0
homogenous catalyst
in the same state (phase) as the reactant
in what region of the em spectrum does ozone absorb
infrared, which raises the temperature of the stratosphere, contributing to global warming
why does oxygen have a stronger bond strength than ozone
it has a double bond and bond order of two compared to ozone, which has a bond order of 1.5, so it absorbs shorter wavelengths of light because it requires more energy to break the bonds in oxygen than ozone
standard enthalpy of combustion
enthalpy change that occurs when one mole of substance burns completely under standard conditions
what is combustion always (endo or exo)
exothermic
what is the significance of Q
reaction quotient; measure of relative amounts of products or reactants for a reaction that is not yet at eqm (also indicates in which direction the rxn is likely to proceed)
