4. Mocks revision

Question 1

First ionization enthalpy, example

Answer 1

the enthalpy change when one mole of gaseous (+1) ions are formed from one mole of gaseous atoms
Na (g) -> Na+ (g)

Question 2

Lattice enthalpy, example

Answer 2

the enthalpy change when one mole of a solid ionic compound is made from gaseous ions (increases with the increasing charge on the ions and with decreasing size of ions)
Na+ (g) + Cl- (g) -> NaCl (s)

Question 3

Entropy
what is one common property to spontaneous reactions
what does it change with

Answer 3

the thermodynamic property related to randomness, it is a measure of a reaction’s spontaneity. It occurs only at standard conditions of temperature and pressure
the final state is more disordered/random than the original (increase in randomness)
slightly increases with temperature but more with phase changes: S (gases) > S (liquids) > S (gases)

Question 4

How is entropy calculated
what is it’s value for exo and what for endo reactions

Answer 4

ΔS = sum S (products) – sum S (reactants)
exo reaction has a positive ΔS (less order), endo reaction has a negative ΔS (more order)

Question 5

Gibbs Energy (G) definition and formula
Free energy of formation (∆G˚f)

Answer 5

the total E change for the system minus the E lost disordering the system
ΔG = ΔH – TΔS
it is specific for each compound and is zero for an element

Question 6

How do ΔH and ΔS values determine ΔG and whether a reaction is spontaneous or not?

Answer 6

negative ΔH positive ΔS: spontaneous (product favored), ΔG<0
negative ΔH (exo) negative ΔS: more spontaneous at lower temp, ΔG depends on T
positive ΔH (endo) positive ΔS: more spontaneous at higher temp, ΔG depends on T
positive ΔH negative ΔS: nonspontaneous (reactant favored), ΔG>0

Question 7

Rate of reaction, formula

Answer 7

the change in concentration of a reactant/product per unit of time
(Δ[P])/Δt or (- Δ[R])/Δt
in units mol/Ls

Question 8

How can the change in C be measured?

Answer 8

mass (solid) or volume (liquid) change
change in pH (only one base/acid present)
change in conductivity
use of a spectrometer or colorimeter (complex ions)

Question 9

Collision theory, the activation E

Answer 9

before particles react they must first come together (collide), they must collide with appropriate orientation and with sufficient energy (Ea) for bonds to be broken
the minimum energy that needs to be supplied by molecular collisions for a reaction to occur

Question 10

Factors affecting the rate of reaction

Answer 10

1| Concentration of reactants: proportional, the more particles of the reactant there are, the more collisions occur (stationary point)
2| Pressure: proportional
3| Temperature: proportional (increasing molecular movement), stationary point (R used up)
4| Surface area: proportional (powdered form=greater SA)
5| Catalysts: decrease the activation energy

Question 11

How is the general reaction rate calculated?

Answer 11

dividing rate expressions by stoichiometric coefficients
r = - 1/a (Δ[A])/Δt = = 1/c (Δ[C])/Δt

Question 12

The rate law, what is the expression

Answer 12

relates the rate of reaction to the concentrations of reactants r = 〖k[A]〗^m 〖[B]〗^n (the exponents establish the order of a reaction), zero-order doesn’t affect the rate, first-order changes it proportionaly, etc.

Question 13

Draw rate/time and concentration/time graphs for zero, first and second order

Answer 13

…

Question 14

What does the rate constant k depend on?

Answer 14

temperature (proportionally), and the presence/absence of a catalyst (increased by a catalyst) but not on the concentration of reactants – it increases the rate of reaction when T is raised/catalyst added

Question 15

Half-life, formula

Answer 15

the time taken for the concentration of a reactant to fall to a half of its initial value
t_(1/2)= 0.693/k

Question 16

Arrhenius equation, what does it state

Answer 16

lnk= -E_a/RT+lnA, the rate constant increases with increasing temperature

Question 17

Dynamic chemical equilibrium

Answer 17

when the forward reaction and the reverse reaction proceed at the same rate and when the amounts (concentrations) of each reactant and product remain constant, it can be achieved from any direction

Question 18

Formula for reaction quotient and equilibrium constant, explain the differences between them

Answer 18

Q= K=(〖[C]〗^c 〖[D]〗^d)/(〖[A]〗^a 〖[B]〗^b )
equilibrium constant can only be calculated using C(R) and C(P) at the equilibrium and C for quotient can be measured at any time of the reaction

Question 19

What influences the equilibrium constant
how do values of Q and K affect the equilibrium position

Answer 19

it can only be changed with temperature because it is based on the reaction rate expressions of the reactions (constant k)
if Q < K_eq (K>1) equilibrium is shifted to the right (product favored) and if Q > K_eq (K<1) equilibrium is shifted to the left (reactant favored)

Question 20

How does a change in reactant or product concentrations affect the equilibrium/K?

Answer 20

increasing C(R) and decreasing C(P) shifts it to the right, and decreasing C(R) and increaisng C(P) shifts it to the left, doesn’t affect K value