Unit 3 - Chapter 7

Question 1

Spontaneous reactions

Answer 1

spontaneity is not related to rate
a spontaneous reaction is one that will occur all by itself (without being driven by external forces)
this may be so slow as to appear that nothing is happening, this is common for reactions with high activation energies

Question 2

Enthalpy change

Answer 2

the change in chemical energy of a system as reactants are converted to products during a reaction
spontaneity is driven by a system achieving lower energy, therefore exothermic reactions favour spontaneity as they increase entropy of surroundings

Question 3

Entropy

Answer 3

measures the spontaneous dispersal of energy at a given Kelvin temperature
entropy increases as energy becomes more dispersed

Question 4

Entropy change

Answer 4

measure of the kinetic energy and phase change that has been dispersed in a system as a result of a reaction at a given Kelvin temperature
spontaneity is driven by the dispersal of energy, and therefore a positive change in entropy, increases entropy of the system

Question 5

Signs of increasing entropy

Answer 5

there are more particles in the products than in the reactants
a phase change occurs that increases the disorder of the particles (solid to liquid, liquid to gas)
a solute dissolves in a solvent (pure substance to aqueous solution)

Question 6

Gibb’s free energy

Answer 6

measure of total energy that is spread out in the universe due to a reaction that has taken place in a system
spontaneity is drive by an exergonic (negative) Gibb’s energy

Question 7

Gibb’s equation

Answer 7

change in enthalpy = energy dispersed between system and surroundings
(temperature)(change in entropy) = energy that is dispersed within the system, products compared to reactants
for a reaction to be spontaneous, the entropy of the universe must increase (universe = system plus surroundings)

Question 8

Reaction spontaneity

Answer 8

negative H, positive S = always spontaneous (enthalpy and entropy driven)
positive H, negative S = never spontaneous
negative H, negative S = sometimes spontaneous (enthalpy driven), can be true at low temperatures
positive H, positive S = sometimes spontaneous (entropy driven), can be true at high temperatures

Question 9

Reaction reversibility

Answer 9

under the right conditions, some reactions can be spontaneous, and the reverse of the reaction may also be spontaneous
reactions like these are reversible
reversible reactions, in a closed system under constant temperature and pressure conditions will reach a state of dynamic equilibrium

Question 10

Equilibrium

Answer 10

equilibrium = the state that exists when the rate of the forward reaction equals the rate of the reverse reaction
at equilibrium, the macroscopic properties of the system remain constant, and the reaction appears to have stopped
the same equilibrium concentrations are reached whether equilibrium is approached from the forward or the reverse direction

Question 11

Solubility equilibrium

Answer 11

by stirring, we can remove the concentration gradient which increases the dissolution, as there are more water molecules around the solid
dissolving and recrystallization is always occurring, even at equilibrium

Question 12

Process of reversible reactions

Answer 12

A and B are added to a closed flask and allowed to react
the forward reaction will begin at a rapid rate, as there is a high concentration of A and B, and no AB present
as the concentration of A and B decrease, the rate of the forward reaction decreases
as this occurs, the reverse reaction begins and increases in rate
the reaction rate of both reactions eventually meet and equilibrium is established

Question 13

The equilibrium constant

Answer 13

constant at constant temperatures
large value of ke = at equilibrium, large concentration of products, small concentration of reactants, the forward reaction leads to greater than 50% yield
small value of ke = at equilibrium, small concentration of products, large concentration of reactants, forward reaction leads to less than 50% yield

Question 14

Heterogenous equilibrium

Answer 14

pure solids and pure liquids have fixed concentrations based on the number of particles per unit of volume
changing the mass or moles therefore does not alter the density, and concentration of solids and pure liquids therefore cannot change
rates involving pure solids/liquids are not affected by the amount of these pure substances
the density and surface area factors associated with the pure solids/liquids is incorporated into the rate constant

Question 15

LeChatelier’s principle

Answer 15

when a stress is applied to an equilibrium system, the system will respond by temporarily favouring the reaction that will offset the stress and return the system to equilibrium
stress = any factor that asymmetrically alters the rates of the opposing reactions

Question 16

Concentration as a stress

Answer 16

increasing concentration of reactants favours forward reaction
decreasing concentration of reactants favours reverse reaction
increasing concentration of products favours reverse reaction
decreasing concentration of products favours forward reaction

Question 17

Temperature as a stress

Answer 17

changing the temperature will affect the rate of both the forward and the reverse reactions, but the reactions are affected differently
higher temperature favours the endothermic reaction
lower temperature favours the exothermic reaction

Question 18

Catalyst as a stress

Answer 18

a catalyst increases the rate of both forward and reverse reactions equally
a catalyst will cause a system to reach equilibrium faster, but has no effect on a system that is already at equilibrium, and is therefore not a stress

Question 19

Pressure as a stress

Answer 19

increasing the pressure by decreasing the volume favours the reaction that produces fewer moles of gas
decreasing the pressure by increasing the volume favours the reaction that produces more moles of gas
increasing pressure by adding inert gases does not affect the equilibrium, and is therefore not a stress

Question 20

Haber process

Answer 20

nitrogen and hydrogen gas are added to a closed reaction chamber
in the reaction chamber, there is a catalyst, high temperatures, high pressure
as the reaction occurs, the ammonia is removed