EQUILIBRIUM

Question 1

reversible reactions

Answer 1

• some chemical reactions occur in both forward and reverse directions → reversible reactions
  
  • many reactions don’t proceed to completion
  • physical changes are also reversible
    
    • changes in states

Question 2

equilibrium

Answer 2

• some reversible reactions reach a point where the concentrations of reactants and products remain constant
  
  • occur when the rate of the forward and reverse reactions are equal
  • both reactions are still occurring, but there is no observable net change
• a state of equilibrium has been used when the overall concentration of reactants and products don’t change over time

Question 3

open vs closed systems

Answer 3

• open systems
  
  • matter and energy can be exchanged between systems and surroundings
• closed systems
  
  • only energy can be exchanged between system and surroundings
• equilibrium can only be reached in CLOSED SYSTEMS

Question 4

dynamic equilbrium

Answer 4

• equilibrium is described as being in the dynamic state (constant activity)
• forward and reverse reactions haven’t stopped → are occuring simultaneously at the same rate
• reaction is ‘incomplete’ and reactants and products are present in the equilibrium mixture
• molecular level
  
  • bonds are constantly being broken and new bonds are being formed
  • reactants and products continue to be converted from one to another

Question 5

reversibility of the reaction

Answer 5

• once product forms, reverse process may occur
  
  • if the product particles collide with enough energy at the correct orientation, bonds may break and the original reactions may reform

Question 6

2 graphs for equilibrium

Answer 6

• rate vs time
• concentration vs time
  
  • remember to double check the axis to check which graph is being shown

Question 7

observing extent of reaction using colour

Answer 7

if one of the products of reaction is coloured, progress of reaction can be monitored by observing a colour change

Question 8

observing extent of reaction using conductivity

Answer 8

• can be used if the reaction involves the production of ions
• different reactions proceed to different extents
  
  • ratios of reactants to products are different for different equilibrium systems
• HCl: strong acid - easily ionised in water - higher conductivity
• CH3COOH: weak acid - doesn’t ionise easily → low conductivity
  
  • HCl reaction will proceed to a greater extent even if starting concentrations of both are the same

Question 9

extent vs rate of reaction

Answer 9

• extent of reaction
  
  • describes how much product is formed when the system reaches equilibrium
  • won’t give any info on how fast a reacion will happen
• rate of reaction
  
  • a measure of change in concentration of reactants and products oVer time
  • rate will determine how quickly an equilibrium is reached
  • in reversible reaction, the rate changes as the reaction progresses

Question 10

reaction quotient (Q)

Answer 10

• concentration fraction of the mixture
• measures the relative amounts of products and reactants present during a reaction at a particular point in time
• concentration of products divided by the concentration of the reactants
• the coefficient of the reactants and products form indices

Question 11

reaction quotient (Q) vs equilibrium constant (K)

Answer 11

• the reaction quotient can be calculated for any micture of reactants and products at any time during a reaction
  
  • value of Q changes as the reaction progresses
• at equilibrium the value will become constant as concentrations no longer change - known as K
  
  • uses concentrations at equilibrium

Question 12

equilibrium constant K

Answer 12

• will give a consistent value when reactions are in equilibrium despite the different concentrations
• is different for different chemical reactions and at different temperatures

Question 13

what does Q tell us

Answer 13

if the reaction quotient (Q) is GREATER than K

• the system will shift to the left to reach equilibrium
• there will be a net reverse reaction with more reactants formed

if the reaction quotient (Q) is SMALLER than K

• the system will shift to the right to reach equilibrium
• there will be a net forward reaction with more products formed

if the reaction quotient Q is the same as K

• the system is at equilibrium

Question 14

how pressure affects volume

Answer 14

volume of gas is inversely proportionate to its pressure

Question 15

total pressure

Answer 15

• sum of partial pressure

Question 16

changes to equilibrium systems

Answer 16

• the extent of a reaction is affected by the reaction conditions
• relative amounts of reactants and products at equilibrium is referred to as position of equilbrium
  
  • are often changed to maximise production of a desired product
  • don’t confuse w equilibrium constant (K) - only changed by temp

Question 17

le chateliers principle

Answer 17

• if an equilibrium system is subject to change, the system will adjust itself to partially oppose the effect of change
  
  • establishes new equilibrium
• LCP doesn’t provide explanation for these changes
  
  • useful generalisation to help predict effect of changes to equilibrium systems

Question 18

reactant/product change for equilibrium

Answer 18

adding a reactant

• formation of more products
• net forward reaction
• equilibrium position shifts to the right

adding a product

• formation of more reactants
• net reverse reaction
• equilibrium position shifts to left

removing a product

• formation of more products
• net forward reaction
• equilibrium position shifts to the right

Question 19

effect of changing pressure/volume

ON EQUILIBRIUM SYSTEMS

Answer 19

only affects equilibrium positions of systems in the gas phase not solid or liquid

• particles are too tightly packed for an increase in pressure to have a noticeable effect on volume

when pressure is increased

• LCP states that the system will respond by reducing the pressure
• position of equilibrium will favour the side of the reaction with the least number of gas particles
• increased pressure → decreased volume of container → increase in partial pressures → increase in concentration

explaining using equilibrium law

• if volume is halved - partial pressures of all reactants and product gases double so concentration doubles
• when there are equal numbers of reactant and product particles, change in pressure won’t shift the position of equilibrium

Question 20

effect of adding inert gas

Answer 20

• overall pressure is increased
• partial pressures and concentrations of reactants and products is unchanged
• no effect on reaction quotient

Question 21

effect of dilution

Answer 21

• adding water decreased the concentration
• this shifts positon of equilibrium toward the side that produces the greatest number of dissolved particles

Question 22

effect of increasing temp

on equilibrium systems

Answer 22

increasing temp

• exothermic
  
  • increases the energy of the substances in the micture
  • applying LCP, the reaction opposes the increase in energy by absorbing the energy
  • reaction will move to the left producing more reactants
  • concentration of reactants increase and concentration of products decrease
  • K decreases
• endothermic
  
  • increases the energy of the substances in the mixture
  • applying LCP, the reaction opposes the increase in energy by absorbing the energy
  • reaction will move to the right producing more products
  • K increases

Question 23

effect of decreasing temp on position of equilibrium

Answer 23

• Exothermic
  
  • decreases the energy of the substances in the mixture
  • applying LCP, the reaction opposes the increase in energy by releasing the energy
  • reaction will move to the right producing more products
  • K increases
• endothermic
  
  • decreases the energy of the substances in the mixture
  • applying LCP, the reaction opposes the increase in energy by releasing the energy
  • reaction will move to the left producing more reactants
  • K decreases
  gradual decrease/increase in concentration not instantaneous when drawing graph

Question 24

effect of catalyst on equilibrium

Answer 24

lowers activation energy for both the forward and reverse reaction by the same amount
SAME AMOUNT SPECIFICALLY FOR THE ACTIVATION ENERGY NOT THE REACTION RATE

• lower activation energy increases the number of effective collisions for both forward and reverse reactions
• increases rate of both equally
• will not affect the position of equilibrium or equilibrium constant
• will increase rate that equilibrium is reached - allow system to reach equilibrium more quickly

Question 25

what does value of K mean

Answer 25

• indicates the extent of a reaction at equilibrium
  
  • how far the forward reaction proceeds to reach equilibrium and the equilibrium yield (amount of products present at equilibrium)

Question 26

K and relative proportions of reactant and products

Answer 26

very large greater than 10^4

• almost complete reaction occurs
• concentration of products are much higher than the concentration of reactants at equilibrium

very small, less than 10^4

• negligible reaction occurs
• concentration of products are much lower than the concentration of reactants at equilibrium

between 10^-4 and 10^4

• extent of reaction is significant
• considerable concentrations of both reactants and products are present at equilibrium

Question 27

structuring equilibrium responses

Answer 27

• decide what change is immediately occurring
• explain how the system will oppose the change
• explain what effect that will have on the position of equilibirum, yield of products, or K - depends on qs

Question 28

reversible vs irreversible reactions

Answer 28

• in reversible reaction, there is continuous interconversion between the chemicals on either side of the reaction equation.
• In an irreversible reaction, the reactants are converted to products, but the products cannot be converted back to reactants.