3.1.1 Chemical Equilibrium

Question 1

Define an open system

Answer 1

energy and matter exchanged with surroundings

Question 2

Define a closed system

Answer 2

energy only exchanged with surroundings

Question 3

Is a physical change reversible?

Answer 3

mostly yes, as only altering states
- eg. ice melting and freezing

Question 4

is a chemical change reversible?

Answer 4

mostly not (irreversible) as stronger forces are needing to be broken: covalent, ionic and metallic
- eg. combustion

Question 5

Describe the observable physical changes at a molecular level

Answer 5

• Initially there is a rapid change from one state to another that slows as equilibrium is approached
• Once equilibrium is achieved there is no change in the amounts of each state present as te rate of change is equal
• No observable changes at equilibrium

Question 6

Describe the observable physical changes at an atomic level

Answer 6

• When at equilibrium at an atomic level there is still atoms constantly changing between states

Question 7

Describe the observable chemical changes at a molecular level

Answer 7

• initially there is a rapid loss of reactants and formation of products that slows as equilibrium is approached
• once equilibrium is achieved there is no change in the concentrations of reactants and products as the rate of change is equal
• no observable changes at equilibrium

Question 8

Describe the observable chemical changes at an atomic level

Answer 8

• when at equilibrium at an atomic level there is still a constant reacting of the reactants into products and products into reactants
• the rate is equal no overall net change in concentration is seen

Question 9

what is the symbol used in balanced chemical equations that symbolizes equilibrium equations?

Answer 9

refer to picture

Question 10

is dynamic equilibrium only reached in an open or closed system?

Answer 10

closed

Question 11

what happens overtime to physical changes and reversible chemical reactions in a closed system?

Answer 11

they reach a state of dynamic equilibrium, with the relative concentrations of products and reactants defining the position of equilibrium

Question 12

what is dynamic equilibrium?

Answer 12

• when the rate of the forward reaction is equal to the rate of the reverse reaction

Question 13

how can you tell when dynamic equilibrium has been reached?

Answer 13

No observable changes in the reactants or products:
- the forwards and reverse reactions occur at the same time
- the forwards and reverse reactions occur at the same rate
- the concentration of the reactants and products have reached a balance
- the concentration of the reactants and products aren’t changing

Question 14

which 2 graphs display dynamic equilibrium?

Answer 14

• a rate graph
• a concentration graph

Question 15

how would the activation energies of the forward and reverse reactions influence the reversibility of chemical reactions?

Answer 15

exothermic enthalpy:
- the forward reaction requires less activation energy than the reverse reaction, and thus an exothermic reaction will always favor the forward reaction

endothermic enthalpy:
- the reverse reaction requires less activation energy than the forward reaction, and thus an endothermic reaction will always favor the reverse reaction

Question 16

what does the concentration graph look like when displaying dynamic equilibrium?

Answer 16

refer to photo

Question 17

how can you identify the equilibrium position on a concentration graph?

Answer 17

Where all of the individual elements reach equilibrium on the graph evident by the plateau
eg. At a time of 2mins:
- H2 reached equilibrium at a concentration of 2M
- N2 at 0.6M
- NH3 at 0.4M at a time of 2mins

Question 18

How do you calculate the equilibrium constant?

Answer 18

use the Kc formula