5.6 EQUILIBRIA

Question 1

what does it mean if a chemical reaction is reversible?

Answer 1

it can go backwards and forwards

Question 2

what does the enthalpy change for a reversible reaction always refer to?

Answer 2

the forwards reaction
for the backwards reaction, delta H is the same number but with the opposite sign

Question 3

common reversible reaction 1:

Answer 3

• dehydration of copper sulfate
• to make the reaction go forwards you heat hydrated copper sulfate crystals, the crystals go from blue to white as they lose all their water
• to make the reaction go backwards, you add water to anhydrous copper sulfate, the white powder absorbs water and turns blue in the process

Question 4

common reversible reaction 2:

Answer 4

• thermal decomposition of ammonium chloride
• to make the reaction go forwards, you heat ammonium chloride
• to make the reaction go backwards, you allow NH3 and HCl to mix in the absence of heat, white ammonium chloride forms immediately wherever the cold gases have met

Question 5

how can common reversible reaction 2 be demonstrated?

Answer 5

• all at once by heating NH4Cl in a test tube
• the ammonium chloride at the bottom of the tube decomposes to form invisible NH3 and HCl
• the invisible NH3 and HCl cool as they move up the tube, recombining to form ammonium chloride

Question 6

what is dynamic equilibrium?

Answer 6

when a reversible reaction is left in a sealed container, the forwards and backwards reactions will eventually ‘balance out’, from that point onwards, the mixture of reactants and products in the container is referred to as being in dynamic equilibrium
- the forwards and backwards reactions are still happening, just at the same rate

Question 7

what is equilibrium?

Answer 7

the concentrations of reactants and products remain unchanged

Question 8

what is the equilibrium position?

Answer 8

the resulting balance of product and reactants in an equilibrium mixture

Question 9

how can you tell where an equilibrium is?

Answer 9

by observing or measuring on eo the substances

Question 10

what can the position of equilibrium be shifted by?

Answer 10

• amounts of reactants or products
• gas pressure
• temperature

Question 11

what is Le Chatelier’s principle?

Answer 11

“The equilibrium shifts to counteract any change imposed on it”
- it is not an explanation of why the equilibrium shifts
- it’s just a rule of thumb for working out which way it shifts

Question 12

how would adding a substance affect equilibrium?

Answer 12

in whichever direction uses it up

Question 13

how would removing a substance affect equilibrium?

Answer 13

in whichever direction creates it

Question 14

how would increasing gas pressure affect equilibrium?

Answer 14

in whichever side has the fewest moles of gas
will shift to lower it back down

Question 15

how would decreasing gas pressure affect equilibrium?

Answer 15

in whichever side has the most moles of gas
will shit to bring it back up

Question 16

how would increasing temperature affect equilibrium?

Answer 16

in whichever direction is endothermic
will shift to cool it back down

Question 17

how would decreasing temperature affect equilibrium?

Answer 17

in whichever direction is exothermic
will shift to heat it back up

Question 18

what is the easiest way to change the concentration of a substance in equilibrium?

Answer 18

• add or remove some of it
• if you add something, the equilibrium will shift to remove the thing you just added
• if you remove something, the equilibrium will shift to create more of it

Question 19

do catalysts have an effect on the position of equilibrium?

Answer 19

no