U3AOS2 - Extent of Chemical Reactions

Question 1

Irreversible Reactions

Answer 1

reactions where the reactants can’t be recovered post reaction

signed with a unidirectional arrow ->

common example: combustion reaction

Question 2

Reversible Reactions

Answer 2

reactions where the reactants can be recovered post reaction

signed with an equilibrium arrow: <->

common example: change of state

Question 3

Extent of Reaction (and complete vs incomplete)

Answer 3

how much of the reactants convert to products

completion - where all the reactants are converted to products
incomplete - when both reactants and products are present

Question 4

Rate of Reaction

Answer 4

How fast the reaction proceeds

Question 5

Do catalysts influence Rate or Extent of Reaction

Answer 5

Only Rate of Reaction

Question 6

Dynamic Equilibirum

Answer 6

Occurs when forwards and backwards reactions are occurring at the same, constant rate, known as dynamic equilibrium

Question 7

5 requirements for a system at dynamic equilibrium

Answer 7

1. the amounts of reactants and products are constant
2. the concentration of reactants and products are constant
3. the pressure of the system is constant
4. the temperature of the system is constant
5. the forward and backward reactions are occurring at the same, constant rate

Question 8

Considerations when using Molar Ratio and Irreversible Reactions

Answer 8

Do not assume that the initial number of reactants will equal the final number of products - as the extent of the reaction will not be complete (use RICEC instead)

However, you can assume that the molar ratios at equilibrium are accurate (i.e. if u have 2 moles of a reactant, and the molar ratio is the same, then you have 2 moles of the product)

Question 9

Concentration-Time Graphs

Answer 9

Used to show the concentration of the products/reactants over time

At equilibrium: the concentration will be constant

Note: Stoich ratios are represented on the graph, so ensure that the spacing of various products/reactants is consistent with the stoich ratios

Question 10

Equilibrium Expression

Answer 10

Constant that defines the concentration of products and reactants at a specific temperature

Question 11

Impact of water (or other solvent) on equilibrium expressions/LCP

Answer 11

Do not include water (when solution is aqueous), or other solvents, in the equilibrium constant, and don’t consider water when discussing LCP
(Q7 VCAA 2019 NHT)

Question 12

Equilibrium Expression (inc units)

Answer 12

Actual value when the concentration of the reactants the products are entered

Units: given that each concentration is measured in M, simplify the number of concentrations (considering the powers), to find the unit - note - it can be M⁰, in which case no units are required

Question 13

Reversing Equilibrium Constant

Answer 13

Use the expression: Kc reverse = 1/Kc

Question 14

Changing Molar Constants with Equilibrium Constant

Answer 14

Multiple the Kc value by factor of molar constant changed - so if molar constants are halved:
Kc = (Kc)^1/2

Question 15

How to find moles at equilibrium from initial?

Answer 15

a RICEC table

R - reactants
I - initial moles
C - change in moles
E - equilibrium moles
C - concentration

Question 16

Reaction Quotient

Answer 16

A value calculated with the equilibrium expression, when the system is not actually at equilibrium

Referred to as Q_c

Question 17

If Q_c is smaller than K_c

Answer 17

K_c needs to increase its value - convert reactants to products - favor the forward reaction

Question 18

If Q_c is bigger than K_c

Answer 18

K_c needs to decrease its value - convert products to reactants- favor the backwards reaction

Question 19

Le Chatelier’s Principle

Answer 19

States that when a system is subject to a change which disrupts equilibrium - the system will move to partially oppose the change, by favoring the forwards or the backwards reaction

Question 20

Factor that can change K_c

Answer 20

Only a change in temp can change K_c!