3.1.6Chemical equilibria, Le Chatelier's principle and Kc

Question 1

What two procedures occur when a reversible reaction is at equilibrium?

Answer 1

• forward and reverse reactions proceed at equal rates
• the concentrations of reactants and products remain constant(dynamic )

Question 2

What is the definition of Le Chatelier’s principle?

Answer 2

If a reaction at equilibrium is subjected to change in
Concentration, pressure or temperature, the position
Will contract the change

Question 3

How does Le Chatelier’s principle qualitatively contribute to the effects of equilibrium for changes in temperature?

Answer 3

• increase temp - add heat - equilibrium shift endothermic ( positive ^H)
• decrease temp - remove heat - equilibrium shift - exothermic (- ^H)
  exo forward then endo backward(vise versa)

Question 4

How does le Chateliers principle qualitatively contribute to the effects of equilibrium for changes in concentration?

Answer 4

Increase con of reactant - equilibrium shift right - more product made
Increase con of product - makes reverse reaction fast - equilibrium shift to the left
decrease con ( opposite effect)

Question 5

Why catalyst have no effect on equilibrium

Answer 5

Can’t increase yield , allows reaction go faster

Question 6

Why for a reversible reaction used in industry, a compromise temperature & pressure may be used?

Answer 6

• 300 degrees ( reasonable yield & fast reaction)
  , 60-70 atm( higher - lots of energy - expensive - need special equipment )

Question 7

Information about Kc( equilibrium constant )?

Answer 7

• deduced from equation for a reversible reaction.
• concentration( in mol dm-3), of a species X - expression for Kc is represented by [X]
• value of the equilibrium constant - no affect by changes in concentration/ add a catalyst.

Question 8

What is the expression for Kc for a homogeneous system in equilibrium?

Answer 8

aA+ bB → dD + eE
[D]^d[E]^e / [A]^a [B]^b

Question 9

Steps to calculate the value of kc from equilibrium concentrations for the homogeneous system at constant temp?

Answer 9

1) stick concentrations into the expression for Kc
2) work out the units (moldm^-3), put units in calc then numbers
- no units if balance out

Question 10

Qualitative effects of changes in temperature on the value of kc?

Answer 10

• temp changes of the system - change equilibrium concentration of product/reactant - Kc change
• more product at equilibrium Kc rise. - if less product, Kc decrease

Question 12

Oxidation definition

Answer 12

Process of electron loss & oxidising agents are Electron receptors

Question 12

Reduction Definition

Answer 12

Process of electron gain & reducing agents are electron donors