Unit 1-7 Flashcards
(44 cards)
What are reversible reactions?
Continue indefinitely
Concentrations of reactants and products remain constant
What is dynamic equillibrium?
Rate of backwards and forward reactions are equal
There is no further observable change
What are the steps leading to dynamic equillibrium?
- Forward reaction is fastest at the start
- Forward reaction slows down as reactants are used up and reverse reactions start to increase
- Dynamic equilibrium is reached
What is Le Chatelier’s principle?
If a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise that change
What is the effect of concentration on equillibrium?
A higher concentration of reactants will favour the forward reaction
Equilibrium moves to the right
A higher concentration of products will favour the reverse reaction
Equilibrium moves to the left
What is the effect of pressure on equillibrium?
Increasing the pressure will favour the side with fewer gaseous molecules
Equilibrium shifts to the side with fewer gaseous molecules
Number of molecules is decreased
Decreasing the pressure will favour the side with more gaseous molecules
Equilibrium shifts to the side with more gaseous molecules
Number of molecules is increased
No change when equal numbers of gaseous molecules appear on both sides
What is the effect of temperature on equillibrium?
Increase in an exothermic (-ΔH) reaction will move equilibrium to the left
Increase in an endothermic (+ΔH) reaction will move equilibrium to the right
The reverse reaction is always the opposite enthalpy (ΔH)
What is the effect of a catalyst on equillibrium?
Don’t affect the position of equilibrium
Cause equilibrium to be achieved faster
What is the equillibrium constant Kc?
A constant for concentration (moldm-3)
Given by the ratio between products and reactants
aA+bB→cC+dD
Kc=[C]c[D]d/[A]a[B]b
What is the effect of concentration on equillibrium constant (Kc)?
No effect
What is the effect of pressure on equillibrium constant (Kc)?
No effect
What is the effect of temperature on equillibrium constant (Kc)?
Kc is larger when the forward reaction is favoured
What do square brackets [] denote?
Concentration
What does a large value for Kc indicate?
The reaction is mostly on RHS and favours the products
What does a small value for Kc indicate?
The reaction is mostly on LHS and favours the reactants
What does a medium value for Kc indicate?
“Equalish” reactants and products
What is the unit for Kc?
Varies between reactions
[H2]x[Br2]/[HBr2]2
[moldm-3]x[moldm-3]/[moldm-3]2
No unit
[Cu2+]x[NH3]4/[Cu(NH3)42+]
[moldm-3]4
mol4dm-12
What is heterogenous equillibria?
A system consisting of more than one phase (state of matter)
What is the Haber process?
Nitrogen is obtained from the atmosphere
Hydrogen is obtained from methane
Equation for equilibrium N2(g)+ 3H2(g) ⇄ 2NH3(g)
To separate ammonia from the reaction mixture ammonia is cooled under pressure until it condenses into other liquids
Nitric acid (NH3) is produced from the oxidation of ammonia
What are the conditions for the Haber process?
400-450C°
200atm
Catalyst
Is poisoned over time and must be replaced (deactivation)
Why is 400 - 450C° used for the Haber process?
Low temperature favours the forward exothermic reaction
Rate is too slow so temperature is increased
Why is 200atm of pressure used for the Haber process?
High pressure is optimal, equilibrium shifts to the side with less gaseous molecules
Very high pressures are to expensive and dangerous
What is the conversion of the Haber process?
Conversion of 15%
During the Haber process, what happens to the hydrogen and nitrogen that isn’t converted?
Recycled back into the reaction
