Physical Chemistry Flashcards
What is the Equilibrium Position ?
When reactants are mixed together a reaction will proceed towards products until this point is reached.
The equilibrium is dynamic- expand on this…
Reactions are still taking place at equilibrium in the forward and reverse reactions but the concentrations of the reactants/products remain constant.
What happens to the rate when a system is not at equilibrium ?
The forward and reverse reactions both occur but at different rates and hence there is a change of concentration.
Name an example on an equilibrium reaction involving Nitrogen:
Haber Process
N2 + 3H2 <=> 2NH3
Equilibrium favours the products when K is …
Greater than 1 (K>1)
Equilibrium favours the reactants when K is …
Less than 1 (K<1)
How do you calculate the equilibrium constant ?
K = [C]^c[D]^d/ [A]^a[B]^b
What is used to calculate the equilibrium constant ?
Molar concentration
K(back)=
1/ K(forward)
How do you calculate the equilibrium constant of the reverse reaction ?
K = [A]^a[B]^b/ [C]^c[D]^d
How do you calculate K(muit) ?
K(muit)=( [A]^a[B]^b/ [C]^c[D]^d )^n
When K<1 then…
equilibrium lies to the left hence there is no yield
When K >1 then …
equilibrium lies to the right hence there is a good yield.
When K~1
[Reactants]~[products] - minimal yield
What is Homogeneous ?
All reactants and products in a single phase
What is Heterogenous ?
Reactants and products exist in different phases.
Why is liquid and solids not included in equilibrium ?
They have a fixed concentration
How do you combine equilibria ?
- Combine reactants
- Reverse reaction if need (Kback= 1/Kforw)
- Balance (Kmulti = K^n)
- Add reactions and cancel species
- Find K
What is an Exothermic reaction ?
Releases heat energy (-ve ΔH)
What is an Endothermic reaction ?
Absorbs heat energy (+ve ΔH)
What affects the equilibrium constant ?
Temperature
How does temperature effect K ?
Rate almost always increases at higher temperature
How does K relate to rate ? (formula)
rate (forward) = K forward [Reactants]
rate (backwards) = K backwards [Products]
What is an ice table ?
Reactants, Products
Initial concentration
Change in concentration
Equilibrium concentration
What are the steps for calculating equilibrium using ICE table ?
- Balance Equation
- ICE table
- Find initial data
- Find change in stoichiometry
- Find equilibrium conc. by solving for X
- use equilibrium constant formula
How do you calculate Qc ?
Qc = [Concentration products]/ [concentration reactants]
What does Qc < Kc mean ? (forward or reverse)
forward
What does Qc = Kc mean ?
Reaction is at equilibrium
What does Le Chateliers Principle state ?
“If a system at chemical equilibrium is disturbed by changing the reaction conditions, the composition of the system will change in order to minimise the effect.”
What effect does adding reactant have on the K ?
Adding reactant, increases concentration of product
What effect does removing product have on K ?
Removing product, decreases concentration of reactant
What effect does increasing pressure have on a reaction where there are 3 moles of reactant and 2 moles of product ?
Increases product yield
What effect does adding an Inert gas have on equillibria ?
No effect - as the concentration is unaffected
What two things in a reaction does temperature effect ?
- Equilibrium position
- The rate at which equilibrium is reached
What effect does increasing temperature have on an exothermic reaction ?
Decreases yield and reverse reaction is favoured
What can reduce the time taken for a reaction to reach equilibrium and what are the exceptions to this ?
- Adding a catalyst
- Increasing temp - if reaction is exothermic- this must be controlled between speed and yield.
What is a catalyst ?
A catalyst reduced the activation energy hence speeds up the rate at which equilibrium is reached. A catalyst is involved in the reaction but if not involved in the reaction.
What are some examples of HETEROGERNOUS catalysts and how do they work ?
Example: Solids such as metals or metal oxides.
The catalyst absorbs onto the surface of the molecule and the reaction takes place at interface.
Exothermic is +ve true or false ?
False
Hess’s law states…
“If a chemical reaction can be written as the sum of two other reactions, the heat absorbed at constant pressure will be the sum of values for separate reactions.”
What is ‘Enthalpy of formation’ ?
Enthalpy for a reaction in which a compound is formed directly from its elements
What is the ‘First law of thermodynamics’ ?
“The total energy of an isolated system is constant - energy can be transformed by not destroyed.”
What does Won stand for ?
Work done ON the system
What does qIN stand for ?
Heat transferred INTO the system
What is the delta U ?
Internal energy
What is the equilibrium constant for water ?
K = [H3O][OH-]
What is an acid ?
Capable of donating protons
What is a base ?
Capable of accepting protons
Amphiprotic meaning
Can function as both an acid and a base
How do you calculate the equilibrium constant for a WEAK ACID ?
K = [H+][A-]/[HA]
As Ka increases what happens to strength of acid ?
increases
As Ka increases what happens to strength of base ?
Decreases
Some compounds have multiple Ka values, why does this happen ?
The negative charge on the anion makes it harder to remove protons
What is the Henderson-Hasselbalch equation (H-H) ?
pH = pKa + log10 [conj base]/[conj acid]
What makes a buffer ?
Weak acid and salt of conj base
Weak base and salt of conj acid
