10 - Reaction Rates And Equilibrium Flashcards
What is rate of reaction
Indicates how fast the reactants are being used up or how fast the products are being formed
Graph of ror
- steepest as reactants are in large concen
- less steep as reactants get used up and concen decreases
- hori when reactants fully used up and concen stops changing so ror is 0
Factoring altering ror
- concentration/pressure: more particles in volume, and closer tog so more collisions more successful (effective) higher ror
- temperature: more particles have activation energy or more, increased vibrations and collisions more successful (effective) and higher ror
- use of catalyst: provides alternate reaction pathway with lower activation energy so more particles have activation energy or more, more energy to collide, more frequently and more successful (effective)
- surface area: more reactant particles available for collisions, more successful..
Collision theory
Two particles must collide for reaction to occur
Why are some collisions ineffective
- must collide with correct orientation
- particles have sufficient energy to overcome activation energy barrier of the reaction
Measuring ror
- monitoring removal of product or measuring formation
Measuring ror for gas
- measuring volume of gas produced using gas syringe or upturned water flask
- measuring decrease in mass on a balance using wool (until unchanged mass)
What is a catalyst
Substance that changes (increases) the ror without being used up itself and unchanged
- not used up
- Reacts with reactant to form intermediate/ provide surface for reactants to react on
- at end of reaction is regenerated
What does it actually do?
Provides alternate reaction pathway, with a lower activation energy
Homogeneous catalyst
Has the same physical state as the reactants
E.g. creating esters with sulphuric acid catalyst
Ozone depletion using Cl radicals (g) as a catalyst
How does catalyst for intermediates work - HOMO
Catalyst reacts with reactant to form intermediate and then that breaks down to give product and regenerate catalyst
Heterogeneous catalyst
Catalysts that have a different physical state than the reactants
E.g. hydrogenation of alkenes using Ni catalyst
Hager process using Fe
How do hetero work
Usually solid in contact with gaseous reactants. They are adsorbed (weakly bonded) onto the surface of the catalyst where the reaction takes place. Then product mols leave by desorption
Catalysis - sustainability and economic importance
Around 90% of reactions use catalysts
As they lower activation energy, less temp and pressure requirements needed
Less electricity and fossil fuels burnt and so economic advantages outweigh any costs associated
Why is Boltzmann used
Shows the spread of molecular energies in gas
What does Boltzmann show
- no mols have zero energy - the curve starts at the origin
- area under graph is total no of mols
- no max energy for a mol - curve would need to reach infinite energy to reach x axis
Temperature effect on Boltzmann
As temp increases, average energy also increases, more mols will have higher energy. Graph is now shifted right and lower on y as the peak is at a higher energy. Area is still the same
Catalyst effect on Boltzmann
Lowers activation energy so again, more mols have the new lower, activation energy or more.
What is a dynamic equilibrium
- indicates that rate of forward is = rate of backwards so equal
- however mols still moving and reacting so dynamic
- concentration is constant and not changing
Reversible reactions
Have both forward and backwards reaction
Le chat principle
When an external change is made to the system in equilibrium, the system will adjust to counter the change
Effect of changing concen on eq
If you increase concentration of reactants, the system will shift eq to right to products to increase concen on that side and vise versa
Changing concentration can also occur…
By adding another substance that reacts to decrease thr concentration of one of the reactants on one side
Changing temperature on eq
If increase temp, eq will shift to endothermic side and vise versa
Changing pressure on eq
As you increase pressure, eq shifts to side with less mols (IN GAS)
Effect of catalyst on eq
Catalyst only increases ror without changing eq position and yield
Limitations of changing eq
Increasing pressure: has to be high enough to have a profitable yield and reasonable ror however not too high to be hazardous or expensive
Changing temp: has to be low enough to have reasonable yield however not too low as to have slow ror and not react at all
How to calculate equilibrium constant
- shows actual position of eq
- magnitude shows if eq is with products/reactants
Equilibrium law
Kc = Cc . Dd /Aa . Bb
What does value of Kc show
- 1 = position of eq is halfway between reactant and products
- > 1 = eq position is towards products (higher concen and yield)
- < 1 = eq position is to reactants (higher concen and yield)
