Chapter 10 - Reaction Rates + Dynamic Equillibrium Flashcards
State the collision theory
for a chemical reaction to take place the particles need to collide with each other in the correct orientation and with enough energy
What is the activation energy
• reactant particles need to overcome a minimum amount of energy
Draw a enthalpy profile for a endothermic reaction
Draw a enthalpy profile for an exothermic reaction
Effect of increased concentration on rate of reaction
• more concentrated = the greater the number of particles in a given volume of solvent
• increased collision frequency and therefore an increased rate of reaction
Effect of increased pressure on rate of reaction
• When the pressure is increased, the molecules have less space in which they can move
• number of effective collisions increases due to an increased collision frequency = increases the rate of reaction
Effect of increased temp on rate of reaction
• Increase temp = greater KE = higher collision frequency
• Greater PE = greater proportion meets activation energy = more successful collisions
What happens to rate of reaction if you double concentration
double the number of particles per unit volume and double the frequency of effective collisions
Rate of Reaction = formula
Why is the rate of the reaction not the same throughout the reaction
• During a reaction, the reactants are used up and changed into products
• This means that as the reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing
Define a catalyst
• substance that increases the rate of reaction by providing the particles with an alternative mechanism with a lower activation energy = doesn’t get used up
Draw a reaction profile with two lines = one with a catalyst + one without
Two types of catalyst
o Homogeneous catalysts
o Heterogeneous catalysts
What are homogenous catalysts
the catalyst is in the same phase as the reactants
E.g. = both in solution
What are heterogeneous catalysts
catalyst is in a different phase to the reactants
E.g. - reactants are gases, but the catalyst used is a solid
What are the benefits of using a catalyst
speed up the rate of reaction = lower temperatures and pressures can be used
What are the advantages of lower pressures + temps
o save energy costs as there is reduced energy demand for providing high temperatures and less electrical pumping costs for producing the high pressures usually required
o fewer CO2 emissions from burning fossil fuels
What is the Boltzmann distribution curve
graph that shows the distribution of energies at a certain temperature
What is a key feature about a sample of gas
few particles will have very low energy, a few particles will have very high energy, but most particles will have energy in between
Particles with low energy will…
Cause ineffective collisions only as they don’t have enough energy to cause a chemical reaction
Draw a normal random Boltzmann distribution curve = just so you know shape
What does this graph show
only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction to take place
Why does the line go through the origin
NO molecules with NO energy
Why does the line never meet the x axis
no maximum energy for molecules
What does the area underneath the graph represent
total number of particles present
What happens to the Boltzmann distribution curve if you increase temp
flattens and the peak shifts to the right
Why does the peak flatten and shift to the right when increase temp
• proportion of successful collisions increases, meaning a higher proportion of the particles possess activation energy to cause a chemical reaction
Draw a Boltzmann distribution curve with two lines - one normal and one with increased temp
What needs to stay the same when increasing temp
total area under the curve should remain constant because the total number of particles is constant
How do catalysts effect the Boltzmann distribution curve
• By lowering Ea, a greater proportion of molecules in the reaction mixture have the activation energy, and therefore have sufficient energy for an effective collision
Draw a Boltzmann distribution curve of a catalysed and uncatalysed reaction
Define reversible reaction
products can react to reform the original reactants
Symbol for reversible reaction
o ⇌
Define dynamic equilibrium
• rate of the forward reaction is the same as the rate of the backward reaction in a closed system, and the concentrations of the reactants and products are constant
Draw two graphs that both show dynamic equilibrium
What is a closed system
none of the reactants or products escape from the reaction mixture
What is an open system
matter and energy can be lost to the surroundings
Technically…could equilibrium be reached in an open system
• reaction takes place entirely in solution = equilibrium can be reached in open flasks as a negligible amount of material is lost through evaporation = NOT IF GASES THOUGH
What is the position of equilibrium
• refers to the relative amounts of products and reactants in an equilibrium mixture
What does it mean if the position shifts to the left
concentration of reactants increases
What does it mean if the position shifts to the right
concentration of products increases
What is Le Chatelier’s principle
says that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change
How does the equilibrium shift if you increase concentration of anything
- moved away from increased conc
How does the equilibrium shift if you increase concentration of reactants
How does the equilibrium shift if you decrease concentration of reactants
Why is changes in pressure special when talking about equilibrium
only affect reactions where the reactants or products are gases
How does the equilibrium shift if you increase pressure
Shifts towards side with fewest gaseous moles
How does the equilibrium shift if you decrease pressure
Shifts towards side with largest gaseous moles
How does the equilibrium shift if you increase temp
Shifts towards endothermic = positive H value
How does the equilibrium shift when you decrease temp
Shifts towards exothermic reaction = negative H value
How does the catalyst effect position of equilibrium
IT DOESNT
What does a catalyst do then in terms of equilibrium
• only cause a reaction to reach equilibrium faster
Outline method to investigate changes to the equilibrium position with concentration
- solution with aqueous chromate ions, CrO42-, and dichromate ions, Cr2O72-
-• By adding dilute sulfuric acid, increase the concentration of H+ (aq) in the solution
• increases the rate of the forward reaction causing the equilibrium position to shift to minimise the change in H+ (aq) concentration
= should turn orange
• By adding aqueous sodium hydroxide, we can decrease the concentration of H+ (aq) in the solution
• The added OH-(aq) ions react with H+(aq) ions forming water
• H+ (aq) + OH- (aq) → H2O (l)
• This decreases the rate of the forward reaction causing the equilibrium position to shift to minimise the change in H+(aq) concentration = yellow
What colour are chromate ions
Yellow
What colour are dichromate ions
Orange
If you increase Sulfuric acid…
o Equilibrium shifts to the right
o Solution turns orange due to the formation of more Cr2O72- (aq)
If you add sodium hydroxide
o Equilibrium shifts to the left, making more H+ (aq) reactant
o Solution turns yellow due to the formation of more CrO42- (aq)
Equation for the Harber process
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1
How to maximise yield of ammonia
N¬2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1
Using pressure…
• An increase in pressure = shifts towards the right so the yield of ammonia increases
• Very high pressures are expensive + dangerous
• compromise pressure
How to maximise yield of ammonia
N¬2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1
Using temperature
• = exothermic reaction, = shift to the right if the temperature is lowered
• at a low temp = the gases won’t have low KE = too slow
• compromise temperature
What is the formula for the equilibrium constant
What are ignored in equilibrium expressions
Solids
When does the Kc value change
only changes if the temperature of the reaction changes
What do square brackets mean
concentration of
What does it mean if the Kc value is very large
Kc>1
equilibrium lies to the RHS so the reaction mixture contains mostly products
What does it mean if the Kc value is very small
Kc<1
equilibrium lies to the LHS so the reaction mixture contains mostly reactants
What happens if the Kc value is 1
contains same concentration of both reactant and products
