Chapter 10 Flashcards
Describe the effect of increasing the concentration on rate of reaction, in terms of frequency of collisions.
When the concentration of a reactant is increased, the rate of reaction generally increases.
Higher concentration - more particles in a given volume - more frequent successful collisions, faster ROR
Describe the effect of increasing the pressure on rate of reaction, in terms of frequency of collisions.
When the pressure of a reactant is increased, the rate of reaction generally increases.
Higher pressure - more particles in a given volume - more frequent successful collisions, faster ROR
What is collision theory ?
A reaction will only occur between particles which collide with the correct orientation and with enough energy to overcome the activation enthalpy.
Define the term “ rate of reaction “
Change in concentration of a product or reactant per unit time.
What is the activation enthalpy of a reaction ?
The minimum amount of energy required by colliding particles before a reaction will occur
How do you calculate the rate of reaction from a graph ?
Draw tangent at specific point
Calculate gradient
Gradient = rate of reaction
What is the role of catalysts in reactions ?
They speed up the reaction without being used up. They provide an alternative reaction pathway and the activation enthalpy is lowered. Enthalpy change is unaffected. They speed up both the forward and reverse reactions by the same rate. A greater proportion of molecules have more energy / greater than and equal to the activation energy.
Draw the enthalpy profile diagram of a reaction with and without a catalyst
…
What is meant by a homogenous catalyst ?
reactants and catalyst are in same physical state
(aqueous solution etc.) e.g. using an acid catalyst to break down proteins
What is meant by a heterogenous catalyst ?
reactants and catalyst are in different physical state
e.g. use of zeolite catalysts in the cracking of hydrocarbons in the petroleum
industry, catalytic converters in car exhaust systems.
Describe the 4 steps of the mechanism of heterogenous catalysts
- Reactants are adsorbed onto the catalyst surface ( the reactants form weak bonds with the catalyst surface )
- Bonds in the reactants weaken and break
- New bonds form, making products
- The products diffuse away from the catalyst surface (desorption)
Explain how catalysts have great economic importance and benefits for increased sustainability
Catalysts lower temperatures and reduce energy demand from combustion of fossil fuels with resulting reduction in CO2 emissions
List and describe the techniques and procedures used to investigate reaction rates
Monitoring the formation ( increase in conc ) of a product eg monitoring volume of gas produced at regular time intervals using gas collection.
Monitoring the removal ( decrease in conc ) of a reactant eg Monitoring the loss of mass using a balance
Explain the Boltzmann distribution and its relationship with activation energy? ( draw graph )
Graph : x axis = energy, y axis = number of molecules with a given energy
Peak = modal speed, most probable speed
Right of peak = mean speed, average speed
Area under curve = total number of molecules
Lower and further right = Ea, area to right of Ea shows molecules with energy greater than Ea
Explain, using Boltzmann distributions, of the qualitative effect on the proportion of molecules exceeding the activation energy and hence the reaction rate, for temperature changes?
At higher temperatures, the peak is lower and shifted to the right
At higher temperatures, a greater number of molecules have an energy equal to or greater than the activation energy so greater number of successful collisions will lead to increased ROR. Collisions will also be more frequent as molecules are moving faster which also increases ROR.
Shown on graph with 2 curves
Can you explain, using Boltzmann distributions, of the qualitative effect on the proportion of molecules exceeding the activation energy and hence the reaction rate, for catalytic behaviour?
In the presence of a catalyst, a greater proportion of molecules exceed the new lower activation energy.
More molecules collide with an energy greater than the activation energy
More effective collisions, increasing ROR.
Shown as one curve, Ec is to the left of the original Ea
Explain what is meant by dynamic equilibrium
Dynamic equilibrium exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change
What is Le Chatelier’s principle ?
The equilibrium position will shift to oppose the change : when a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of the change.
What is the effect on the position of equilibrium of increasing the concentration of the reactant ?
Equilibrium shifts to product side
What is the effect on the position of equilibrium of increasing the pressure ?
Equilibrium shifts to side with fewest moles ( gaseous )
What is the effect on the position of equilibrium of increasing the temperature ?
Equilibrium shifts in the same direction as the endothermic reaction
What is the effect on the position of equilibrium of adding a catalyst ?
No change of position of equilibrium because a catalyst increases the rate of both forward and reverse reactions in an equilibrium.
Reaches equilibrium faster.
Explain the importance to the chemical industry of a compromise between chemical equilibrium and reaction rate in deciding the operational conditions
Conditions compromise eg Haber Process :
Low temp = slow ROR
High temp = high ROR but low yield of product
High pressure = high ROR and yield but costly due to equipment and extra energy
Have to find compromise temp and pressure to find a balance between ROR, yield and price
How do u find the expression for the equilibrium constant of a reaction and calculate it ?
aA + bB ⇌ cC + dD
Kc = ( [C] to the power of c X [D] to the power of d ) / ( [A] to the power of a X [B] to the power of b )
How do you estimate the position of equilibrium from the magnitude of Kc ?
The larger the value of K c , the further to the right the position of equilibrium lies.
If K c = 1 the position of equilibrium lies halfway between the reactants and products ( [reactants] = [products] )
* If Kc > 1 the position of equilibrium lies on the right of the equation ( [products] > [reactants] )
* If Kc < 1 the position of equilibrium lies on the left of the equation ([reactants] > [products] )
If solid present in reaction, it is not included in Kc
How can you determine the value of Kc experimentally ?
Allow the reaction to reach equilibrium
Measure the equilibrium concentrations of all reactants and products
Substitute the values into the Kc expression
