Rates ✅ Flashcards
What is the equation for rate
Rate=change in concentration/time
Rate (moldm-3s-1)
What factors affect the rate of a reaction
Concentration (or pressure when reactants are gases)
Temp
Use of a Catalyst
SA of solid reactants
Under what circumstances will an effective collision occur
The particles collide with correct orientation
Particles have sufficient energy to overcome activation barrier of the reaction
How does increasing the concentration affect the rate of reaction
When conc increases ROR is increased, as increase in conc increases the number of particles in the same volume
. The particles are closer together and collide more frequently, in given period of time there will therefore be more effective collisions
How does increasing pressure of a gas affect ROR
ROR increases as conc of gas molecules increases as same number of gas molecules occupy a smaller volume, molecules are closer together and collide more frequently, leading to more effective collisions in same time
How can the progress of a chemical reaction be followed by
Monitoring the removal of a reactant
Following formation of a product
With gases monitor volume of gas producing at regular time intervals or loss of mass of reactants using a balance
Explain monitoring production of gas using gas collection practical
- Hydrogen peroxide added to conical flask and bung placed
- Initial volume of gas in measuring cylinder recorded
- Manganese dioxide catalyst quickly added to conical flask and bung replaced stopwatch started
- Volume of gas produced in measuring cylinder recorded at regular intervals until reaction complete
- Plot graph etc to find results
Changing concentration of reactant alters ROR, state 3 other factors that affect rate. Explain how an increase in conc increases the rate (5 marks)
Temp, SA, catalyst
When conc of a reactant increases ROR increases, increase in conc means more reactant molecules in same volume, reactant molecules are closer together and collide more frequently leading to more effective collisions
What is a catalyst
A substance that changes the rate of a chemical reaction without undergoing any permanent change
How does a catalyst increase the rate of reaction
By providing an alternative reaction pathway of lower activation energy
How/what happens in a reaction with a catalyst
Catalyst not used up in the chemical reaction
Catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
At end of reaction the catalyst is regenerated
Know how to draw an enthalpy profile diagram with a catalyst
Activation energy is lower
What is a homogeneous catalyst
Has the same physical state as the reactants, catalyst reacts with the reactants to form an intermediate, the intermediate then breaks down to give the product and regenerates the catalyst
What is a heterogeneous catalyst
Has a different physical state from reactants, usually solids in contact with gaseous reactants or in solution. Reactant molecules are absorbed into surface of catalyst, where reaction takes place, after reaction product molecules leave surface of the catalyst by desorption
State the difference between a homogeneous and heterogeneous catalyst (1 mark)
Homogenous catalyst has same physical state as reactants, heterogeneous catalyst has different physical state from reactants
Describe the effect of a catalyst on activation energy of a chemical reaction and on enthalpy change of reaction (2 marks)
Lowers activation energy by providing an alternative route for the reaction, no effect on 🔺H
What are features of the Boltzmann distribution
No molecules have 0 energy- curve starts at the origin
Area under curve is equal to total number of molecules
No max energy for a molecule- curve does not meet x axis at high energy
Graph has a sharp increase rounds off and then decreases sharply then curve flattens
What is the effect of temperature on Boltzmann distribution
More molecules have an energy greater than or equal to activation energy
Therefore greater proportion of collisions will lead to a reaction, increasing the ROR
Collisions will also be more frequent as molecules are moving faster, increases energy of molecules is much more important than increased frequency of collisions
What is effect of a catalyst on Boltzmann distribution
Activation energy with a catalyst means greater proportion of molecules now have an energy equal or greater then lower activation energy, on collision more molecules will react to form products
Explain what is meant by term activation energy (1mark)
Minimum energy required to start a reaction by the breaking of bonds
Describe and explain how ROR is affected by a decrease in temp (2 marks)
ROR decreases as fewer molecules have energy greater than the activation energy, collisions are less frequent as molecules moving slower but much smaller effect then decrease in molecules exceeding activation energy
What is the rate constant
Concentrations of species in rate equation to overall rate of reaction
What are the units of rate
=K[A]2
Moldm-3s-1/moldm-3 moldm-3
= dm3mol-1s-1
What are the graphs for 0,1,2 order for rate and [A]
0= straight line parallel with x axis
1=straight line from origin where time and conc are equal
2= increase in gradient
What is half life
Calculated from graphs (0,1,2 order), time taken for initial concentration of reactants to half (t=1/2)
Gradient calculated by rate/[A]=K
K=ln2/t1/2
What is rate determining step
Determined by slowest step, rate equation contains all species involved in stages up to and including rate determining step
What is the equation for K (Arrhenius equation)
K= -Ea/RT to the power of Ae
What is the equation for lnK, how do you get the values from a Lnk and 1/T graph
LnK= -Ea/RT + lnA
ln A= intercept of Y axis
-Ea/R= gradient
What is the equation for activation energy
Ea= (lnA-lnK) RT
what are the economical advantages of using a catalyst
Economic: reactions are faster and occur at lower temperatures, energy costs are reduced
Sustainability: the alternative pathway provided by a catalyst has lower energy requirements and more particles on a Maxwell-Boltzmann distribution curve will pass this threshold, this reduces demand for fossils fuels and reduce CO2 emissions
how does increasing the surface area of a reactant increase rate of reaction
the more particles that are on the surface, the more particles that are available to react, the higher the chance of an effective collision
turning a solid lump to powder increases SA
what is Le Chateliers principle
if a system at equilibrium is changed by an external factor, the system adjusts to reduce the effect of that change
when does an equilibrium form
when a reversible reaction takes place in a closed system and the rate of the forward reaction is equal to the rate of the reverse reaction
if you increase the temperature of an equilibrium that’s forward reaction is endothermic, what way will the equilibrium shift
equilibrium shifts right, aiming to lower the temperature
Rate of reaction is also increased
why do different factors need to be weighed up when determining optimal conditions, give example with production of ammonia
production of ammonia is exothermic, so lower temps favour production of ammonia. However this decreases the rate of reaction
higher pressures favours production of ammonia however this is expensive
what is Kc, what is the equation, what do values over 1 mean
Kc is the equilibrium constant
aA+bB=cC+dD
Kc=[C]c [D]d
[A]a [B]b
if Kc greater then 1 there will be more products
if Kc below 1 then there will be more reactants
what are the graphs for 0,1,2 order for concentration of reactants and time
0= straight decreased line
1= curve decreased line
2= steeper deeper curved line
what is Kp
the equilibrium constant for a system that contains gaseous molecules
equation the same as Kc
what is the equation to find out partial pressure of gas A
partial pressure of gas A= mole fraction of gas A X total pressure
what is the equation to find out mole fraction of gas A
mole fraction of gas A= number of moles of gas A in mixture/ total number of moles of gas in the mixture
