Topic 5: Kinetics Flashcards
Explain how rate of reaction could be increased and how it can be measured
By crushing the solid into a powder surface area increases which increases rate of reaction. More particles are available so more frequent and successful collisions.
or increase conc of solution then increase temperature
Measure gas produced in a given time
true or false - pressure only affects gaseous reactions
true - no change for liquids
rate of reaction
-Reactions take place over a certain amount of time. Some reactions occur more quickly than others ctions occur more qui
-Rate of reaction –> the speed in which reactants are turned into products
equation for rate of reaction
change in conc / time
Activation energy
Activation energy = the minimum amount of energy required for a reaction to take place
Collision theory
-For a reaction to occur –> particles must collide with the correct orientation and particles must have sufficient energy
-Reactants need kinetic energy
-collisions and activation energy are required for a successful reaction to occur
Factors affecting rate
-pressure
-concentration
-surface area to volume ratio
-temperature
-catalyst
how to make a reaction faster
-increase number of collisions
-lower activation energy
-increase energy of particles
concentration
A concentrated solution would have a faster rate of reaction than a diluted solution because it has a higher number of particles due to higher chance of collisions occuring
-decrease rate of reaction using a more dilute solution
explain how using pure oxygen instead of air would affect the rate of burning
-rate of burning would increase using pure oxygen
-greater concentration of O2 particles in a given volume
-more successful and frequent collisions
pressure
-increasing pressure increases the number of particles in a given volume which increases the amount of frequent and successful collisions
-gas = greater pressure = faster rate of reaction
surface area
-increasing surface area will increase rate of reaction as the surface area to volume ratio is higher. This increases the amount of collisions between particles and are more frequent in nature
-Larger surface area to volume ratio means size of particles are smaller so greater rate of reaction due to more successful collisions
Increase the surface area of a solid by crushing it down into a powder/smaller pieces
temperature
-rate of reaction is greater at a higher temperature as particles have more kinetic energy
-At higher temperatures a higher proportion of molecules have sufficient energy = more successful collisions
-Energy –> frequent collisions –> successful collisions
calculating rate of reaction
Gradient = rate of reaction
Rate of reaction = change in concentration (reactant used or product formed) / change in time
measuring rate of reactions experimentally
-In order to measure rate of reaction, we need to measure the amount of reactant being used or the amount of product being produced
-For example, you can measure the volume of hydrogen gas formed in a reaction
-When talking about measuring products we cannot say amount –> we have to say mass, volume or concentration
what is rate of reaction proportional to
Rate of reaction is proportional to 1/time
initial rate experiments
-Silver nitrate was added to three test tubes containing different haloalkanes
1 – 20mins and 50 seconds
2 – 9 mins and 15 seconds
3- 5 seconds
Rate of reaction is proportional to 1/time
1/1250 : 1/555 : 1/5 = 1:2:25:250 (divide by smallest) = 1/1250
graphs/curves for powder
volume = half
slope = shallow
what is correct about gas molecules
at a fixed temperature, their average kinetic energy is constant
explain why the reaction is fastest at the start
-larger concentration of reactants at the start
-this means that there were more frequent and successful collisions
explain how a catalyst increases the rate of reaction
-catalyst provides a different pathway with a lower activation energy
true or false - the lower the temp the higher the equillbrium yield
true
explain why an increase in temperature increases the rate in which gas decomposes
-more molecules with activation energy
-higher rate of successful collisions
y and x axis
x = energy
y = number of molecules
State what happens to the rate of reaction when temperature is increased
-average kinetic energy of molecules increases
-more molecules have sufficient energy greater than or equal to the activation energy
-this means there are a greater proportion of more frequent and successful collisions
true or false - as temperature increases more particles have sufficient energy
true
How does the curve change when temperature is increased
the peak decreases in height and moves to the right
What would be the effect to the curve if reactant concentrations were increased
there would be no change
what would the effect on the curve if a catalyst was added to the activation energy
activation energy would move to the left and the peak would be unchanged
if the temperature were lowered what would be the effect on the shape of the curve
the peak would shift to the left and be higher
what would shift the activation energy line to the right
the removal of a catalyst
Use your understanding of collision theory to explain why the student did not obtain a straight line
-as concentration increases, rate of reaction increases as there are more particles in a given volume so more frequent and successful collisions occur so gradient becomes steeper.
-however, the reaction is not a straight line as the amount of pure magnesium remains unchanged
reversible reaction
two directions each with different outcomes to a reaction
-If the forward reaction has an enthalpy change of –92KJ/mol the enthalpy change of the backwards reaction will be +92KJ/mol
thermodynamically feasible
a reaction which is most likely to occur based on the enthalpy changes of reaction
An exothermic reaction is more thermodynamically feasible as it does not require energy overall to occur
kinetically stable
Some reactions do not occur unless high temperatures are provided. It is therefore said to be kinetically stable and has a high activation energy
what does a maxwell boltzmann distribution graph show
A maxwell-boltzmann distribution graph shows us the energy and speed different particles have bc not all particles have the same amount of kinetic energy
maxwell boltzmann curve
-Area under the curve represents the total number of particles in the sample
-The line to the right represents the activation energy. The number of particles with at least the activation energy to the right of the line is shaded (these are the reacting particles)
effect of temperature on maxwell boltzmann curve
-changing the temperature of the reaction will change the shape of the energy distribution curve without changing the activation energy
-When we increase the temperature, the proportion of particles with higher energies increase and so the graph shifts to the right and the peak is lower
-The area under the curve remains the same and therefore the number of molecules remains the same
temp on curve
As we increase temp, the curve shifts right
As we decrease temp, the curve shifts left
As temperature increases the area to the right of the activation energy increases
catalysts
-not used up during the reaction but increases rate of reaction
-starting mass of a catalyst equals the final mass of a catalyst
definition of catalyst
Catalysts provide an alternative reaction pathway with a lower activation energy so a higher proportion of molecules have an energy greater than the activation energy
what is catalysis
One way a catalyst can work is by changing during the reaction and then changing back to reform the catalyst (alternative reaction pathway) -present in reactants and products
types of catalysts
Heterogeneous –> the catalyst and reactants are in different phases
Homogenous –> catalyst and the reactants are in the same phase
heteregenous catalysts
-different phase to reactants
-A catalytic converter is a catalyst which reduces toxic emissions from a car engine by converting harmful exhaust products into harmless ones
-reactants = gas catalyst = solid (catalytic converter)
steps for heterogeneous catalysts
1) During heterogenous catalysis the reactants adsorb on the surface of the catalyst onto an active site
2) A reaction occurs when another reactant molecule collides with the adsorbed molecule. This forms the products
3) The products desorb from the surface of the catalyst
adsorption, reaction, desorption
Adsorption –> a molecule forms weak bonds with the surface of the catalyst
Desorption –> the weak bonds with the surface of the catalyst break and the molecule leaves the surface
1) adsorption of reactants
2) reaction occurs
3) desorption of products
effect of adsorption on activation energy
Effect of adsorption of activation energy –> adsorbing reactant in a particular orientation and weaking bonds in the reactant
steps - catalysts
-adsorption increases the chance of collision by holding one or more of the reactants in a stationary position
-by holding the reactants in the best geometry for reaction or by weaking bonds in the reactants, this process creates an alternative reaction pathway with lower activation energy
-once the reaction has taken place on the surface of the catalyst the product desorbs
Step 1 (fast) –> gasesous reactants + active site –> adsorbed reactants
Step 2 (slow) –> adsorbed reactants –> adsorbed product
Step 3 (fast) –> adsorbed product –> gaseous product + empty active site
role of catalytic coverters
Catalytic converters are designed to remove CO and NO produced by internal combustion engines.
Explain why a small decrease in temperature can lead to a large decrease in the rate of reaction
fewer molecules have sufficient energy greater than or equal to the activation energy. Less frequent and successful collisions occur
State two ways of speeding up a gas phase reaction other than by changing the temperature
-increase pressure
-add a catalyst
suggest two factors that suggest hydrogen bromide is behaving as a catalyst
-unchanged
-offers and alternative reaction route
explain why most collisions do not result in a reaction
not sufficient energy
maxwell boltzmann distribution
(spread of energies at a specific temperature)
-only few particles have energy greater than Ea
-Emp = most probable energy (peak)
-area under curve = total number of molecules present
-mean energy is past the peak
-starts at origin (0) because there are no molecules with no energy
higher temp
at higher temperatures most probable and mean energy shift higher but less molecules have this much energy (right but lower)
-total number of particles however is constant but their energy is different
gradient of curve
=rate of reaction
higher concentration = faster rate = steeper gradient
as temperature increases
a larger proportion of molecules have energy greater than or equal to the activation energy
catalyst
Definition: Catalysts increase reaction rates without getting used up.
Explanation: They do this by providing an alternative route or mechanism with a lower
activation
with catalyst activation energy on Maxwell boltzmann distribution shifts left
using the reactants suggest why small amounts of the reactants are used in this experiment
SO2 is toxic
suggest why the student chose not the carry out experiments at low temperatures
reaction would be too slow
why does rate of reaction decrease over time
reactants used up = less frequent and successful collisions
enthalpy of combustion for homologus series
When comparing the enthalpies of combustion for successive members of a homologous series such as
alkanes or alcohols there is a constant rise in the size of the enthalpies of combustion as the number of
carbon atoms increases.
As one goes up the homologous series there is a constant amount and type of extra bonds being broken and
made e.g. 1C-C, 2C-H and 1.5 O=O extra bonds broken and 2 C=O and 2 O-H extra bonds made, so the
enthalpy of combustion increases by a constant amount.
calculated values
Remember that calculated values of enthalpy of combustions will
be more accurate if calculated from enthalpy of formation data than
if calculated from average bond enthalpies. This is because
average bond enthalpy values are averaged values of the bond
enthalpies from various compounds.
explain how your results could be used to calculate the enthalpy of solution
Stage 1 Method
(1a) Measures water with named appropriate apparatus (1b) Suitable volume/mass / volume/mass in range 10 - 200 cm3 /g
(1c) Into insulated container / polystyrene cup (NOT just ‘lid’)
(1d) Add known mass of MgCl2(s) (1e) Use of ‘before and after’ weighing method. NOT ‘added with washings’ Stage 2 Measurements (could mark from diagram)
(2a) Record initial temperature (min 2 measurements) (2b) Record T at regular timed intervals for 5+ mins / until trend seen
(2c) Plot T vs time
Stage 3 Use of Results
(3a and 3b could come from diagram)
(3a) Extrapolate lines to when solid added (to find initial and final T)
(3b) Tfinal - Tinitial = ∆T / idea of finding ∆T from graph at point of addition
(3c) q = mc∆T (3d) amount = mass/Mr (0.80/95.3 = 8.39 x 10-3 mol) (3e) ∆Hsoln = q/8.39 x 10-3 or in words
