Kinetics Flashcards
what is the collision theory
The collision theory states that for a chemical reaction to take place the particles need to collide with each other in the correct orientation and with enough energy in order to react
what happens to the bonds in reactions
they are made or broken
if molecules collide at high speeds, then (bonds)
the bonds can be broken and made
what constitutes to an unsuccessful collision
An ineffective collision is when particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction
what is orientation
reactant molecules must collide with “favorable” orientation such that there is direct contact between the molecules to induce the breaking or forming of a bond in space
ie the same charge cannot be together as it will repel.
what diagram is used to show the progress of a reaction
a reaction profile
what is activation energy
the minimum energy with which particles need to collide to cause a reaction
what is the symbol for activation energy
Ea
Reaction profile for endo and exo
In exothermic reactions the reactants are higher in energy than the products
In endothermic reactions the reactants are lower in energy than the products
what is the purpose of catalysts
A catalyst is a substance that increases the rate of reaction without taking part in the chemical reaction by providing the particles an alternative mechanism with a lower activation energy
what is the Maxwell-Boltzmann energy distrubution
A Maxwell-Boltzmann distribution curve is a graph that shows the distribution of energies at a certain temperature
what does the M-B graph show
The graph shows that only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction to take place
what is the effect of temp on the M-B graph
When the temperature of a reaction mixture is increased, the particles gain more kinetic energy
This causes the particles to move around faster resulting in more frequent collisions
Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction
With higher temperatures, the Boltzmann distribution curve flattens and the peak shifts to the right
how does temp increase rate
Therefore, an increase in temperature causes an increased rate of reaction due to:
There being more effective collisions as the particles have more kinetic energy, making them move around faster
A greater proportion of the molecules having kinetic energy greater than the activation energy
how do reactions with low Ea carry out
they are faster as less temp is needed so the particles have a lower energy that they need to get to to react
these molecules have above threshold energy
what are some points to note for M-B graph
-area under curve gives total number of molecules
-no molecules can have 0 energy
-very few molecules have high energy
-the mode is the most probable energy
-mean of energy is not the peak of curve it is slightly to the right of it as the curve is not symmetrical
-Ea is activation energy
what is the defintion of rate of reaction
rate of reaction is the change in conc of reactants or products per unit time
what are the standard units for rate
moldm-3 s -1
what affects rate of reaction
temp
pressure for a gas/ conc for a solution
surface area of solid
presence of catalyst
what is the effect of temperature
-graph is lower but further right- as total number of molecules stay the same
-higher temp more molecules have an energy greater to or equal to Ea
-more frequent collisions
-rate is increased
-doubling the temp will mean many more molecules have E>/ Ea
an increase in temp of —– will approximately —— the rate of reaction
10 degrees
double
what is the effect of pressure/ conc
-area under curve increased
-more particles present in same volume, so more frequent collisions
-more collisions with E>/ Ea
-rate is increased
the Emp does not change - draw graph higher but peak at same point
what is the effect of SA
-more particles exposed, more frequent collisions
-more collisions with E >/ E
-rate increased
what is a catalyst
substance that increases the rate of a chemical reaction without being used up or changed chemically at the end of the reaction
how do catalysts work
provide an alternative reaction route with lower activation energy
what is the effect of a catalyst
-lower activation energy
- more collisions with E>/ Ea
-rate is increased
-position of equilibrium is unchanged
what is a heterogeneous catalyst
one with reactants in a different state eg catalytic converter
what is homogenous catalyst
reactants in the same state eg chlorine radical in ozone depletion
what are the 6 ways to measure the rate of a reaction
-collecting a gaseous product
when gas is insoluble - use a downward-displacement of water
when gas is soluble use gas syringe
-measure mass loss
cotton wool used to allow gas to leave but retains liquid product
-light absorption of coloured products
calorimetry
-clock reactions
use of indicator
-formation of precipitate
cross method
-measure change of ph with time
-change in electrical conductivity
rate of reaction calculation
change in reactants or products/time
required practicle 3
.Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s)Measure rate of reaction as 1/time.
Method
1. Using a measuring cylinder, measure 10cm3 of the hydrochloric acid to an ‘acid’ tube.a. 10 cm3 of sodium thiosulfate into another tube and place this tube over the cross and also put in a thermometer
.2. Note the starting temperature (room temperature) then add 1cm3 of the acid to the sodium thiosulfate and start the timer.
3. Look down at the tube from above and record the time it takes for the cross to disappear. Also record the temperature of the reaction mixture at the end, pour the contents into the sodium carbonate solution.
4. Measure another 10.0cm3 of sodium thiosulfate solution into a clean tube and place itin a water bath (large beaker) (no hotter than 55 oC otherwise too much SO2 produced) to warm uo for 3 minutes.
5. Repeat the steps 2 and 3.
6. Repeat these steps for 5 different temperatures.
how do u minimise the effect of sulphur escaping rp 3
● To minimise the escape of toxic sulfur dioxide during the experiment a lid is advised. Two holes should be made in the lid using a hot wide cork borer. These holes should securely hold the glass tubes and vertically in the plastic container. Could also perform the experiment in a fume cupboard.
● Containers of sodium carbonate solution and phenolphthalein (stop baths) should be available to students so that the acid and sulfur dioxide can be neutralised at any point during the experiment.
● Once the colour of the solution in the stop bath changes, the sodium carbonate has been used up and the stop bath will need to be replenished.
● The stop bath should be placed in a fume cupboard, if available.
