Rate Of Reaction Flashcards
Rate of reaction
The change in concentration per unit time of any one reactant or product
Factors that affect the rate of a reaction
Nature of the reactants
Surface area
Concentration
Temperature
Catalyst
how do we draw the rate of reaction diagram
product line starts on 0
reactants start high
x axis = time
y axis = concentration
when is the rate of reaction at its fastest?
start
why is the rate of reaction fastest at the start
there is the greatest concentration of reactants thus more effective collisions occur
why does the rate of reaction decrease as the reaction progresses
conc of reactants is reducing, less effective collisions
why do both the reactants and products level out at the same time on the graph?
when all of the reactants are used up, no more product is produced so the conc of both remains constant
effective collision
a collision that results in the formation of products.
the rate of a reaction depends on the number of effective collisions per second
what is required for a reaction to occur
particles to collide
when will collisions result in the formation of products
if a certain minimum energy is exceeded in the collision (activation energy). this is an effective collision1
what happens in an effective collision
bonds break (endothermic) and new bonds form (exothermic)
activation energy
the minimum energy that colliding particles must have for a reaction to occur
what does the size of the AE depend on
the nature of the reactants ie ionic or covalent
what happens to the rate of reaction if you decrease activation energy
increases
how can the activation energy be changed
catalysts
what does the max Boltzmann distribution curve show
the relationship between the number of particles and activation energy
explain how the max boltzmann distribution curve works
the peak of the graph is the average kinetic energy of gaseous particles
from the graph, the activation energy is higher than the average kinetic energy therefore, one a small portion of the particles have enough energy for successful collisions
what affect does heat energy have on activation energy
does not change
what affect does increasing temp have on rate of reaction
increases
why is the rate of reaction increased as the temp increases
at higher temperatures, molecules have more kinetic energy and are moving faster this leads to an increased number of collisions
there are more effective collisions at higher temperatures- more molecules have the activation energy so when they collide, they collide more effectively and product is formed
a reaction profile diagram
a graph which shows the change in energy of a chemical reaction over time as the reaction progresses
draw the diagram for exothermic reactions
y axis= energy
x axis = time
reactants start higher than products
show activation energy (from reactants to peak)
show change in activation energy from reactants to products
label reactants and products
draw the diagram for the endothermic rate of reaction
y axis= energy
x axis = time
reactants start lower than products
show activation energy (from reactants to peak)
show change in activation energy from reactants to products
label reactants and products
what is drawn if they ask for the reaction profile diagram of a catalyst
smaller peak
why does a catalyst lower activation energy
less energy is needed for effective collections thus more collisions are effective and more product is formed
why does the energy of reactants products and delta H the same with or without a catalyst
the same reactants are used and the same products are formed ie the nature is the same bonds are the same so energy will be the same
the only difference is the size of the activation energy
how do you monitor the rate of reaction
monitoring the removal or decrease in concentration of a reactant ie the change in mass of the reactant with time
monitoring the formation ie the increase in concentration
how can the volume of a gas be determined
downward displacement of water or using a gas syringe
average rate
calculated by dividing the total volume by the total time taken to go to completion
instantaneous rate
the rate of reaction at any one particular time during the reaction. draw a tangent to the curve at that particular time + find slope of that line
which nature of reactants results in a faster rate of reactants
ionic
why are ionic reactions faster
when 2 ionic substances react, the oppositely charged ions combine to form the precipitate
there are no bonds broken when ionic substances react as when in solution, the crystal lattice has already broken up
when 2 ionic substances react, the product forms immediately and a colour change is seen
what happens when acidified sodium dichromate reacts with ammonium iron 2 sulphate
when these 2 ionic substances react, the product forms immediately and a colour change from orange (Cr+6) to green (Cr+3) is seen
what happens when acidified sodium dichromate is added to ethanal
ethanoic acid is formed. it takes longer for the solution to change colour from orange to green because ethanal has covalent intramolecular bonds. these covalent bonds in the reactant must be broken first before new bonds can form
what effect do powdered substances have on the rate of reaction
they have a larger surface area thus more surface is exposed and available for reaction. this means more sucsessful collisions occur between reactants
write a balanced equation for the decomposition of hydrogen peroxide
MnO2
H2O2 —-> H2O + 1/2 O2
what are the risks of using or storing hydrogen peroxide and how can it be managed
it is corrosive
dilute it
if you have the same mass of a substance but some is powdered and some is solid how does the quantity of product formed differ
the same amount of product is formed at the end of both reactions because the same mass of marble is used in each reaction
the only difference is the rate of reaction
Give the balanced equation of a the experiment which demonstrates the effect of particle size/ surface area on rate of reaction
CaCO3 + 2HCl ——> CO2 + H2O + CaCl2
What is a the purpose of the cotton wool in the experiment which demonstrates the effect of particle size/ surface area on rate of reaction
Cotton wool allows carbon dioxide to escape but stops acid spray from escaping so that the decrease in mass is from carbon dioxide formation only
Sketch the graph showing how surface area affects the reaction rate
X axis is time
Y axis is change in mass (g)
Reaction rate is faster with powdered calcium carbonate (steeper slope).
• The same mass of calcium carbonate used for both reactions, so same amount of product is produced - this is why they both finish with the same change in mass.
How do you show the affect of surface area on rate of reaction
measure the volume of carbon dioxide produced every 30 seconds using powdered calcium carbonate and marble chips by collecting carbon dioxide in a gas syringe or by downwards displacement of water. Record the change in volume over time.
What affect does increasing concentration have on rate? Explain
Increases- there are more particles in a fixed volume
There are a greater number of collisions in the container
There are more effective collisions and more product is produced
What effect does increasing temperature have on rate of reaction
As temp increases, the kinetic energy of the colliding particles increases.
They collide more often with each other and the sides of the container.
Therefore there is a greater number of effective collisions per unit time therefore the rate of reaction increases
What is a catalyst
A substance that alters the rate of a chemical reaction but is not consumed in the reaction
Properties of catalysts
They are recovered unchanged at the end of an experiment + can be reused
Catalysts are specific
Catalysts only need small amounts to work
They can be poisioned
What is an inhibitor
A catalyst that slows down a chemical reaction
Give an example of an inhibitor
Glycerine
Give an example of a reaction involving a catalyst
MnO2 used in the decomposition of hydrogen peroxide to form oxygen gas
What is an enzyme
A substance that is produced by a living cell and acts as a biological catalyst
How do catalysts effect the rate of reaction
It has no effect on the rate of reaction
It Provides an alternative pathway or route for the reaction
Lowers the activation energy needed for the reaction to occur
More reactant particles have enough energy to collide effectively and produce product
Homogeneous catalysis
Both the reactants and the catalyst are in the same phase ie there is no boundary between the reactants and the catalyst
What is an example of homogeneous catalysis
Iodine snake
Explain the iodine snake experiment
Add concentrated potassium iodide (liquid catalyst) to washing up liquid and hydrogen peroxide.
Observe foaming due to O2 gas produced by the decomposition of H2O2. Oxygen gas is trapped in bubbles and forms a foam
Heterogeneous catalysis
Reactants are in a different phase to the catalyst ie there is a boundary between the catalyst and the reactants
Give an example of heterogeneous catalysis
Formation of ethene
Catalyst Al2O3 - white solid
Ethanol- liquid/vapour/gas
Autocatalysis
Catalysis in which one of the products of the reaction acts as a catalyst for the reaction
Give an example of autocatalysis
Reaction of acidified potassium permanganate and ethanedioc acid
Explain the Reaction of acidified potassium permanganate and ethanedioc acid
When the first few drops were added, the pink potassium permanganate decolourises slowly
After this is decolourises more rapidly because the Mn+2 formed in the reaction acts as a catalyst
What is the immediate formation theory
The catalyst works by forming an intermediate compound, which then reacts with a second reactant to form the product
What are the problems with the intermediate formation theory
The intermediate only forms for a short period of time and may be difficult to detect
What is evidence for the intermediate formation theory
Sodium tartrate + water + pink cobalt
Colour change is observed from green back to pink. The green substance is the intermediate and the Co+2 ions are not used up in the reaction so the pink catalyst is regenerated
Write the mechanism for the intermediate formation theory
Step 1. Reactant + catalyst —-> intermediate
Step 2: intermediate + reactant —-> products + catalyst
Overall reaction :
Catalyst
Reactant + reactant —-> products
Write the mechanism for the iodine snake reaction
H2O2 + I- —-> IO- + H2O
IO- + H2O2 —-> H2O + O2 + I -
I-
Overall : 2H2O2 —> 2H2O + O2
Catalyst= I-
adsorption
means something accumulates on the surface of another substance
what are the 3 steps of the surface adsorption theory
adsorption
reaction on surface
desorbtion
describe the mechanism for the production of water using a catalyst
adsorption: h2 and o2 adsorb onto the surface of the Pt catalyst forming temporary bonds with the surface of the metal- this weakens the covalent bonds in the H2 and O2 molecules.
reaction on surface: there is a greater concentration of H2 and O2 on the surface of the Pt catalyst. they are more likely to collide effectively on the surface and form H2O
desorption: products leave the surface of the catalyst and diffuse away. the surface of the catalyst is now free for more reactant molecules to adsorb and the cycle begins again
catalytic converter
a device in which the exhaust of a motor vehicle that contains catalysts which converts pollutants in the exhaust gases to less harmful substances
how do catalytic converters work?
by lowering the activation energy
what do exhaust fumes consist of?
contain carbon monoxide, nitrogen dioxide, and unburned hydrocarbons which are harmful to human health
source of CO
incomplete combustion of hydrocarbons due to insufficient oxygen
danger of CO
highly poisonous- can lead to death
removal of CO
reacts with O2 to form CO2
source of NOx
during combustion , heat is released ( combustion is exothermic) this causes N2 in the air to react with O2 to form NO and then NO2
danger of NOx
poisonous and causes acid rain
removal NOx
converted to N2 and O2
source of smog
unburned hydrocarbons leads to smog
danger of smog
hazardous if inhaled
causes a range of respiratory conditions
removal of smog
reacts with NOx to form CO2, N2 and H2O on the surface of the catalyst
what is the catalyst in the catalytic converter
thin layer of metal on a ceramic honeycomb layer structure such as platinum palladium or rhodium
what is a catalytic poison
lead
how does lead poison a catalyst
it makes the catalyst inactive. lead binds irreversibly to the catalyst and forms a permanent bond. the lead cant be removed the catalyst is now poisoned
explain how a catalytic converter works
harmful pollutants in exhaust fumes pass through the catalytic converter
thin layer of metal (Pt, Pd or Rh) is coated on a ceramic honeycomb structure
less harmful gases emerge from the other side and are emitted to the atmosphere. all pollutants are converted into CO2 N2 and H2O which occur naturally in the atmosphere
give the balanced equation of how a catalytic converter works
2CO + 2NO—->2CO2 + N2
