2.2 rates of reaction Flashcards
what must occur for a collision to be successful?
the reactant particles must collide with:
- the correct orientation
- energy greater than or equal to the activation energy of the reaction
(chemical reactions occur when particles of reactants collide successfully)
can reaction conditions be changed to increase the likelihood of successful collisions in order to increase the rate of reaction?
yes
what factors can change the rate of reaction?
- surface area
- concentration
- temperature
- catalyst
activation energy definition
the minimum amount of energy needed to start a reaction
increasing the temperature of a reaction, ____ the rate of the reaction
increases
why does increasing the temperature of a reaction increase the rate?
- the reactant particles have greater kinetic energy
- they move a lot faster
- more of the particles have energy that is greater than the activation energy
- more successful collisions
increasing the concentration/pressure of a reaction, ____ the rate of the reaction
increases
why does increasing the concentration/pressure of a reaction increase the rate?
- there are more reactant particles in the same volume
- increase in the number of collisions per unit time
- greater chance of successful collisions
increasing the surface area of a reaction, ____ the rate of the reaction
increases
why does increasing the surface area of a reaction increase the rate?
- only particles on the surface of a solid can collide with other reactant particles
- so increasing surface area increases the number of particles available to collide
- which increases number of collisions per unit time
- so increases the chance of successful collisions occuring
rate of reaction definition
the change in quantity/concentration of a reactant or product per unit time
when is the rate of reaction greatest?
at the beginning of the reaction
why is the rate of reaction the greatest at the beginning of the reaction?
as reactant concentration are at their highest
- as the reactants are used up, the rate of reaction slows until one of the reactants is used up, at which point the reaction stops
how can the rate of reaction at a given time be found?
by calculating the gradient of a tangent to the curve at that time
what are energy profiles?
diagrams that represent the enthalpy changes in both exothermic and endothermic reactions
at a given temperature, not all the particles in a sample have the same energy.
the energy distribution of the particles in a system is shown by a graph known as what?
the Boltzmann energy distribution curve
what are some key features of the Boltzmann energy distribution curve?
- the highest peak of the graph shows the most probable energy at that temperature
- the mean energy of all particles is always slightly higher than the most probable energy, due to the greatest number of particles with an energy greater than the most probable energy
- there are no particles with an energy value of zero, at any temperature
- at higher energies, the number of particles approaches zero but never reaches zero. hence the curve never touches the x-axis
what does the area under the Boltzmann energy distribution curve represent?
the total number of particles in the system
(therefore, the area under the curve must remain constant in a closed system)
as the temperature of a reaction increases, what happens to the Boltzmann energy distribution curve?
- the peak shifts to the right
- the peak moves lower
- the area under the curve remains the same
- the number of particles with energies greater than or equal to the activation energy increases
as the temperature of a reaction increases, why does the peak of the Boltzmann energy distribution curve shift to the right?
- due to higher overall energy of the particles at higher temperature
as the temperature of a reaction increases, why does the peak of the Boltzmann energy distribution curve move lower?
as the range of particle energies becomes wider
as the temperature of a reaction increases, why does the area under the Boltzmann energy distribution curve remain the same?
as increasing temperature does not change the total number of particles
as the temperature of a reaction increases, the number of particles with energies greater than or equal to the activation energy increases.
what does this lead to?
a greater change of successful collisions
does adding a catalyst to a reaction change the shape of the Boltzmann energy distribution curve? why?
no
as the catalyst does not add energy to the reaction
what does adding a catalyst do to a Boltzmann energy distribution curve?
- the position of the activation energy shifts to the left
(showing that adding a catalyst lowers the activation energy)
- as the a.e has been lowered, there are now additional particles with energy greater than the new lower a.e, which increases the chance of successful collisions occuring
how do catalysts lower the activation energy?
by providing an alternative reaction pathway/route that has a lower activation energy
what are the two types of catalysts?
- homogeneous catalysts
- heterogeneous catalysts
what are homogeneous catalysts?
- they are in the same physical state as the reactants
- these catalysts take an active part in the reaction and are then regenerated as the reaction proceeds
what are heterogeneous catalysts?
- different physical state from the reactants
- provides a surface for the reactant particles to adsorb (process of gas or liquid particles forming a thin film on the surface of a solid) to, which holds them in place. the products then desorb (the release of adsorbed particles from the surface of a solid) from the surface
how can rates of reaction be measured?
using any change of reactant or product with time
what are some ways of measuring rates of reactions?
- change in volume of a gas
- change in pressure
- change in mass
- change in colour
when can the change in volume of a gas be used to measure the rate of reaction?
- it can be used for any reaction where gas is produced
(the gas can be collected and measured accurately using a gas syringe at regular time intervals during the reaction)
what can be used to collect and measure gas produced by a reaction?
a gas syringe
when can the change in pressure be used to measure the rate of reaction?
- can only be used for reactions involving gases
- if the number of moles of gas differs between reactants and products, then the pressure will change
how can you measure a pressure change during a reaction?
a pressure change can be measured in a sealed container at regular time intervals using a manometer
when can the change in mass be used to measure the rate of reaction?
- when one of the heavier gases like CO2 is released during a reaction and allowed to escape
how can a change in mass be measured during a reaction?
- a change in mass can be measured at regular time intervals using an accurate balance
by what can a change in colour be measured?
using a colorimeter
how is a colorimeter able to measure a colour change efficiently?
as it’s able to monitor the colour change more accurately and over time using a light sensor
what are some examples of reactions what measure the colour change?
- reaction between sodium thiosulfate and hydrochloric acid (cross on paper disappears)
- acidified propanone and iodine (brown colour will fade and a colorimeter can measure the colour change and therefore the change in conc over time)
measuring the rate by timing how long it takes for a cross below the container to disappear can be used if a ___ forms as a product?
precipitate
what does the method used to determine the rate of reaction depend on?
the reaction itself
(e.g the state, if precipitate formed etc)
(e.g CaCO3 (s) + 2HCl(aq) -> CaCl2 (aq) + H2O (l)+ CO2 (g)
products are a solution, liquid and a gas. none of the reactants are coloured. so a gas syringe would be used to measure the change in gas)
but also measuring the change in pressure would also work as the moles of gas are not the same on the reactant and products side)
why doesn’t using an inverted measuring cylinder in a water trough not work well when measuring the volume of carbon dioxide?
because CO2 is very soluble in water and so will dissolve in the water rather than collect at the top of the cylinder where it can be measured
what three reactions can be monitored by measuring the rate of reaction by gas collection?
- reactions between an acid and a metal carbonate to produce CO2
- reactions between an acid and a metal to produce hydrogen
- the decomposition of hydrogen peroxide using a catalyst (such as manganese (IV) oxide)
what are the steps of how to carry out a gas collection experiment for the reaction of calcium carbonate with hydrochloric acid?
- measure 50cm^3 of HCl solution of known conc into the conical flask using the 50cm^3 measuring cylinder
- weigh 2g of CaCO3 powder using a weighing boat
- set up the gas syringe in a clamp and stand
- put the CaCO3 powder into the conical flask and then quickly put the rubber stopper and delivery tube into the neck of the conical flask. at the same time, start the stopwatch
- stop the stopwatch as soon as the volume of gas in the syringe reaches 100cm^3
- record this result in an appropriate table and calculate the average rate for this reaction in cm^3s^-1
- repeat steps 2-7 using different concentrations of HCl solution
- use your data to plot a graph of rate (volume/time) against concentration of HCl solution
note: if the volume doesn’t reach 100cm^3, you can stop the timer when the volume stops increasing. the reaction rate can then be calculated usign the time and the volume of gas collected
what reaction is a good reaction in order to compare the rates of reaction under different conditions?
iodine clock reaction
what does the iodine clock reaction involve?
- setting up a number of experiments in which initial concentrations are known and the time it takes for each reaction is recorded
what must be produced in the iodine clock reaction?
iodine must be produces as part of the reaction
what happens in the most common iodine clock reaction?
- iodine ions are oxidised by hydrogen peroxide to iodine, and the presence of iodine is indicated using starch solution
- a strong blue colour is obtained with even the smallest amount of iodine
H2O2 (aq) + 2I- (aq) + 2H+ (aq) -> I2 (aq) + 2H2O (l)
if however, a given amount of thiosulfate ions are present, iodine will react very quickly with it, reforming iodide ions
therefore no blue colour will appear until all of the thiosulfate has been used up
2S2O3 2- (aq) + I2 (aq) -> S4O6 2- (aq) + 2I- (aq)
the time taken for this to happen acts as a type of ‘clock’ to measure the rate of iodine ions being oxidised
what are the steps to carry out an iodine clock reaction? example
- add the following reagents to a 250^3 conical flask:
10cm^3 of 1 moldm^3 sulfuric acid
25cm^3 of 0.1 moldm^3 potassium iodide
10^3 of 0.005moldm^-3 sodium thiosulfate
1cm ^3 of 1 moldm^-3 starch solution - add 5cm^3 of 0.1moldm^-3 hydrogen peroxide to the reaction mixture and start the stopwatch immediately after the addition
- stop timing when the solution turns blue-black and record the time
- repeat steps 1-3 using different concentrations of hydrogen peroxide
the temp has to be kept constant as rates vary a great deal with the slightest change in temp. you should test at least three different concs of the peroxide in order to ensure a good spread of results
as the rate of reaction is proportional to 1/time and the total volume is constant for each experiment, conc of peroxide is proportional to the vol of peroxide used in each experiment. plotting a graph of 1/time against conc of peroxide will give the relationship between peroxide conc and rate
in the iodine clock reaction, does the reaction temperature need to be kept the same between experiments if the effect of concentration is being investigated?
yes
why is sodium thiosulfate present in the iodine clock reaction?
to reduce iodine molecules to iodine ions in order to delay the formation of iodine
why is starch present in the iodine clock reaction?
to form a blue-black colour when iodine is present in the reaction
how is the rate of reaction of the iodine clock reaction calculated?
by dividing 1 by the time taken for the reaction mixture to change colour
in the iodine clock reaction, does the concentration and volume have to kept constant for all reagents except for the reagent that is being investigated?
yes
an iodine clock experiment was set up to determine the effect of hydrogen peroxide conc on the rate of reaction.
what is the relationship between the rate and the conc of hydrogen peroxide?
they are directly proportional
an iodine clock experiment was set up to determine the effect of hydrogen peroxide conc on the rate of reaction.
why is the rate directly proportional to the concentration or hydrogen peroxide?
- increased conc of hydrogen peroxide increases the no. of hydrogen peroxide particles which leads to an increases no. of collisions which leads to an increased no. of successful collisions
an iodine clock experiment was set up to determine the effect of hydrogen peroxide conc on the rate of reaction.
why is it important that the concentrations and volumes of the other reagents involved in the reaction remain constant between experiments?
so that the results we collect are only affected by a change in conc of the hydrogen peroxide
an iodine clock experiment was set up to determine the effect of hydrogen peroxide conc on the rate of reaction.
why is it important to keep the temperature constant between each experiment?
as a small change in temperature can have a significant effect on the results
this is due to the fact that a higher temperature increases the kinetic energy of the particles, leading to a greater number of particles with an energy greater than the activation energy.
this leads to a greater chance of successful collisions
an iodine clock experiment was set up to determine the effect of hydrogen peroxide conc on the rate of reaction.
how is it possible to investigate the effect of the concentration of another reagent?
if you keep the conc and volume of all the other reagents, including hydrogen peroxide, the same
an iodine clock experiment was set up to determine the effect of hydrogen peroxide conc on the rate of reaction.
how is it possible to investigate the effect of temperature?
if all concs and volumes for all reagents are kept constant between all experiments
how do you calculate the rate of a reaction?
rate = change in conc/time
units = moldm^3s^-1
if another variable, such as mass or volume is measured, the rate can be expressed in corresponding units such as gs^-1 or cm^3s^-1
what is collision theory?
for a chemical reaction to take place:
- reactant particles must collide with the correct orientation
- reactant particles must have the activation energy
- for a chemical reaction to take place, reacting molecules must collide effectively
- only a small fraction of the total number of collisions leads to a reaction
- but, the greater the number of collisions, the higher the chance that some of them will be effective
- for a collision to be effective the molecules must collide in the correct orientation and have sufficient energy
- any factor that increases the change of effective collisions will also increase the rate of reaction
- the minimum energy needed is called the activation energy
does the curve touch the energy axis (x-axis) in the Boltzmann energy distribution curve?
no
do homogeneous catalysts take an active part in a reaction?
yes
(rather than being an inactive spectator)
what are some examples of homogenous catalysts?
- concentrated sulfuric acid in the formation of an ester from a carboxylic acid and an alcohol
- aqueous iron (II) ions, Fe 2+ (aq), in the oxidation of iodide ions, I- (aq), by peroxodisulfate (VI) ions, S2O8 2- (aq)
what are some examples of heterogeneous catalysts ?
- iron in the Haber process for ammonia production
- vanadium (V) oxide in the contact process within sulfuric acid manufacture
- nickel in the hydrogenation of unsaturated oils in the production of margarine
what type of catalyst is often favoured in industry?
heterogeneous catalysts
because they are easily separated from the products
why does industry rely heavily on catalysts?
to reduce costs
less energy is required for the molecules to react, and this saves energy costs
how do catalysts have benefits for the environment?
- much of the energy for reactions is taken from electricity supplies or burning fossil fuels so a catalyst
- if less fossil fuel is burnt, less carbon dioxide will be released during energy production
what are some benefits of using enzymes as catalysts?
- lower temperatures and pressures can be used, saving energy and costs
- they operate in mild conditions and do not harm fabrics or food
- they are biodegradable. disposing of waste enzymes is no problem
- they often allow reactions to take place which form pure products with no side reactions, removing the need for complex separation techniques
marble chips react with hydrochloric acid to produce calcium chloride, water and carbon dioxide. how could you increase the rate of this reaction?
- increase the surface area of the marble chips by turning them into a powder
- increase concentration of acid
- increase temperature of the reaction
