Unit 2.2 - Rates of reaction Flashcards
what do all reactions occur at different?
speeds or rates
Give an example of a reaction with a slow rate
rusting of steel
Give an example of a reaction with a fast rate
firework reactions
List 6 factors which increase the rate of a reaction
increasing the concentration of the solution
increasing the pressure of the gas
increasing the surface area of the solid
increasing the temperature
using a catalyst
light (for some reactions)
How do we measure the rate of a reaction?
change in quantity of reactant or product per unit time
What’s the most common equation for calculating a rate?
rate of reaction = change in concentration
———————————
unit time
What is the rate of reaction equation regarding concentration used for?
for a reactant or product in solution
What’s the unit of a rate?
depends on the specific quantity being measured
Unit of rate of reaction when concentration is involved
moldm^3s^1
Unit of rate of reaction when mass is involved
gs^1
How can a rate of reaction be measured?
A change in concentration of the reagent or the product - has to be found be experiment
What change in a reaction is usually difficult to follow?
concentration, so usually other properties such as colour change is measured
What do we need to find in order to calculate the rate of a reaction?
a physical or chemical property which varies over time
What are all of the experiments for calculating a rate done at the same?
temperature
How do we follow the rate of reaction?
measure the concentration of reactant or product over time and plot a graph
How do we measure the rate of a reaction at an exact point?
gradient on a graph
What happens to the gradient of an ‘amount of product’ v.s time graph show over time?
the gradient gets less steep over time - the rate of reaction is decreasing over time
Why does the rate of a reaction decrease over time?
there’s less reactant available for collisions to make products
What does a flat line on an ‘amount of product’ v.s time graph represent?
the reaction has stopped
What does a steeper line on an ‘amount of product’ v.s time graph represent?
A greater rate of reaction
When is the rate of a reaction at its greatest and why?
at the beginning as is decreases as the reagent is used up
What does a horizontal straight line on an ‘amount of product’ v.s time graph represent?
the rate of the reaction is zero - the reaction has stopped
At which point on a graph can we see that the reaction has stopped?
horizontal straight line on an ‘amount of product’ v.s time graph
Is concentration directly proportional to the rate of a reaction?
not always - sometimes its independent of it, other times is actually is directly proportional
What do we always need to mention when discussing measuring rates of reaction?
the word “time”
How do we know which way to measure the rate of a reaction?
we need to pay attention to the equation
What are the 5 ways in which we can measure the rate of a reaction?
change in…
gas volume
pressure
concentration
colour
mass
Example of a reaction in which the change in gas volume can be measured to work out the rate
decomposition of hydrogen peroxide
What is the concentration of gas directly proportional to?
volume
What do we plot on our graph when measuring the change in gas volume?
Volume of oxygen gas against time
How is the volume of a gas measured?
using a gas syringe
How do we use a gas syringe when measuring gas volume in order to calculate a rate?
at regular intervals
Why is using a syringe useful for measuring a change in gas volume?
The volume is measured at constant pressure
What type of reactions can measuring the change in pressure only be used for?
reactions involving gases
What causes pressure to change?
if the number of moles of gas differs between reactants and products, then the pressure will change
How can the pressure change of a reaction be measured?
in a sealed container at regular time intervals using a manometer
What does a manometer measure
pressure
What does a gas syringe measure?
gas volume
How do we measure the change in concentration of a reaction?
Sampling and quenching
Is sampling and quenching an effective method for measuring the rate of a reaction?
It’s laborious, but yes, it’s accurate
What does the process of sampling and quenching measure?
Change in concentration
Example of a process that can be monitored using sampling and quenching to measure its concentration
Hydrolysis of 1-bromobutane
How does sampling and quenching work?
As the reaction proceeds, the concentration of the reactant decreases
At given time intervals, a sample of the mixture is removed, cooled rapidly to stop the reaction and titrated against HCL of known concentration
Plot volume of acid v.s time
(The volume of HCl used is directly proportional to the concentration of hydroxide)
How do we measure the change in colour in a reaction to measure the rate?
Colorimeter
What are colorimeters used for?
Measuring the change in colour of a reaction
How can we measure the initial rate of reaction for a colour change reaction?
1/time
What’s an example of a reaction in which a colour change occurs?
Sodium thiosulfate and HCL
How can we draw a graph when experimenting with a change of colour? From which data>
By measuring the initial rate of reaction (1/time) for different volumes (concentration) of the thiosulfate, a plot of rate of reaction against concentration can be drawn - shape shows their relationship
What does a colorimeter monitor and how?
The rate of reaction by monitoring the concentration of the substance changing colour
How does a colorimeter actually measure the concentration of a substance changing colour?
As a precipitate forms, light is blocked, so the absorbance falls, which is measured by the colorimeter
Draw and label a diagram of how a colorimeter works
(See notes)
When can we measure the change in mass to measure the rate of a reaction?
When a heavier gas like CO2 is released during the reaction and allowed to escape
Give an example of a heavy gas that could cause a change in mass when released in a reaction
CO2
How can we measure the change in mass when CO2 is allowed to escape during a reaction?
At regular time intervals using an accurate balance
Sketch a graph showing the mass lost of a substance that released CO2 in a reaction over time
(See notes)
Instead of measuring the mass remaining, how else could we have measured the rate of the reaction that released heavy CO2 gas?
Measured the volume of CO2 gas released against time using a gas syringe
Why does increasing the temperature, or pressure, or surface area etc increase the rate of a reaction?
Cause more successful collisions of particles per unit time
When do molecules react?
When 2 molecules collide with sufficient energy to overcome the energy barrier to the reaction
What must molecules have in order to react?
A definite energy
Define activation energy
The minimum energy that molecules must posses for a successful collision to occur
Describe the activation energy of a slow reaction
High
Describe the activation energy of a fast reaction
Low
Draw and label an exothermic energy profile
(See notes)
Draw and label an endothermic energy profile
(See notes)
Describe the general activation energy of an exothermic reaction
Small
Describe the general activation energy of an endothermic reaction
Large
Do lots of collisions between molecules lead to reactions? Why?
No, only a small fraction of molecules have sufficient energy to overcome the energy barrier to a reaction - few collisions actually result in a chemical reaction
What is a reaction rate really a measure of?
How frequently effective collisions occur
What are explanations of rates of reaction based on?
Collision theory
What does collision theory lead to?
The explanation of rates of reaction
Collision theory
For a reaction between 2 molecules to occur, an effective collision must take place (i.e - a collision that results in the formation of product molecules)
Not all WHAT result in chemical reactions?
Collision between molecules
What do we need to increase in order to increase the rate of reaction and why?
The number of collisions between molecules, as the greater the number of collisions, the higher the change that some of them will be effective
What makes a collision effective?
The molecules must collide in the correct orientation and with enough energy to react
What factor will increase the rate of a reaction?
Any factor that increases the rate of effective collisions
Name the factors that effect the rate of reaction
Concentration (pressure for gases)
Temperature
Particle size
Catalysts
Light
How do you increase the rate of reaction in terms of concentration?
Increase the concentration
How do you increase the rate of reaction in terms of pressure?
Increase the pressure
What type of substances can you increase the pressure of?
Gases
Give an example of using a more concentrated reactant in order to increase the rate of reaction
Concentrated instead of weak acid for magnesium to produce hydrogen gas gives a more vigorous effervescence
Why does increasing the concentration of reactants increase the rate of reaction?
Increased concentration
More molecules in a given volume
Distance between molecules is decreased
Increase in number of collisions per unit time
Greater chance that the number of effective collisions increases
The rate of reaction increases
Why does increasing the pressure increase the rate of reaction?
It’s the same as increasing the concentration, as pressure is proportional to concentration
How do we increase the rate of reaction in terms of temperature?
Increase
Give an example of increased temperatures increasing the rate of a reaction
Milk going sour faster on a hot day
Why does increasing the temperature increase the rate of a reaction?
Increase in temperature
Increase in kinetic energy of molecules
Move faster
More molecules have enough energy to react on collision
Increased reaction rate
What must happen to particle size to increase the rate of a reaction?
Decrease
How can we practically decrease our particle size to increase the rate of reaction?
In a reaction involving a solid, break down the solid into smaller pieces
In what type of reactions does changing the particle size effect the reaction rate?
Ones involving solids
Give an example of changing the particle size to increase the rate of reaction
Using powdered instead of a ribbon of metal reacts faster in acid
Why does changing the particle size effect the rate of a reaction?
Reducing the particle size
Increased surface area
Molecules are closer together
Increase in the number of collisions per unit time
Increase in reaction rate
How do catalysts effect reaction rates?
Adding a catalyst increases the rate of reaction
Example of a catalyst increasing the rate of reaction
Manganese (IV) oxide helps hydrogen peroxide decompose at room temperature
What does light do to the rate of reaction?
(Some reactions) - added bright light leads to a more vigorous reaction
Example of a reaction in which added bright light increases the rate of reaction
Photochlorination of methane
Catalyst definition
A substance which increases the rate of a chemical reaction but is unchanged itself at the end of the reaction
List 4 things that a catalyst does
Increases the rate of reaction and so decreases the time taken to reach equilibrium
Does not change the position of equilibrium
Provides an alternate pathway for the reaction of lower activation energy
Increases the rate of the forward and backward reactions to the same extent
What are the 2 types of catalyst?
Heterogenous
Homogenous
Heterogenous catalyst
In a different physical state as the reagents
Example of a heterogenous catalyst
Iron in the Haber process for the manufacture of ammonia
Haber process equation
N2 (g) + 3H2 (g) —> 2NH3
<—
Iron oxide catalyst (s)
How do heterogenous catalysts work?
Provides a surface for the reactant particles to absorb to, which holds them in place to react
The products then desorb from the surface
Adsorb
The process of gas or liquid particles forming a thin film on a solid surface
The process of gas or liquid particles forming a thin film on a solid surface
Adsorb
Desorb
The release of adsorbed particles from the surface of a solid
The release of adsorbed particles from the surface of a solid
Desorb
What are often heterogenous catalysts and why?
Transition metals because they have empty d orbitals available of suitable energy for forming temporary bonds with the reactant molecules
What are transition metals often used as? Why?
Heterogenous catalysts
They have empty d orbitals available of suitable energy for forming temporary bonds with the reactant molecules
Homogenous catalysts
In the same physical state as the reagents
How do homogenous catalysts work?
Take active part in the reaction and are then regenerated as the reaction proceeds
Example of a homogenous catalyst in a reaction
Concentrated sulfuric acid in the reaction between ethanol and ethanoic acid to form an ester (esterificiation)
What does concentrated sulfuric acid do in esterification and what is this process exactly?
Protonates the alcohol
The reaction between ethanol and ethanoic acid to form a ester
What are enzymes?
Biological catalysts
What conditions do enzymes work under and why is this useful?
Mild conditions (roughly 40 degrees Celsius - body temperature), meaning less energy consumption
What type of catalysts require severe conditions to react?
Inorganic catalysts
What do inorganic catalysts require to react?
Severe conditions
Give an example of an inorganic catalyst in action?
The breakdown of hydrogen peroxide in the liver
Draw an energy produce showing both a catalysed and uncatalysed reaction, labelling their activation energies too
(See notes)
What might catalysts help us achieve?
Some of the aims of green chemistry
Give some examples of developments in green chemistry that catalysts could help us achieve
The development and discovery of new resources
Developing the infrastructure and technology require for new methods of energy generation in a green and sustainable manner
Re-engineering reaction methodologies to allow for the possibility of lower temperatures and pressures
Which group of catalysts will have a large role to play in green chemistry?
Enzymes
What’s different between particles at any temperature?
At any temperature, not all particles in a liquid or gas have the same velocity, as they don’t all have the same kinetic energy
Under which conditions do not all particles in a liquid or gas have the same velocity and hence not the same kinetic energy?
Even at the same temperature
What does a max week-Boltzmann energy distribution curve show?
The energy distribution of the particles in a system
What shows the energy distribution of particles in a system?
A maxwell-Boltzmann energy distribution curve
Draw and label a Maxwell-Boltzmann distribution curve labelling…
Most probable energy
Mean energy
Activation energy
(See notes)
What is the highest peak of a Maxwell-Boltzmann energy distribution curve?
The most probable energy at that temperature
Where is the mean energy placed on a Maxwell-Boltzmann energy distribution curve and why?
Slightly to the right of the most probable energy as the mean energy of all the particles is always slightly higher than the most probable energy, due to the greatest number of particles having an energy greater than the most probable energy
What are there none of on a Maxwell-Boltzmann energy distribution curve?
Particles with an energy value of zero at any temperature
Where does the curve of a Maxwell-Boltzmann energy distribution graph never touch and why?
The x-axis as the number of particles approaches zero at higher energies but never reaches it
At which point on a Maxwell-Boltzmann energy distribution curve does the number of particles approach zero?
At higher energies (but, it never reaches zero)
What does the area under a Maxwell-Boltzmann energy distribution curve represent?
The total number of particles in the system
What represents the total number of particles in a system on a Maxwell-Boltzmann energy distribution curve?
The area under the curve
What must the area under the curve of a Maxwell-Boltzmann energy distribution curve remain when in a closed system? Why?
Constant as the area represents the total number of particles in the system
In what type of system does the area under the curve of a Maxwell-Boltzmann energy distribution curve remain constant and why?
Closed system as the area underneath represents the total number of particles in the system
Closed system
A chemical reaction in which none of the reactant or product particles can be removed
What does increasing the temperature do to the rate of a reaction?
Increases the rate of a reaction
When do molecules react on collision?
If they have energies greater than the activation energy
How many molecules have energies greater than the activation energy?
Only a small fraction
Explain how increasing the temperature increases the rate of reaction
Increasing the temperature
Increase in kinetic energy of molecules
Molecules move faster
More have energies greater than the activation energy
Number of collision which result in a reaction increases considerably
Increased rate of reaction
What leads to an increase in the rate of a reaction and what must it not be confused with?
An increase in the number of collisions between molecules with sufficient energy to react
NOT
The increase in the number of collisions in general
Draw a Maxwell-Boltzmann energy distribution curve of the same reaction at 2 different temperatures
(See notes)
In which direction does the peak distribution of a Maxwell-Boltzmann energy distribution curve shift at higher temperatures and why?
To the right
Higher overall energy of particles at a higher temperature (mean velocity increases)
What happens to the peak height of a Maxwell-Boltzmann energy distribution curve at higher temperatures and why?
Moves lower as the range of particle energies becomes wider
What happens to the area underneath of a Maxwell-Boltzmann energy distribution curve at higher temperatures and why?
Remains the same
Increasing the temperature does not change the total number of particles
What happens to the number of particles with energies greater than or equal to the activation energy on a Maxwell-Boltzmann energy distribution curve at higher temperatures and what does this lead to?
Increases, leading to a greater chance of successful collisions
What happens to ALL reactions as the temperature increases?
They all go faster, including exothermic reactions
What can catalysts have an effect on?
The activation energy of a reaction
How does a catalyst actually effect a reaction?
Increases the rate of a reaction
Provides an alternative pathway of a lower activation energy
(Basically - makes a reaction occur more easily)
What does adding a catalyst do to the Maxwell-Boltzmann energy distribution curve and why?
It doesn’t change its shape as it doesn’t add energy to the reaction
What happens when adding a catalyst to the position of the activation energy on a Maxwell-Boltzmann energy distribution curve and why?
It moves to the left
Adding a catalyst lowers the activation energy
Draw and label a Maxwell-Boltzmann energy distribution curve showing the activation energy of a catalysed and uncatalysed reaction
(See notes)
What occurs to the left of the activation energy on a Maxwell-Boltzmann energy distribution curve?
No reaction
What happens to the right of the activation energy on a Maxwell-Boltzmann energy distribution curve?
Reaction
Why can the particles to the left of the activation energy on a Maxwell-Boltzmann energy distribution curve nt react?
They have low energies
How does a catalyst lowering the activation energy increase the rate of a reaction?
Lower activation energy
Additional particles with energy greater than or equal to the new, lower activation energy
Increases the chance of successful collisions occurring
What happens to the reactant particles as time passes during a reaction? What does this lead to?
They get further apart (concentration decreases)
Less frequent collisions
Lower rate
What would we do to determine if one catalyst is better than another?
Time how long it takes to produce a fixed volume
Use the same volume and concentration of reactant
Use the same amount of catalyst
The better catalyst will increase the rate by more
What does a Maxwell Boltzmann distribution describe?
Describes the distribution of energies among particles in a sample of gas at a given temperature
What’s happens to iodine ions at a measurable rate?
can be oxidised to iodine
What can be used to measure the rate at which it’s ions are oxidied?
iodine
In what type of solution does iodine turn a strongly coloured blue complex?
starch solution
What does iodine do in starch solution?
turn a strongly coloured blue complex
In what situation will iodine not immediately turn blue in starch solution?
if a given amount of thiosulfate ion - with which iodine reacts very rapidly as it reforms iodide ions - is added, no blue colour will appear until enough iodine has been formed to react with all of the thiosulfate
what type of ion does iodine react very quickly with and to do what?
thiosulfate ions
to reform iodide ions
When will the blue colour appear for iodine with thiosulfate ions?
when enough iodine has been formed to react with all of the thiosulfate
What acts as the ‘clock’ to measure the rate of iodide ions being oxidised?
the time taken for enough iodine to be formed to react with all of thiosulfate ions to turn the solution blue
What does the time taken for enough iodine to be formed to react with the thiosulfate ions in order to turn the solution blue act as?
a ‘clock’ to measure the rate of iodide ions being oxidised
what’s a suitable example of an iodine clock reaction?
oxidising iodide ions by hydrogen peroxide in acid solution
Thiosulfate ion
S203^2-
S2O3^2-
Thiosulfate ion
equation for oxidising iodide ions by hydrogen peroxide in acid solution
H2O2 (aq) +2H+ (aq) + 2I- (aq) –> 2H2O (l) + I2 (aq)
equation for iodine reforming iodide ions with thiosulfate ions
I2 (aq) + 2S2O3^2- (aq) –> 2I- (aq) + S4O6^2- (aq)
What are we varying and measuring in an iodine clock reaction?
varying - concentration
measuring - rate
what can the iodine clock reaction be used for?
measuring the dependence of rate on concentration for any reactant
what should always be kept constant during an iodine clock reaction and why?
temperature - rates vary rapidly with temperature
What are the 2 ways of completing an iodine clock reaction?
FIRST WAY
Rate is directly proportional to 1/time
volume is constant, so concentration of H2O2 is equal to the volume of peroxide used
graph of 1/t v.s volume of peroxide
shows the relationship between concentration of H2O2 and rate
SECOND WAY
varying the concentration of potassium iodide to find the affect of concentration of iodide ions on reaction rate
BUT we muse add peroxide solution last every time
Hydrogen peroxide
H2O2
H2O2
hydrogen peroxide
Why is measuring the time taken to produce excess iodine in an iodine clock reaction only an approximation for the rate as the reaction proceeds?
The reaction is faster at first (higher concentration of reactant)
Average rate is measured
Where do we take the gradient on a graph if asked for INITIAL rate of reaction?
time = 0
What is the catalyst involved in the Haber process for the manufacture of ammonia?
Iron
What is the catalyst used in the production of esters?
Concentrated sulphuric acid
How do you measure the rate of a reaction where a precipitate is formed (measuring temperature change for example)?
- Time how long it takes a cross to disappear from a piece of paper under the flask (look from the top)
- 2nd time - increase the temperature (keep other quantities the same)
- Time to the same end point (same density of precipitation)
- Can be repeated for many temperatures
Why can transition metals and their compounds act as catalysts?
Ability to form co-ordinate bonds
Partially filled 3d energy levels
Variable oxidation states
Ability to absorb “molecules”
How do we calculate a rate at an exact point on a graph?
Gradient
Why would someone use cotton wool at the neck of a flask when completing an experiment that measures the change in mass over time?
Allows gas to escape, but stops acid from spraying
What does doubling the acid concentration but halving the volume do to the value of change in temperature and why?
Double the Change in temperature
Enthalpy change/q = constant, but the mass has decreased
What happens to the change in temperature of a reaction if we use nitric acid instead of hydrochloric?
Same change in temperature
Both strong
Same number of moles H+
Why do we measure out the reactants as a single batch at the beginning of before starting the iodine clock reactions?
For efficiency
Why do we measure the peroxide out in the boiling tube at the beginning before adding to the solution during an iodine clock reaction?
So that it can be added quickly, since the reaction starts immediately after it’s been added
What’s the alternative method for measuring the rate of reaction using an iodine clock reaction?
Measure the intensity of the colour of the iodine over time using a colorimeter
How does a graph show that the rate of a reaction decreases as the reaction proceeds?
it becomes less steep
where is the transition state on an energy profile?
at the top of the curve before it drops
Summarise the iodine clock reaction
1.) 5 different conical flasks of different concentrations peroxide and water
2.) in a separate conical flask, add the 2 other reagents
3. Add starch to these separate flasks
4.) rapidly add H2O2 to the first flask and stir
5.) stop timing when blue
6.) repeat for other concentrations
