UNIT 6: Kinetics
eactivity 2.2 How fast? The rate of chemical change
R
.2.1: Measuring rate of reaction
2
- The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.
- As a reaction progresses, the rate of reaction decreases over time because the frequency of collisions between reactant particles
decreases.When examining the effect of a factor onrate of reaction, the instantaneous rate of reaction is used for comparison,
rather than the average rate.
● Determine rates of reaction.
∆[𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠]
- 𝑟𝑎𝑡𝑒 = ∆𝑡𝑖𝑚𝑒
∆[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]
- 𝑟𝑎𝑡𝑒 = ∆𝑡𝑖𝑚𝑒
● Calculation of reaction rates from tangents of graphs of concentration, volume or mass against time.
𝑟 𝑖𝑠𝑒
- 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 = 𝑟𝑢𝑛
𝑟𝑖𝑠𝑒
- Draw atangent linefor a specific point to then use 𝑟𝑢𝑛 (instantaneous)
- What methods are used to provide data to determine the rate of reactions?
- Gas volume over time: using a syringe to measure the volume of gas produced over time
- The clock reaction: time for enough precipitation to form to cover ‘X’
- Mass change over time: mass over time and change in mass over time
- Pressure sensor: volume of gas produced over time
- Change in intensity of color: graph of absorbance vs time, use of calibration curve to convert from absorption to
concentration
- Quenching reaction: Find the actual concentration for ‘freeze-frames’ of the reaction
- pH changes/conductivity: for any neutralization reaction/for a reaction where there is a significant change in conductivity.
- What experiments measuring reaction rates might use time as
i) a dependent variable:
The clock reaction
ii) an independent variable
Almost all the other experiments
2.2.2: Collision theory
- Species react as a result of collisions of sufficient energy and proper orientation.
- Explain the relationship between the kinetic energy of the particles and the temperature in kelvin, and the role of collision geometry
- All particles move randomly as a result of kinetic energy
- The average kinetic energy of the particles is proportional to temperature in kelvin
- For two species to react together they must come into contact with each other. They first have to collide, and then theymay
react. For a collision to be successful, this will depend on two factors:
- The particles must collide in the right orientation and with enough energy to react.
1) Orientation:
- The collisions between particles are random and so these will happen in a range of different orientations. Only the
reactions where the reactive parts of the molecules collide will be successful.
2) The energy of the collision:
- Particles must collide with a certain minimum kinetic energy which willovercome the forces of attractionbetween the
reactant molecules and break bonds. Only the moleculeswith kinetic energy equal to or more than the Ea will react.
, - According to collision theory the rate of reaction depends on the frequency of successful collisions of reactants influenced by the
orientation of the particles and the proportion of particles with energy greater than or equal to the activation energy.
2.2.3: Factors affecting rate of reaction
- Factors that influence the rate of a reaction include pressure, concentration, surface area, temperature and the presence of a
catalyst.
- Predict and explain the effects of changing conditions on the rate of a reaction
- Temperature:
- As the average kinetic energy of the particles (temperature) increases then the rate of reaction increases.
- A minor reason for this is that theparticles are moving faster and are therefore colliding more often.This higher
collision frequency leads to more successful collisions.
- A more important reason for this is that there is an increased number of particles with high energy. This leads to an
increased number of particles with energy greater than or equal to the activation energy.
- As a general rule, an increase of 10K doubles the rate of reaction
- Concentration:
- Rate of reaction increases as concentration increases.
- Increasing the concentration of reactantsincreases the frequency of collisions between particles. More collisions
in total means more successful collisions in a certain time.
- Pressure:
- Rate of reaction increases as pressure increase
- For reactions involving gases, an increase in pressureeffectively increases its concentration and so the rate
increases.
- Particle size/surface area
- Decreasing particle size leads to an increasing surface area leads to an increase in rate of reaction.
- Increasing the surface area of a reactant, especially a solid, enhances the frequency of collisions between reacting
particles.This heightened collision frequency leads to more successful collisions.
- Presence of a catalyst:
- A catalyst increases the rate of reaction
- A catalyst provides an alternative pathway with lower activation energy. This means that more particles will have
higher kinetic energy than the Ea and so will collidesuccessfully, causing a reaction, even at lower temperature.
- hat variables must be controlled in studying the effect of a factor on the rate of a reaction?
W
The rest of the factors that affect the rate of reaction must be controlled when studying one of them.
- How can graphs provide evidence of systematic and random error?
Through error bars we can tell the random error of an experiment, if they are big then there has been a large amount of variation
between results. If a line of best fit doesn’t pass through the origin and we know that it should, this demonstrates a possible
systematic error.
2.2.4: Activation energy
- Activation energy, Ea, is the minimum energy that colliding particles need for a successful collision leading to a reaction.
- Example of a normal Maxwell-Boltzmann energy distribution curve:
eactivity 2.2 How fast? The rate of chemical change
R
.2.1: Measuring rate of reaction
2
- The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.
- As a reaction progresses, the rate of reaction decreases over time because the frequency of collisions between reactant particles
decreases.When examining the effect of a factor onrate of reaction, the instantaneous rate of reaction is used for comparison,
rather than the average rate.
● Determine rates of reaction.
∆[𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠]
- 𝑟𝑎𝑡𝑒 = ∆𝑡𝑖𝑚𝑒
∆[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]
- 𝑟𝑎𝑡𝑒 = ∆𝑡𝑖𝑚𝑒
● Calculation of reaction rates from tangents of graphs of concentration, volume or mass against time.
𝑟 𝑖𝑠𝑒
- 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 = 𝑟𝑢𝑛
𝑟𝑖𝑠𝑒
- Draw atangent linefor a specific point to then use 𝑟𝑢𝑛 (instantaneous)
- What methods are used to provide data to determine the rate of reactions?
- Gas volume over time: using a syringe to measure the volume of gas produced over time
- The clock reaction: time for enough precipitation to form to cover ‘X’
- Mass change over time: mass over time and change in mass over time
- Pressure sensor: volume of gas produced over time
- Change in intensity of color: graph of absorbance vs time, use of calibration curve to convert from absorption to
concentration
- Quenching reaction: Find the actual concentration for ‘freeze-frames’ of the reaction
- pH changes/conductivity: for any neutralization reaction/for a reaction where there is a significant change in conductivity.
- What experiments measuring reaction rates might use time as
i) a dependent variable:
The clock reaction
ii) an independent variable
Almost all the other experiments
2.2.2: Collision theory
- Species react as a result of collisions of sufficient energy and proper orientation.
- Explain the relationship between the kinetic energy of the particles and the temperature in kelvin, and the role of collision geometry
- All particles move randomly as a result of kinetic energy
- The average kinetic energy of the particles is proportional to temperature in kelvin
- For two species to react together they must come into contact with each other. They first have to collide, and then theymay
react. For a collision to be successful, this will depend on two factors:
- The particles must collide in the right orientation and with enough energy to react.
1) Orientation:
- The collisions between particles are random and so these will happen in a range of different orientations. Only the
reactions where the reactive parts of the molecules collide will be successful.
2) The energy of the collision:
- Particles must collide with a certain minimum kinetic energy which willovercome the forces of attractionbetween the
reactant molecules and break bonds. Only the moleculeswith kinetic energy equal to or more than the Ea will react.
, - According to collision theory the rate of reaction depends on the frequency of successful collisions of reactants influenced by the
orientation of the particles and the proportion of particles with energy greater than or equal to the activation energy.
2.2.3: Factors affecting rate of reaction
- Factors that influence the rate of a reaction include pressure, concentration, surface area, temperature and the presence of a
catalyst.
- Predict and explain the effects of changing conditions on the rate of a reaction
- Temperature:
- As the average kinetic energy of the particles (temperature) increases then the rate of reaction increases.
- A minor reason for this is that theparticles are moving faster and are therefore colliding more often.This higher
collision frequency leads to more successful collisions.
- A more important reason for this is that there is an increased number of particles with high energy. This leads to an
increased number of particles with energy greater than or equal to the activation energy.
- As a general rule, an increase of 10K doubles the rate of reaction
- Concentration:
- Rate of reaction increases as concentration increases.
- Increasing the concentration of reactantsincreases the frequency of collisions between particles. More collisions
in total means more successful collisions in a certain time.
- Pressure:
- Rate of reaction increases as pressure increase
- For reactions involving gases, an increase in pressureeffectively increases its concentration and so the rate
increases.
- Particle size/surface area
- Decreasing particle size leads to an increasing surface area leads to an increase in rate of reaction.
- Increasing the surface area of a reactant, especially a solid, enhances the frequency of collisions between reacting
particles.This heightened collision frequency leads to more successful collisions.
- Presence of a catalyst:
- A catalyst increases the rate of reaction
- A catalyst provides an alternative pathway with lower activation energy. This means that more particles will have
higher kinetic energy than the Ea and so will collidesuccessfully, causing a reaction, even at lower temperature.
- hat variables must be controlled in studying the effect of a factor on the rate of a reaction?
W
The rest of the factors that affect the rate of reaction must be controlled when studying one of them.
- How can graphs provide evidence of systematic and random error?
Through error bars we can tell the random error of an experiment, if they are big then there has been a large amount of variation
between results. If a line of best fit doesn’t pass through the origin and we know that it should, this demonstrates a possible
systematic error.
2.2.4: Activation energy
- Activation energy, Ea, is the minimum energy that colliding particles need for a successful collision leading to a reaction.
- Example of a normal Maxwell-Boltzmann energy distribution curve:
