Physical chem - Kinetics
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Definition: Rate of reaction + units
The change in concentration of a substance per unit of time. Measured in moldm-3 s-1 (moles per decimetre cubed per second)
LIST only the ways rate of reaction can be increased
1) Increasing temperature 2) Increasing concentration of reactants 3) Adding a catalyst 4) Increasing the pressure (in a gaseous reaction) 5) Increasing the surface area of a solid reactant
What kind of graph can rate of reaction be measured from and how?
Concentration-time graph • the gradient of the curve represents the rate of reaction • Draw a tangent to the curve at a chosen time • Measure change in Y which is then divided by change in X between two points on the line chosen
Collision theory: What is collision theory?
Particles must collide with sufficient energy and the correct orientation for a reaction to occur.
Collision theory: Name and describe the two types of collisions between particles
Successful and unsuccessful Unsuccessful collisions - transfer energy but don’t result in the formation of new products (no reaction) Successful collisions - do result in the transfer of reactants into products. Successful collisions only occur if both particles have energy greater than or equal to the Ea.
Definition: What is activation energy (Ea)?
The minimum amount of energy needed for a reaction to occur
The effect of changing concentration on the rate of reaction
• When concentration is increased, more moles of particles are being added into a given volume/space • This increases the frequency of collisions/ there will be more collisions per second • This then also increases the frequency of successful collisions
The effect of changing pressure (gaseous reactions) on the rate of reaction
The ideal gas equation shows how pressure is inversely proportional to volume; as one increases the other decreases. If you increase the pressure, then the volume must be decreasing. • increased pressure and decreased volume means the particles are pushed closer together • this increases the frequency of collisions • which will then also increase the frequency of successful collisions
The GENERAL effect of changing temperature on the rate of reaction
• At higher temperatures, particles have more energy and this increases the frequency of collisions, leading to more effective collisions • At lower temperatures, particles have less energy and this decreases the frequency of collisions, leading to less effective collisions
Which graph shows the ADDED effect of changing temperature on the rate of reaction?
The Maxwell Boltzmann Distribution Curve
The Maxwell Boltzmann Distribution Curve: • What does the area under the graph represent?
The total number of molecules in the system
The Maxwell Boltzmann Distribution Curve: • Where on the graph can the most common energy (mode) be found and what is another name for this? • Where can the average energy be found and why is it found here?
• the most common energy is known as the most probable energy, found to the left of the graph/ aka the peak of the graph • the mean energy is found to the right the graph/most probable energy because the particles at very high energy skew the graph to the right
The Maxwell Boltzmann Distribution Curve: Where does the graph always start and why? How does the graph always end and why?
Always starts at the origin because no molecules can have zero energy. At the end, the graph touches the x-axis because there is no theoretical maximum energy a particle can have
The Maxwell Boltzmann Distribution Curve to show increased temperature: What generally happens to the peak of the curve?
Goes lower and shifts towards the right side of the graph
The Maxwell Boltzmann Distribution Curve to show increased temperature: What is the difference in particles in correlation to in Ea, between T1 and T2?
With T2, a greater proportion of molecules can overcome the Ea (greater area under the curve) Therefore a greater proportion of collisions now lead to a reaction.
The effect of adding a catalyst on the rate of reaction: What is the transition state?
The point in the reaction where existing chemical bonds are in the process of breaking and new chemical bonds are in the process of forming. It is unstable and goes on to form products
The effect of adding a catalyst on the rate of reaction: Definition of a catalyst
A substance which increases the rate of a reaction without being used up in the process. Represented on graph as Ec
The effect of adding a catalyst on the rate of reaction: Explain how a catalyst works
It provides an alternative route for the reaction which has a lower activation energy than the un-catalysed route.
Catalysts: What are the two types of catalysts?
Homogeneous - catalyst in the same physical state as the reactants. Heterogeneous - catalyst is in a different physical state to the reactants.
Catalysts: How do homogeneous catalysts work?
They react with the reactants to form intermediates. The intermediates then react to form the final products The catalyst is regenerated
Catalysts: Industrial example of a heterogeneous catalyst and the reaction it catalyses.
Fe(s) - Involved in making ammonia (Haber process) N2(g) + 3H2(g) <—Fe(s)—> 2NH3(g) reversible reaction
The advantage of catalysts on the environment
reduce demand for fossil fuels; if a chemical process can run at the same efficiency at a lower temperature in the presence of a catalyst as it did at the higher temperature, then the demand for energy is lower. Meaning less fossil fuels burnt and so less greenhouse gas emissions