Chemical kinetics Flashcards
What is kinetics
It is a study of the rates of reaction. This is measured by a decrease in the concentration of reactants per unit time and an increase in the concentration of products per unit time.
What is collusion theory
Collision theory states that when suitable particles of the reactant hit each other, only a certain amount of collisions result in a perceptible or notable change; these successful changes are called successful collisions. This is for reactants in gaseous, liquid or aqueous states.
Substances in liquids, aqueous solutions and gasses are in constant motion. When they collide some substances react. These successful collisions are based on three factors- the collision frequency, energy and orientation. Changing any of these factors effects the rate of a chemical reaction.
This is dependant on the frequency, energy and orientation of collisions. Changing any of these factors also changes the rate of the reaction.
What is collision frequency?
If a reaction is to occur, two particles must first collide. The number of these collisions is the CF.
The CF can be affected by concentration pressure temperature and surface area.
What is collision energy?
Not all collisions result in a reaction due to insufficient energy to break bonds. The combined energy of particles colliding is called the CE
What is the correct orientation?
Different molecules have functional groups on various parts of the molecule and these must interact for a reaction to occur.
What is activation energy?
The minimum energy needed to start a chemical reaction. collision energy must be equal or greater than the AE.
What are the key features of a maxwell Boltzmann distribution curve?
Gas in a sample moves at different speeds depending on the amount of Ke
It depicts the energy distribution of a sample of gas particles (no of mol with particular energy on Y and molecular energy on X).
The graph starts at (0,0) as 0 molecules have no energy
The area under the graph represents the total number of molecules
A point along the graph represents the number of molecules with that given energy.
The apex represents the most likely energy a particle would have (mode) whilst the average is found right of the peak (mean)
How does temperature affect the MBDC?
The temp increases and the peak moves down and to the right (broader and flatter curve) as there is a greater distribution of molecules with a higher energy meaning more would molecules would have AE. The area under the graph would remain the same but the proportion of molecules to the right of the Ea line increases.
How do catalysts affect RR?
Catalysts reduce the AE needed by providing an alternative pathway for the reaction to progress. They actively help the bond breaking and forming processes. This makes it easier to turn reactants into products and requires less energy.
Therefore more particles have the required AE to react resulting in more successful collisions. CF/CO/CE are unchanged.
What are the different kinds of catalysts
Homogeneous catalyst- The cat is in the same physical state as the reactants eg H+ ions in the esterification reaction.
Heterogeneous catalyst- The cat is in a different physical state as the reactants eg iron in the Harber process where the reactants are nitrogen and hydrogen- iron assists in bond breaking and forming to rearrange molecules on the iron surface and produce ammonia
Catalyst remain chemically unchanged at the end of a reaction.
How does concentration affect RR?
This affects a gas or liquid mixture. Effecting the concentration alters the particle per unit volume. The more particles there are in a given volume the more likely there will be a successful collision. CF increases.
How does pressure affect RR?
This affects gas mixtures. Effecting the pressure alters the particles per unit volume. Increasing pressure is usually done by decreasing volume. The more particles in a given volume the more frequent the collisions the higher the rate of successful collisions
How does surface area affect RR?
If a solid is broken down smaller then more particles are exposed to other reactants, increasing the likelihood of a successful collision. CF increases
How does temperature affect RR
There are two main effects from raising the temp
Increase in collision energy- thermal energy is converted to kinetic energy which can be further converted into potential energy. Therefore this increases the overall collision energy and the chance of the CE being above the AE increases, increasing the likelihood of a SC. Increases the energy colliding particles have which in turn increases CE
Increase collision frequency- Thermal energy is converted to kinetic energy and therefore move faster and collide more often, increasing the likelihood of a SC.
What is transition state theory?
When two molecules are moving towards each other their electron clouds repel and they move apart before bond breaking for new bond formation.
If the kinetic energy of the molecules was increased then the molecules are able to rearrange electrons in the outer shell to form new bonds. The Ke is converted to potential energy.
A highly energetic and unstable species is formed at the point of maximum potential energy or the transition state (this is the intermediate compounds in trasition states)
Activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (the transition state) and particles of reactants in their initial state
