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Chemistry 2.8

AS edexcel chemistry - kinetics

How can you measure the rate of reaction? Change in concentration of reactants (mol/dm3) divided by change in time OR change in concentration of products (mol/dm3) divided by change in time
Draw a graph for the concentration of reactants and products over time and what does this show? Steep gradient at start- fast rate of reaction - high concentration of reactant, lots is being converted into product. At the end, gradient is 0 - no reaction occurs when all reactant has been converted into product. Overall decreasing rate of reaction
Draw the reaction profile diagrams for endo and exothermic reactions There should be: delta H, activation energy, transition state, reactants, products, enthalpy on y axis
What is a phase? 2 substances are in different phases if a boundary can be seen between them e.g. oil and water
What is a homogeneous reaction? Takes place in 1 phase
What is a heterogeneous reaction? Takes place in multiple phases
What must occur for a reaction to happen? (collision theory) The molecules of the reactants must collide - frequency of collisions determines rate of reaction. The collisions must have KE >/= activation energy. Molecules must collide with the correct orientation.
What is Ea? The minimum amount of energy that the colliding molecules must possess for a successful collision
What is the effect of a high activation energy? Reaction is slow - there are few molecules with KE that is greater than/equal to the activation energy, so the system is kinetically stable.
What is a Maxwell-Boltzmann model? Model of distribution of molecular energies is a graph with the number of molecules with a particular energy against the KE
What factors affect the rate of reaction? What type of reactions do they affect? Pressure (homogeneous reaction of gases), concentration (reaction of solutions), temperature (all), surface area (heterogeneous reaction involving a solid), catalysis (all, however the catalyst is specific to reactions)
How does pressure affect the rate of reaction? How do you make high pressure? High pressure = more reactant molecules per unit of vol = increased frequency of collision = by probability increased frequency of successful collisions = increased rate of reaction. Pump more reactant gas into container/ make volume of container smaller
How does concentration affect the rate of reaction? High conc = more reactant molecules per unit of vol = increased frequency of collisions = by probability, increased frequency of successful collisions = increased rate of reaction. Concentration and rate of reaction are directly proportional
How does temperature affect the rate of reaction? Higher temperature = increased rate of reaction. 1) molecules have higher average KE, so a greater proportion of collisions have KE >/= Ea and are successful. 2) increases chance of collisions and frequency of collisions
Draw a Maxwell-Boltzmann graph of two temperatures, where T2 > T1, and explain the curves. Both start at origin, end by tending towards x axis. Number of molecules on y, KE on x. Same area under curves. Peak of T2 right and lower than T1= at higher temperatures, more energy levels are accessible to the system, so the number of molecules...
Continued at each energy level is lower, and the most common energy level is of a higher kinetic energy. The area under the curves to the right of the activation energy is the number of molecules with sufficient KE for a successful collision.
How does surface area affect the rate of reaction? Greater surface area of solid e.g. powder = increased number of reactant particles exposed at one time = increased frequency of collisions = by probability increased frequency of successful collisions = higher rate of reaction
How do catalysts affect the rate of reaction? They increase ROR by providing an alternative path with lower Ea. The number of molecules with KE >/= Ecat is higher than the number of molecules with KE >/= Euncat so a greater proportion of collisions are successful, higher rate of reaction
How does a heterogeneous catalyst work? It promotes a reaction through its active sites (places where reactants can bond). Gaseous reactant + active site -> adsorbed reactant. Adsorbed reactant -> adsorbed product. Adsorbed product -> gaseous product + free active sites
How does a homogeneous catalyst work? The catalyst bonds to one reactant and forms an intermediate compound, which then reacts with the other reactant to reform the catalyst
Enzymes are? Biological catalysts, lock and key hypothesis, active sites are highly specific for their substrate
How can you experimentally determine rate of reaction? Limitations? Measure time taken for a certain mass of reactant to react or product to form, 1/t is rate of reaction. Observable changes: time taken for a certain volume of gas to be formed, time taken for a colour change in a colorimeter, time taken to produce...
continued enough solid to hide a cross on a piece of paper under the reaction apparatus. Limitation: rate of reaction starts decreasing as soon as reactants are mixed!
Draw the Maxwell-Boltzmann graph for catalysed and uncatalysed reactions 1 graph, Ecat to the left of Euncat
Created by: 11043



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