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Chemistry 2.8
AS edexcel chemistry - kinetics
| Question | Answer |
|---|---|
| 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 |
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