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# AQA A2 Kinetics

Question | Answer |
---|---|

Explain why rate of a reaction decreases as the reaction proceeds. | The concentration of the reactants decrease thus there are fewer particles per unit of volume so fewer collisions per second between reactant particles. |

Describe the graph rate V concentration for i) a zero order reaction, ii) a first order and iii) a second order reaction. | zero=horizontal line; first= straight line going up; second=curve going up and up |

Calculate the value of the rate constant from the rate equation Rate=k[O2]^1[NO]^2 where the rate= 7.1 mol dm-3 s-1 and the concentrations of O2 and NO are 0.001 mol dm-3. | Answer: k=rate/[O2][NO]^2 k=7.1/0.001^3 = 7.1x10^10 |

Explain how to determine the order of a reaction with respect to a reactant from a concentration vs. time graph. | Calculate the time taken for the concentration to decrease by a half – find the half-life. If half-life remains constant the order of reaction is 1 however if it doubles then the order is 2. |

What is the two steps involved in the reaction: NO2+CO->NO +CO2 where Rate= k[NO2]^2 | 2 moles of NO2 in rate equation means slow step is NO2+NO2àNO+NO3(NO3=intermediate); CO, reactant in equation, must appear in fast step so NO3+ COàNO2+CO2 |

A catalyst is added to an equilibrium A+2B<=>C+3D, in what direction does the equilibrium shift in? | Neither, catalyst is speedin up reactions in both direction,equilibrium is achieved faster. |

State the order of the reaction with respect to NO where Rate= K[NO2]^2[NO][O2]^2 | 1 |

What is the rate equation? When Cl. + O3 -> ClO. + O2 (slowest step) and ClO. + O. -> Cl. + O2 | Rate = K [Cl.][O3] |

If [A]=4.55x10-2 mol dm-3 and [B]=1.70x10-2 mol dm-3 and the initial rate=6 .64x10-5 mol dm-3 s-1, using the rate equation rate=k[A][B]2, calculate the rate constant. | k=5.05mol-2 dm6 s-1 |

Describe what happens to the half-life in zero and first order reactions. | Zero order – half-life decreases over time. First order – half-life is constant. |

Explain what is meant by the term second order reaction. | rate ∞ [A]2 – If the conc. of A increases by 2 times, the rate increases by 22=4 times. |

Describe how to find the rate of a reaction at time=t from a graph [A] vs time. | Draw the longest tangent possible at t and work out the gradient of the line to find the rate. |

Explain what is meant by the term rate determining step | The slowest step in the reaction mechanism of a multi-step reaction. |

Work out the rate equation of the following equation: A + 2B -> C + D | The rate equation can NOT be worked out from the stochiometric equation alone |

Explain what would happen to the rate of reaction if the concentration of reactant B is doubled when order with respect to B is 2 | when the [B] is doubled, the rate of reaction is quadrupled due to reagent B having second order (2^2=4) |

Define the term half life | The time it takes for the concentration of reactants to half. |

Explain what happens to the rate constant when the temperature is increased and give the reason behind it | When the temperature is increased, rate constant value increases because molecules have more kinetic energy. This means more frequent collisions, and more of collisions exceed the activation energy of the reaction. |

Describe the term catalyst | A substance that increases the rate of a chemical reaction without being used up in the process, by providing an alternative pathway of lower activation energy |

Define rate of reaction (2) | The change in concentration of a reactant or product per unit time. |

Work out the units of k in a reaction that has rate eq rate=k[A]2[B] | dm^6.mol^-2.s^-1 |

Paracetamol has a biological half life of 380s. How long will it take for the level of paracetamol in the body to fall to 1/16th of its original value? | 1520s. |

Explain the effect of increasing the temperature on the rate of reaction | The proportion of particles with an energy above the activation energy increases, so there will be more collisions with sufficient energy for bonds to break, so rate will increase |

Describe how to find the rate of a reaction at time=t from a graph [A] vs time. | Draw the longest tangent possible at t=0sec (make it cross the axis) and work out the gradient of the line to find the rate. |

when working out the order of reaction from a table of initial rates, what are the pitfalls? | check possible factor like x10^-3 in the initial rate column |

when working out the order of reaction from a table of initial rates, what do you do if two reactants must be x2? (instead of having one x2, whilst the other one stays constant) | you'll have worked out the order of one in a previous question, so work out the initial rate when you double that reactant; then observe what happens to the rate when you double the second reactant |

Suggest a Mechanism for 2 NO + O2-->2NO2 with rate = k [NO]2 | FOR EXAMPLE!!! slow step= NO + NO --> NO2 + N; fast step= N + O2 --> NO2 |

How do you work out a two step mechanism given the rate equation and the overall equation? | 1) the chemicals that appear in the rate eq are the reactants for the slow step; 2) take these reactants from the overall eq, the rest of the reactants are the reactants for the fast step; 3) choose one product from the overall equation and place as produ |

What is Arrhenius equation? | k=A x e^-(Ea/RT) |

What is the derived equation of Arrhenius? | ln k = lnA - (Ea/RT) |

How would you work Ea from the ln k vs 1/T graph? | The gradient of the (straight) line of best fit is Ea/R |

Created by:
UrsulineChem