Rate of Reaction - Lecture Note

CHEMISSCHEMISTRY RATE OF REACTION: How foot or slow a reaction w Rate [reactant] Rate product time Cime product time time Techniques for measuring rate of reaction: 1 Mesour ing volume of a quo (collection over water for gases insoluble in water, or using a gas syringe) 2. Measure the contents in the reaction misture 3 monitoring colour (Colorimetry) how fast colour changes 4. Analysk by titration (to find concentration at certain levels) 5. Measuring Bectical conducuviky ions read to form solid, so electrical conductivity reduces as solids won't conduct elkarricity. But it won't be O, because not all ians will have reacted completely to form a solid. KATE CQUATION Relationship between rate of reaction and concentration of the redaant Rate of reaction is directly proportional to the concentration of the maciant Ra [A] R= K[A] where: R rate of reccion K Fate Lonstant [A] concentration of the reactant Order of a reaction: The order of a reaction in the power to which the concentration of a reactant raved to, in a rate equation. R= K[A] [B] - The order of the reaction, with respect to A, 102. The order of the reaction, with respect to 8, 61 The overall order of reaction for this equation is 0+13. R-KCAJ RK We can have zero order reaction, first, second and third order reaction. The rate constant will always remain the same, and can only be affected by temperature Unit for rate of reaction will always be moldm's The unit for the rate conatane will depend on the order of the reaction RK[A] Zejo order K-R mol dm-s- When the order of a reaction is O, the change in concentration of reactant does not affect the rate RK[A] First order in the case, when the concentration of a reaction in doubled, the rate gr reaction also gets doubled, if it in tripled, the rate is suso tripled A] [ ond dim-1 radem = 51 *RK[A] second ander In the cave, when the concentration of a reactant is doubled, the rate incrmones by 4 times (quadrupled). KR [A] mildm (moldm-3) = moldmsdra mol don *RK[A]? Reaction mechanics: - A reaction mechanum refers to the steps invaved in a reaction This inckides the formation of intermediate In the steps invowed, some steps cur past, while others are slow.. The slowest step in the mechanism will determine the overall rate of reaction The slowest step to referred to as the rate-determining step The reactants before the slowest step and the ones in the slower step will be the ones to appear in the rate equation. Determining orders of reaction The order of a maction can be determined epoimentaling, woing the pollbuing techniques: 1. Continuous method 2. using inicial race method. 3 using rate-concentration graph 1. Continuous method: This involves drawing a anembronion-time graph CAJ seondly, we worn out the half-life from the concentration-time graph. Half life time taken for the concentration of the reactant to decreque by haly If the half life is wotone, then it is a first order reaction. If the half lifes are deciraning, then it is a zero-order reaction If the half lifes are increasing, then it is a second order reaction 2 hitial rate method: This involves soing tabulated data with different experiments showing how the rate of reaction varies with changes in the concentration of different reactants when investigating one of the reactants, the concentration of the other weste reactants must be constant. 3. Rate concentration graph: Rx2(x) (Rato the reciprocal of time) t Rate to inversely proportional to time From the equation given we can plot a graph of / against the concentrationg reactants The shape of the graph will then tell uw the order of the reaction CAJ [A] EAJ Zero ardar first order second ander Question: 1. A compound P decomposes when heated the graph shows the change in concentration when a sample of P in mated Pg 13 a state what is meant by the term half-life of a reaction It in the time taken for the concentration to decrabe by half b. Ust the graph to show that the decomposition of Pu a pint order reaction Half life comtant C. Explain the effect on the half life of doubling the initial concneration of p No effect Haiclipe a independent of concentration of raccone's