Kinetics | Using Graphs to Determine Reaction Orders | A level H2 Chem | Making Sense Chem

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In today's video, we will be re-visiting at the most important concepts in the chapter Kinetics!

0:00 Introduction
0:13 Recapping Rate equations
1:48 Shapes of graphs
8:43 Pseudo first order reactions
12:05 Determining order through graph

Shapes of conc-time graphs
DO NOT memorise shape of graphs! Use mathematical approach to determine depending on the order of reaction.
t1/2 is the time required for the concentration of a REACTANT to decrease by HALF.
t1/2 of first order reactions are CONSTANT; value of t1/2 (1st order) = ln2/k
All orders of reactions have half-lives, but only those of first order reactions are CONSTANT.

Pseudo first order reactions
Given a reaction A + B = C with the rate = k[A][B]
To create a pseudo first order reaction wrt A, concentration of all other reagents in rate equation must be CONSTANT (large excess/catalyst).
Group the constants k and [B] together, expressing the new rate constant as k’ (read as k prime)
With [B] constant, the rate equation is now: rate = k' [A] where k' = k[B]

Determining order through graph
For [reactant]-time graphs, shape of the graph determines order: downward sloping straight line (zero order), downward sloping curve (1st or 2nd order)
If the curve is a downward sloping curve, determine the t1/2 of the curve
To find t1/2 of a [reactant]-time curve: first t1/2: time taken for [reactant] to decrease from 100% to 50%; second t1/2: time taken for [reactant] to decrease from 50% to 25%.
If constant t1/2 = 1st order; if non-constant t1/2 = 2nd order.
For [product]-time graph, reaction is only complete when gradient of graph is zero (i.e. graph is flat/plateaus)
Upward sloping straight line (zero order rxn), upward sloping curve (1st or 2nd order - check t1/2 to determine whether 1st or 2nd order)
To find t1/2 of a [product]-time curve: first t1/2: time taken for [product] to increase from 0% to 50%; second t1/2: time taken for [product] to increase from 50% to 75% (50% or 75% of the total amount - 100% - of [product] formed)
Order of reaction wrt product = order of reaction wrt limiting reactant.

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