A-level Chemistry AQA [New Spec] - The Rate Determining Step | A-level Chemistry | OCR, AQA, Edexcel

The Rate Determining Step in a Snap! Unlock the full A-level Chemistry course at http://bit.ly/2mpyD7x created by Barney Fidler, Chemistry expert at SnapRevise. SnapRevise is the UK’s leading A-level and GCSE revision & exam preparation resource offering comprehensive video courses created by A* tutors. Our courses are designed around the OCR, AQA, SNAB, Edexcel B, WJEC, CIE and IAL exam boards, concisely covering all the important concepts required by each specification. In addition to all the content videos, our courses include hundreds of exam question videos, where we show you how to tackle questions and walk you through step by step how to score full marks. Sign up today and together, let’s make A-level Chemistry a walk in the park! The key points covered in this video include: 1. Rate-determining step analogy 2. Rate determining step 3. Reaction mechanisms from rate equation 4. Hydrolysis of chloromethane 5. Proposing Reaction Mechanism 6. Validity of Reaction Mechanisms Rate-Determining Step Analogy We’re going to use the analogy of baking a cake to understand some of the key concepts: A reaction mechanism is the series of steps that make up a reaction - we’re going to think of this as our recipe. The rate-determining step is the slowest step in a reaction - for our cake this will be the baking stage. An intermediate is a species which is formed in one step but used up in another, it is not seen as a reactant or product in the overall equation - we can think of this as our cake mix. Rate Determining Step The slowest step in the reaction will dictate how fast the whole reaction can happen. The rate-determining step is the slowest step in the reaction mechanism of a multistep reaction. The rate-determining step (RDS) is so much slower than all the other steps that when you measure the rate of reaction you are measuring the rate of the RDS. Reaction Mechanisms from Rate Equations As we’ve seen, some reactants are order 0 and have no effect on the rate of reaction. These reactants cannot be involved in the RDS. For any reactant in the rate equation the order tells us how many molecules of the reactant are involved in the RDS (either directly or by forming an intermediate). If we double the concentration of X then the process above is twice as likely to happen. If we double the concentration of A, the chance of each red species successfully attaching increases by two. Because we have two species attaching this means the rate increases by four. Hydrolysis of Chloromethane This implies that the chloromethane alone is involved in the RDS. Proposing a Reaction Mechanisms The overall reaction equation tells us nothing about the reaction mechanism. Considering our analogy: If you were just given the ingredients and the final product that doesn’t tell you how to make anything. However if you’re an experienced baker, with this information and a time for how long you should spend making - you could have a good gue at the recipe. This is the same for chemists, they ensure: The RDS is given that has the same numbers of molecules as the order for the reactant. The other steps in the mechanism eventually generate the products in the balanced equation. All intermediates which are created are used up in later steps. Validity of Reaction Mechanisms Reaction mechanisms are proposed to fit the rate equations derived from experimental data and to balance the over equations. Mechanisms can be proven wrong if more data becomes available. They are educated guesses as we cannot see exactly what happens during in any given reaction. If a baker comes up with a way to turn ingredients into a desired cake then until someone finds a problem with it - that is the recipe to work from. Summary We know a reaction mechanism is the steps that make up our overall reaction We know an intermediate is a species created in the mechanism which is then used up again We know the rate-determining step is the slowest step in the reaction mechanism and controls the rate We can predict the rate equation consistent the rate-determining step We can suggest possible mechanisms consistent with the rate equation and overall reaction equation