Grade achieved: A* I have put so much hard work into making this set of notes. Achieving an A* and constantly getting top grades at school proved that my hard work paid off. My notes are mostly based on 10+ years of mark schemes from past papers, and some from revision guides + online resources. Pl...
, OCR A level Chemistry notes Module 5: Physical chemistry and transition elements
5.1 Rates, equilibrium and pH
5.1.1 How fast?
Rate of reaction
𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (𝑚𝑜𝑙 𝑑𝑚!" )
𝑅𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 = = 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡
𝑡𝑖𝑚𝑒 (𝑠)
Initial rate: rate at t = 0
Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction at
other times during the experiment (1)
• (At start) the concentrations are known
Start how the initial rate is obtained from a graph of the concentration of the product against time (2)
• (Calculate) gradient (of tangent) at t=0
Rate equation
Rate constant
• Constant of proportionality between rate and conc. of reactant
• Only temp affects rate constant
o ↑ temp ↑ rate constant
What happens to the rate constant if ↑ pressure? (1)
• No change
Order: the power to which the conc. of a reactant is raised to
Overall order: sum of all orders in the rate equation m + n
Order of reaction: rate ∝ [A]n
Zero order First order Second order
• rate ∝ [A]0 • rate ∝ [A]1 • rate ∝ [A]2
• Has no effect on the rate • If [A] × 2 ⇒ reaction rate × 2 • If [A] × 3 ⇒ reaction
• Constant negative gradient (21 = 2) rate × 9 (32 = 9)
in conc-time graph • Concentration of half-lives is
constant in conc-time graph
• Straight diagonal line via
(0,0) in rate-conc graph
2
, OCR A level Chemistry notes Module 5: Physical chemistry and transition elements
Concentration-time graph Rate-concentration graph
1st order reactions & half life (t1/2)
• Time for [reactant] is constant
𝐼𝑛2
𝑘=
𝑡#/%
Suggest how conc of bromine can be monitored? (1)
• Measure reduction of colour of bromine
Suggest a different experimental method to allow reactant rates to be followed overtime (1)
• Measure volume of CO2 produced
3
, OCR A level Chemistry notes Module 5: Physical chemistry and transition elements
Rate-determining step
Rate-determining step (RDS): slowest step in a multistep reaction mechanism
• Steps in a multi-step reaction take place at a different rate
• Rate equation only includes reacting species involved in RDS
• Orders in rate equation match no. of species involved in RDS
Suggest a reaction mechanism for this reaction
(CH3)3CBr + OH- → (CH3)3COH + Br-
Rate equation: rate = k[(CH3)3CBr]
• Step 1: (CH3)3CBr → (CH3)3C+ + Br- Slow (RDS)
• Step 2: (CH3)3C+ + OH- → (CH3)3COH Fast
4
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller acer. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $7.73. You're not tied to anything after your purchase.
