Reaction Kinetics Flashcards
What is the rate equation?
Rate = k[A]^a[B]^b , determined experimentally
Relationship between concentration and pressure?
Concentration directly proportional to pressure
What is zero-order reaction? Units of k?
Rate= k[A]^0 , mol dm-3 s-1
What is first-order reaction? Units of k?
Rate = k[A]^1 , s-1
Charactersitic of first-order reaction? Give two equations and an observation relating to this characteristic
Constant half-life
t1/2 = ln2/k c/co = (1/2)^n
*Half-life is independent of concentration of reactants
What is second-order reaction? Units of k?
Rate = k [A]^2 , mol-1 dm3 s-1
Recall Concentration - Time graphs
Refer to Image 2
Recall Rate - Concentration graphs
Refer to Image 1
How to differ zero,first and second-order reactions?
Check shape concentration - time graphs.
Straight line –> Zero Order
Curve –> First or second –> Check the half life
Three conditions where pseudo-first-order reaction occurs. Common characteristic between them.
- Presence of large excess of reactants
- Reactant is a solvent (1)
- Presence of catalyst
[ ] approximately constant.
How to determine order from continuous method?
Shape of concentration time graph, check the half life
How to determine Rate Equation of first order reaction from continuous method?
Use t1/2 = ln2/k to find k
Given 2 sets of data, how to find order of reaction w.r.t to a reactant?
Initial rates method.
Initial rate = Gradient of graph at t=0min
How to determine order from discontinuous method?
Rate1/Rate2 = k[A]^a[B]b/ k[A]^a[B]b –> Solve
Compare 2 experiment where only 1 [reactant] changes
One characteristic about the slow step/ rate determing step
Has the highest activation energy
Three criteria to meet when proposing a reaction mechanism
- For slow step, aA + bB –> C ===> rate = [A]^a[B]^b
- All the elementary steps must add up to the overall balanced equation
- Rate equation should involve all the reactants in the overall balanced equation (Excluding intermediates)
5 things to note when drawing Energy Profile diagram
- Correct axes (Energy,Reaction pathway)
- ___ Step Mechanism = ___ Peaks
- Slow step –> Highest Ea
- Endothermic vs Exothermic
- Label species (Start,intermediate,end)
Maxwell Boltzmann Curve for increased temperature
Refer to image 3
- Start at origin
- Indicate T1 > T2
- Maximum curve shifts right and lowered
- Key
- Correct Axes
Explain how increase in temperature increases rate of reaction.
- Average KE of molecules increases
- No. of reactant molecules with energy ≥ Ea increases
- Frequency of effective collision increases
- Rate constant K + Rate increases
Maxwell Boltzmann Curve for presence of catalyst
Refer to image 4
- Start at origin
- Ea and Ea’
- Key
- Correct axes
Explain how presence of catalyst increases rate of reaction.
- Catalyst provides alternate pathway of lower Ea.
- More molecules with energy ≥ lowered Ea (Ea’)
- Frequency of effective collision increases
- Rate constant K + Rate increase
Define activation energy
Minimum energy needed by reactants for reaction to take place
What are the 3 types of catalysis?
- Hetrogeneous
- Homogenous
- Autocatalysis
How does hetrogeneous catalyst increase rate?
2 ways
- Adsorption increases concentration of reactant
- Adsorption weakens exisiting bonds within reactant molecules, lowers Ea
Kinda explain homogeneous catalysis
Converted into reactive intermediate to take part in reaction, regenerated afterwards
What is autocatalysis?
Explain the charactersitics of autocatalysed reaction. (4 points)
Products formed acts as catalyst
- Initially slow
- Rate increases as product (autocatalyst) formed
- Rate reaches max
- Rate decreases towards end as concentration of reaction decrease. Reaction stops when reactant used up.
Condition for maximum rate of enzyme-substrate reaction
All active sites of enzyme occupied by substrate
When and why is rate first order wrt substrate?
Rate - [substrate] graph
Initially, when active sites of enzyme not fully occupied
When and why is rate zero order wrt substrate?
Rate - [substrate] graph
After certain concentration, when active sites of enzyme occupied
Using the inital rates method when given volume
One thing to note when changing total volume
Make sure total volume of solution is same, volume directly proportional to concentration
- Rate does not change when changing total volume*
- -> Concentration remains the same
