Reaction Kinetics Flashcards
What is rate of reaction?
Change in concentration of reactant or product per unit time
What is the gradient of concentration-time graph?
rate of reaction
If concentration-time graph is a 1/x² graph, what does this imply?
- gradient decreasing with time
- reaction is slowing down
- rate of reaction is decreasing with time
What is initial rate?
reaction rate at the start of the reaction where an extremely small amount of reactant has been used up
How to obtain initial rate?
determining the gradient of tangent drawn to the curve at time t=0
What is instantaneous rate?
rate of reaction at a particular instant in time
How to obtain instantaneous rate?
gradient of tangent drawn to the curve at that time
What is average rate of a reaction?
Calculated from final change in concentration of reactant or product over a particular period of time
How to obtain average rate of reaction?
Measuring gradient of line drawn between two points on the curve for the time interval
What happens if your calculate rate of reaction is negative?
- rate of reaction always positive
- add a negative sign to make it positive
What is rate equation or rate law?
Experimental relationship between the reaction rate and the concentration of reactants
What is the rate equation of aA + bB → cC + dD
rate of reaction = k[A]ᵐ[B]ⁿ
For rate of reaction = k[A]ᵐ[B]ⁿ, what is k, m, n and m+n?
- k is rate constant
- m is the order of reaction with respect to A
- n is the order of reaction with respect to B
- (m+n) is overall order of reaction
What is order of reaction?
It is the power of the reactant’s concentration term in the experimentally-determined rate equation is the order of reaction with respect to that reactant
What is rate constant, k?
proportionality constant in the experimentally determined rate equation
When is rate constant a constant for a given reaction?
At a given temperature
What affects rate constant, k?
- k increases when temperature increases
- k increases when Ea decreases (using a catalyst)
What is zero-order reaction?
rate of reaction is independent to [reactant]
What does the concentration time graph look like for zero-order reactions? How to interprete the graph?
- downward sloping linear line
- gradient = -k
- gradient is constant which means that rate is constant
What does the rate-concentration graph of zero order reaction look like?
horizontal straight line at k (vertical intercept)
What does the rate-time graph of zero order reaction look like?
horizontal straight line at k (vertical intercept)
What is a first-order reaction?
rate of reaction is directly proportional to [reactant]
What does the concentration time graph look like for first-order reactions?
curve where gradient is getting less steep over time
What is the rate-[concentration] graph of first-order reaction?
- upward sloping linear graph
- gradient = k
What is half-life?
time taken for the concentration of a reactant to be halved
What is unique to first-order reactions?
half-life is constant
What is the formula for half-life?
t₁/₂ = ln 2/k
How to calculate the fraction of reactant remaining for a first order reaction?
- using number of half-lives passed (n)
- fraction of reactant remaining = (1/2)ⁿ
What is second-order reaction?
rate of reaction is directly proportional to [reactant]²
What does the concentration time graph look like for second-order reactions? And what does it imply?
- curve where gradient is getting less steep over time
- half-life increases with time
- rate of reaction is slower than first order
What is the rate-concentration graph for second order reaction?
upward sloping curve with increasing gradient
What is the rate-concentration² graph for second order reaction?
- upward sloping linear graph
- gradient = k
What is pseudo-order reaction about?
Make a reactant A in large excess with respect to B such that [A] will not change significantly throughout the reaction and can be regarded as constant
How do you describe a pseudo reaction with A in excess?
rate = k[A]ᵐ[B]¹
- pseudo zero order with respect to A
- first order with respect to B
- overall pseudo first-order reaction
(pesudo is like appears like)
What is initial rates method?
It involves several experiments at which initial concentration of the reactant of interest is varied while keeping the concentration of other reactants constant
Why do you use initial rate?
- Initial rate is where an extremely small amount of reactant has been used up in an extremely small amount of time (differentiation)
- thus amount of reactants is close to the inital amount of reactants
- thus can just use initial amt. of reactants which is known (make things much easier)
- By varying concentration and seeing how rate changes, can find order of reaction and k
rate of reaction = k[A]ᵐ
(y=mx+c)
What experimental procedure can be done to determine rate of reaction?
A + B → C
- mix known concentration of A and B, where B is in excess
- determine concentration of reactant A or product C at various time of the reaction
- Plot concentration vs time graph
What does it mean if concentration vs time graph is a straight line?
- reaction is zero order
- rate of reaction = -gradient =k
What does it mean if concentration vs time graph is a curve?
- reaction can be first or second order
- determine half-life to find out
- if half life is approximately constant, reaction is first order
What is the significance of the slowest step?
- rate-determining step
- dictates the rate of the entire reaction
What does collision theory assume?
reactant particles must collide effectively before they can react
What does collision theory say about when a reaction will occur?
A reaction will occur only if the particles collid with:
* minimum amount of kinetic energy called activation energy and
* appropriate collision geometry/orientation
What is the area under the Maxwell-Boltzmann distribution curve?
Total number of particles