Module 5: rates Flashcards
How to calculate rate:
rate= change in concentration/change in time OR rate=quantity reacted or produced/time
What is rate of reaction proportional to?
Rate is proportional to the concentration of a particular reactant raised to a power e.g [A]^2
What does the power represent in [A]^n
Represents the order of reaction
What is a zero order reaction?
Concentration of a reactant has no effect on the rate
What is a first order reaction?
If concentration is doubled, rate of reaction also doubles
What is a second order reaction?
Rate increases by the square of the concentration increase; when concentration is doubled, rate of reaction quadruples
What is the rate equation?
Rate=k[A]^n[B]^n
What is k in the rate equation?
Rate constant
How to calculate the overall order of a reaction?
overall order= sum of orders of each reactant
Rules when writing rate equation:
Zero order reactants are not represented
Don’t need to write power of 1 when representing first order reactants
How to calculate units for rate constant?
- Rearrange equation to make k the subject
- Substitute units into expression for k
- Cancel common units and show final units on a single line
Remember to swap powers if units are on the bottom of the equation!
How can orders of reaction be found from experimental results?
- By monitoring physical quantity changes over time
- This can be done either through a continuous monitoring method or initial rate method
How can continuous monitoring be carried out?
- Colorimeter
- Measure volume of gas produced over time
- Change in pH
Rate of reaction definition:
Change in the amount of reactants or products per unit time
How can initial rate be measured?
- Iodine clock
- Track formation of a precipitate
What shape is a zero order time-concentration graph?
Straight line with negative gradient
What shape is a first order time-concentration graph?
Downward curve with a decreasing gradient over time
What shape is a second order time-concentration graph?
Also a downward curve, steeper at the start, but tailing off more slowly
What is half-life?
Time taken for the concentration of a reactant to half
How to calculate the rate constant from the rate?
- Draw tangent to the curve on the concentration-time graph is drawn at a particular concentration
- Gradient of tangent calculated, giving the rate of reaction
- Rearrange equation to find k, and input rate and concentration values
What shape is a zero order rate-concentration graph?
Horizontal straight-line with zero gradient
What shape is a first order rate-concentration graph?
Linear line through the origin
What shape is a second order rate-concentration graph?
Upward curve with increasing gradient
How can rate constant be calculate from iodine clock practical?
- Solution is colourless at the start and the time is measured for the black blue colour of the iodine to appear
- Experiment repeated at varying concentrations
- Initial rate is proportional to 1/t
- A graph of rate against concentration plotted
- Shape of graph identified to represent order in respect to one reactant
- Rate equation can be written and rate constant calculated
What is the rate-determining step?
Slowest step in a reaction that requires multiple steps
Why might some reactions occur in more than one step?
Reactions with many reactants will occur in more than one step as it is unlikely for them all to collide together at the same time, and in the correct orientation
How can you check your reaction mechanism is correct?
- The rate equation only includes species in the rate determining step
- The orders involved in the rate equation match the number of species involved in the rate determining step
How can you work out the mechanism of a reaction?
If you know the rate determining step of a reaction, then you can figure out the mechanism
What is the effect of temperature on the rate constant?
As temperature increases, the value of k will also increase
What is the Arrhenius equation?
Equation that can be used to calculate the effect of both temperature and activation energy on the value of k
What is the relationship between first order reactions and half life?
First order reactants have constant half lifes
What is the equation used to determine rate constant using half life in a first order reaction?
k=In/half life
For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?
The order of reactant
What is the effect of a 10*C temperature rise on the rate of reaction (roughly)?
Doubles rate of reaction
How to rearrange Arrhenius equation into useful form for plotting a graph?
In K= -Ea/RT + In A
