Chapter 18 Flashcards
(24 cards)
Define rate of reaction
Change in concentration of a reactant or a product per unit time
What is the order of a reagent in a reaction ?
The power to which the concentration of the reagent is raised in the rate equation for the reaction. It tells us how rate is affected by changing the concentration of a reactant. eg for first order reactant doubling conc doubles rate and for second order reactant doubling conc quadruples rate
What is meant by the overall order ?
The sum of the individual orders of the reagents in a reaction
What is meant by the rate constant ?
The constant of proportionality in the rate equation
What is the half life for a reaction ?
The time taken for the concentration of a reactant to fall to half its original value
What is meant by the rate determining step ?
The step in a reaction that determines the overall rate of the reaction. It is the slowest step in the reaction and determines the rate at which the products are formed.
If the order is zero with respect to a reactant, what does this mean ?
Changing conc of reactant has no effect on rate
If the order is first with respect to a reactant, what does this mean ?
Rate directly proportional to conc
If the order is second with respect to a reactant, what does this mean ?
Change in rate = change in conc squared
Write a generic rate equation and state what each term means
Rate = k[A][B]
k = rate constant for reaction
[A] and [B] are conc of species A and B respectively
Powers on [A] and [B] are orders of reaction with respect to A and B
How to calculate rate constant and related quanitities from a rate equation and determination of units ?
Rearrange rate equation to make k the subject
Use indices laws to work out units
Draw a concentration time graph for a zero order reactant
Straight line with negative gradient
Draw a concentration time graph for a first order reactant
Curve starting at high y value with low x value then decreases and levels off
How can you use a concentration–time graph to calculate reaction rates
Measurement of gradients
How can you use a concentration–time graph of a first order reaction, to measure a constant half-life
Identify the initial concentration
Find half the initial concentration
Read the time
Repeat with different starting concentrations:
Observe the constant half-life
What is the equation used to determine the rate constant using half life in a first order reaction ?
k = ln2 / t1/2
k = ln 2 / half life
Draw a rate concentration graph for a zero order reactant
Flat straight line, doesn’t touch origin
Draw a rate concentration graph for a first order reactant
y = x graph
linear
Draw a rate concentration graph for a second order reactant
Exponential curve shape starting at origin
Draw a rate concentration squared graph for a second order reactant
y = x
linear
How can you use a rate–concentration graph to determine the rate constant for a first order reaction ?
Gradient = k
k = rate / conc
What techniques and procedures used to investigate
reaction rates ?
the initial rates method and by continuous monitoring,
including use of colorimetry?
Describe “ the initial rates method “
1) Set up 5-6 experiments, each with a different initial concentration of the reactant you wish to find the order of
2) Continuously monitor each reaction and plot the progress curve for each one
3) For each curve, measure the gradient of the tangent at t=0 to determine the original rate
4) Take these initial rates and plot against initial concentration of the reactant to find the order of reaction ( with respect to this reactant )
Plot all results on a single concentration time graph
