18 - Rates of reactions Flashcards
What is the order of reaction
The power to which the concentration of a
reactant is raised in the rate equation
what is the overall order of reaction ?
The sum of the individual orders of reactant in the rate equation
What is zero order ?
when the concentration of a reactant has no effect on the rate.
What is first order ?
when the rate depends on its concentration
raised to the power of one
what is second order ?
when the rate depends on its concentration
What is the rate equation?
gives a mathematical relationship between the concentrations of the reactants and the reaction rate
What is the rate constant ?
The number that mathematically converts the rate of reaction and concentration and orders
what is overall order of reaction ?
The overall effect of the concentrations of all reactants on the rate of reaction
units for k when the order is O
Moldm‐³S-¹
units for k when overall order is 1
S-¹
Units for k when overall order is 2
dm³mol-¹s-¹
what is important when comparing the effect of
different concentrations of reactants on
reaction rates ?
That the rate is always measured after the same
time , ideally the Start of the experiment
Three methods for continuous monitoring of rate
Monitoring gas collection , by gas lost and with a colorimeter
what is the gradient on a concentration - time graph ?
The rate of reaction
Zero order reaction on a concentration time graph
A straight line with a negative gradient . The
value of the gradient is the same as k
First order reaction on a concentration time
graph
A downwards curve with a decreasing gradient
over time. Order one can be determined by two
constant half lifes
How do you calculate k from half life ?
ln (2) / half life
what is a graph of a zero order reaction on a rate - concentration ?
Horizontal Straight line. The reaction rate does not change with increasing concentrations.
The y-intercept is the value of k.
The reaction rate does not change with
increasing concentrations
What is the first order reaction on a
rate/concentration graph?
A straight lie through the origin.
Rate is directly proportional to concentration
The rate constant can be determined by
measuring the gradient of the line
What is the second order reaction on a
rate/concentration graph?
An upward curve with an increasing gradient
The rate constant cant be directly obtained
What is a reaction mechanism?
the series of steps that make up an overall
reaction
What is the rate-determining step?
The slowest step in the reaction mechanism
How do you predict the reaction mechanism?
-the rate equation only includes reacting
species involved in the rate-determining
step
-the orders in the rate equation match the
number of species in the rate determining
step
What is the exponential factor in the Arrhenius equation?
The proportion of molecules that exceed the activation energy
What is the pre-exponential term in the
Arrhenius equation?
Takes into account the frequency of collisions with the correct orientation
A student investigates the reaction between ethanoic acid, CH3COOH(l) and methanol, CH3OH(l), in the presence of an acid catalyst. The equation is shown below. CH3COOH(l) + CH3OH(l) ⇌ CH3COOCH3(l) + H2O(l)
The student carries out an experiment to determine the order of reaction with respect to CH3COOH. The student uses a large excess of CH3OH. The temperature is kept constant throughout the
experiment
Explain why the student uses a large excess of methanol in this experiment
To keep [CH3OH] (effectively) constant
OR
Zero order with respect to CH3OH
OR
To ensure equilibrium is far to the right ✓
Aqueous solutions of hydrogen peroxide, H2O2(aq), decompose as in the equation below.
2H2O2(aq) → 2H2O(l) + O2(g)
A student investigates the decomposition of H2O2(aq) by measuring the volume of oxygen gas produced over time. All gas volumes are measured at room temperature and pressure.The student uses 25.0 cm3 of 2.30 mol dm−3 H2O2. From the results, the student determines the concentration of H2O2(aq) at each time. Thestudent then plots a concentration–time graph.
Suggest a different experimental method that would allow the rate of this reaction to be followed over time
Suggest apparatus that would allow oxygne gas volume to be collected, indicating clearly the scale of working
1)Measure mass (loss)
ALLOW weight for mass
ALLOW take samples and titrate (remaining H2O2)
2) Collect in 1000 cm3/1 dm3 measuring cylinder ✓
Aqueous solutions of hydrogen peroxide, H2O2(aq), decompose as in the equation below.
2H2O2(aq) → 2H2O(l) + O2(g)
A student investigates the decomposition of H2O2(aq) by measuring the volume of oxygen gas produced over time. All gas volumes are measured at room temperature and pressure.The student uses 25.0 cm3 of 2.30 mol dm−3 H2O2. From the results, the student determines the concentration of H2O2(aq) at each time. The student then plots a concentration–time graph.
Determine the initial rate of reaction, the order with respect to H2O2, and the rate constant
A comprehensive conclusion using quantitative data
from the graph to correctly determine initial rate AND
half lives / gradient with 1st order conclusion for
H2O2 AND determination of k.
There is a well-developed line of reasoning which is
clear and logically structured.
Clear working for initial rate, half life / gradient and order
and k.
Units mostly correct throughout
Initial rate
* Tangent shown on graph as line at
t = 0 s
* Gradient determined in range: 1.5 − 2.0 × 10−3
* initial rate as gradient value with units: mol dm−3
s−1,
Half life shown on graph
* Half life range 800-1000 s
* Two ‘constant’ half lives ±50 s
2. Two gradients → two rates
* 2 tangents shown on graph at c and
c/2
* Gradient at c/2 is half gradient at c
e.g. c = 2.3 mol dm−3, gradient = 1.6 × 10−3
AND c = 1.15 mol dm−3, gradient = 0.8 × 10−3
* For chosen method, conclusion: H2O2 is 1st
order
Determination of k 2 methods
* k clearly linked to rate OR half-life
Explain how the student could determine the activation energy, Ea, for the reaction graphically using values of k and T.
Plot graph using ln k AND 1/T ✓
(Measure) gradient ✓
Independent mark
Ea = (–)R × gradient OR (–)8.314
× gradient ✓
* Independent mark, even if variables for graph
are incorrect
* Subsumes ‘gradient‘ mark
A student investigates the rate of reaction between iodine, I2, and propanone, CH3COCH3, in the presence of H+ ions. The student uses HCι(aq) to supply H+ ions.
I2(aq) + CH3COCH3(aq) → CH3COCH2I(aq) + HI(aq)
The student follows the method outlined below.
1. The student starts the reaction by mixing the following solutions.
1.00 cm3 of 1.00 mol dm−3 I2(aq)
49.5 cm3 of 1.00 mol dm−3 CH3COCH3(aq)
49.5 cm3 of 1.00 mol dm−3 HCι(aq)
2. The student places a sample of the reaction mixture in a colorimeter, immediately starts a stopwatch, and records the absorbance.
3. The student records the absorbance every 100 s. The results are shown below
Explain why absorbance decreases during the experiment.
Iodine (solution) has a yellow/orange/brown colour
AND
Concentration of I2 decreases/I2 is used up ✓
Hydrogen peroxide reacts with iodide ions in acid conditions, as shown below.
H2O2(aq) + 2I−(aq) + 2H+(aq) → I2(aq) + 2H2O(l)
The student concluded that H+(aq) ions act as a catalyst.
Explain why the student’s conclusion is not correct
H+ ions are consumed / used up
OR
H+ ions are in the (overall) equation ✔
State what is meant by the term rate-determining step.
The slowest / slow step ✔
5I- + IO- + 6H -> 3I2 + 3H20
A proposed mechanism for this reaction takes place in several steps.
Suggest reason why it is unlikely that this reaction could take place in one step.
Collision unlikely with more than 2 ions / species /
particles ✔
Methanoic acid and bromine react as in the equation below.
Br2(aq) + HCOOH(aq) → 2H+(aq) + 2Br−(aq) + CO2(g)
A student investigates the rate of this reaction by monitoring the concentration of bromine over time.
The student uses a large excess of HCOOH to ensure that the order with respect to HCOOH will be effectively zero.
Suggest how the concentration of the bromine could have been monitored
Suggest a different experimental method that would allow the rate of this reaction to be followed over time.
Why would use of excess HCOOH ensure that the order with respect to HCOOH is effectively zero?
Measure reduction of colour of bromine
Measure volume of CO2 (produced)
Concentration of HCOOH would be constant
