18 - Rates of reactions Flashcards by Cherry .x

What is the order of reaction

The power to which the concentration of a
reactant is raised in the rate equation

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what is the overall order of reaction ?

The sum of the individual orders of reactant in the rate equation

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What is zero order ?

when the concentration of a reactant has no effect on the rate.

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What is first order ?

when the rate depends on its concentration
raised to the power of one

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what is second order ?

when the rate depends on its concentration

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What is the rate equation?

gives a mathematical relationship between the concentrations of the reactants and the reaction rate

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What is the rate constant ?

The number that mathematically converts the rate of reaction and concentration and orders

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what is overall order of reaction ?

The overall effect of the concentrations of all reactants on the rate of reaction

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units for k when the order is O

Moldm‐³S-¹

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units for k when overall order is 1

S-¹

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Units for k when overall order is 2

dm³mol-¹s-¹

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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

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Three methods for continuous monitoring of rate

Monitoring gas collection , by gas lost and with a colorimeter

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what is the gradient on a concentration - time graph ?

The rate of reaction

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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

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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