Rates, Orders & Arrhenius (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define rate equation
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Define rate equation
A rate equation is an expression linking the rate of reaction to the rate constant k and the concentrations of reactants raised to their respective orders. It can only be determined experimentally, not from the stoichiometric equation.
Why is a zero-order reactant not included in the rate equation?
Changing the concentration of a zero-order reactant has no effect on the rate of reaction. Its concentration term therefore cancels to 1 and does not appear.
If a reactant is second order, doubling its concentration increases the rate by a factor of .......... .
If a reactant is second order, doubling its concentration increases the rate by a factor of 4.
True or False?
Reaction intermediates can appear in a rate equation.
False.
Intermediates are never included in rate equations. Only reactants (and sometimes catalysts or products) feature, as their concentrations can be measured experimentally.
Why can the rate equation for a reaction not be deduced from its stoichiometric equation?
The order with respect to each reactant reflects the mechanism of the reaction, which is not shown by the stoichiometric equation. Rate equations must be determined from experimental data.
Define overall order of reaction
The overall order of reaction is the sum of the powers (individual orders) of all reactant concentration terms in the rate equation. It describes the combined effect of all reactants on the reaction rate.
How is the rate constant k calculated from initial rate data?
Rearrange the rate equation to make k the subject, then substitute the initial concentrations and corresponding initial rate from one experiment. Units must be derived by substituting units into the rearranged expression.
Define half-life (t1/2)
A half-life (t½) is the time taken for the concentration of a reactant to halve. Its behaviour over successive intervals is used to identify the order of reaction.
What shape is the concentration-time graph for a zero-order reaction, and why?
The graph is a straight line with a negative gradient. The rate does not depend on concentration, so the reactant is consumed at a constant rate throughout the reaction.
For a first-order reaction, the successive half-lives remain .......... throughout the reaction.
For a first-order reaction, the successive half-lives remain constant throughout the reaction.
True or False?
For a second-order reaction, the half-life decreases as the reaction proceeds.
False.
For a second-order reaction, the half-life increases with time, meaning it takes progressively longer for the concentration to halve.
Why does a first-order reaction have a constant half-life?
The rate is directly proportional to concentration, so as concentration falls the rate falls in proportion. The time required for the concentration to halve is therefore always the same.
How is the rate of reaction found at a particular time from a concentration-time graph?
A tangent is drawn to the curve at that point and its gradient is calculated. The magnitude of the gradient equals the rate of reaction at that instant.
True or False?
For a zero-order reaction, the gradient of the concentration-time graph equals the rate constant k.
True.
For a zero-order reaction, rate = *k, so the gradient of the straight-line graph is both the rate of reaction and the rate constant k*.
Define zero-order reaction
A zero-order reaction is a reaction in which the rate does not depend on the concentration of the reactant. The rate equation is rate = *k* and the rate-concentration graph is a horizontal line.
What shape is the rate-concentration graph for a first-order reaction?
A straight line through the origin. The rate is directly proportional to the concentration of the reactant.
The rate equation for a second-order reaction with respect to reactant A is: rate = .......... [A]2.
The rate equation for a second-order reaction with respect to reactant A is: rate = k [A]2.
True or False?
A horizontal line on a rate-concentration graph indicates a zero-order reaction.
True.
A horizontal line shows the rate is constant regardless of concentration, which is the defining feature of a zero-order reaction.
Why does the rate-concentration graph for a second-order reaction curve upward?
The rate is proportional to the square of the concentration, so as concentration increases the rate rises more steeply. This squared relationship produces an upward-curving graph rather than a straight line.
The rate-concentration graph for a second-order reaction is a .......... line.
The rate-concentration graph for a second-order reaction is a curved line.
How is the order of a reaction determined from a rate-concentration graph?
A horizontal line indicates zero order; a straight line through the origin indicates first order; an upward curve through the origin indicates second order.
What is the initial rates method?
The initial rates method is an experimental approach that determines the rate of reaction at t = 0, found by drawing a tangent to a concentration-time graph at the start and calculating its gradient.
How is the initial rate of reaction obtained from a concentration-time graph?
A tangent is drawn to the curve at t = 0 and the gradient of that tangent is calculated. The gradient gives the initial rate of reaction.
In a clock reaction, the initial rate is proportional to .......... , where t is the time for the visual change to occur.
In a clock reaction, the initial rate is proportional to 1/*t , where t* is the time for the visual change to occur.
True or False?
A clock reaction assumes that the rate of reaction does not change significantly between t = 0 and the time of the visual change.
True.
This is the key assumption of a clock reaction. If the rate changes significantly during the measured interval, the calculated initial rate becomes less accurate.
Why does the accuracy of a clock reaction decrease as the measured time gets longer?
Over a longer time interval, the concentration of reactants decreases more, so the rate changes more significantly from its initial value. The assumption of a constant rate becomes increasingly invalid.
What is continuous monitoring?
Continuous monitoring is an experimental method in which data are collected throughout the course of a reaction to plot a concentration-time graph. Examples include measuring gas volume evolved or using a colorimeter.
Why is a calibration curve required before using a colorimeter to monitor reaction rate?
A calibration curve converts absorbance readings from the colorimeter into concentration values. Without it, the raw colorimeter data cannot be used to produce a concentration-time graph.
Define rate-determining step
A rate-determining step is the slowest step in a multi-step reaction mechanism, which determines the overall rate of reaction. Only reactants involved in this step appear in the rate equation.
Why does a reactant's concentration appear in the rate equation?
The reactant must be involved in the rate-determining step, so its concentration directly affects the overall rate of reaction. Reactants not in this step have zero order and do not appear in the rate equation.
If the rate equation for a reaction is rate = k[NO2]2, the reaction is .......... order with respect to NO2 and the rate-determining step involves .......... molecules of NO2.
If the rate equation for a reaction is rate = k[NO2]2, the reaction is second order with respect to NO2 and the rate-determining step involves two molecules of NO2.
True or False?
In the reaction NO2(g) + CO(g) → NO(g) + CO2(g) with rate = k[NO2]2, CO is first order in the rate equation.
False.
CO does not appear in the rate equation, so it is zero order with respect to CO. Only species involved in the rate-determining step appear in the rate equation.
How can the rate equation be used to predict a possible reaction mechanism?
The orders in the rate equation reveal which reactants and how many molecules are involved in the rate-determining step. Steps before the rate-determining step must produce any intermediates consumed in it, while faster steps follow after.
What is a reaction intermediate in a multi-step mechanism?
An intermediate is a species produced in one step of a mechanism and consumed in a subsequent step, so it does not appear in the overall equation. It differs from a transition state in that it has a finite lifetime and does not appear at an energy maximum.
True or False?
The rate-determining step is the fastest step in a reaction mechanism.
False.
The rate-determining step is the slowest step in the mechanism. The overall reaction can only proceed as fast as this slowest step.
Define rate constant
A rate constant (k) is the proportionality constant in the rate equation that links the rate of reaction to the concentrations of reactants. It remains constant only when concentration is the variable changed, increasing in value when temperature rises.
Why does increasing temperature increase the rate constant, k?
Higher temperature increases the proportion of molecules with kinetic energy greater than or equal to the activation energy, raising the fraction of successful collisions. This causes a greater rate of reaction, and since rate = k[A][B], the value of k must be larger.
When temperature increases, two factors contribute to a higher rate: more .......... collisions (particles move faster) and a greater .......... of molecules having kinetic energy above the activation energy.
When temperature increases, two factors contribute to a higher rate: more frequent collisions (particles move faster) and a greater proportion of molecules having kinetic energy above the activation energy.
True or False?
The rate constant, k, is independent of temperature.
False.
The value of k increases with increasing temperature because a greater proportion of molecules possess energy greater than or equal to the activation energy.
Which factor has the greater effect on reaction rate when temperature increases: collision frequency or the proportion of successful collisions?
The increase in the proportion of molecules with kinetic energy greater than or equal to the activation energy has the greater effect. The increase in collision frequency alone accounts for a smaller contribution to the overall rate increase.
What is the shape of a graph showing how the rate constant, k, varies with temperature?
The graph shows an exponential increase, meaning rate constant rises steeply as temperature increases. This reflects the non-linear relationship between temperature and the proportion of molecules exceeding the activation energy.
True or False?
The rate of a reaction always exactly doubles for every 10°C rise in temperature.
False.
This is a useful approximation but does not apply to all reactions. The exact temperature interval required to double the rate varies between reactions and also changes gradually as temperature increases.
Define Arrhenius constant
The Arrhenius constant (A) is a reaction-specific constant in the Arrhenius equation that relates to the frequency and orientation of molecular collisions. It has the same units as the rate constant, k.
Why does a higher activation energy result in a smaller rate constant, k?
A higher activation energy means a smaller proportion of molecules have sufficient energy to react, reducing the fraction of successful collisions. This decreases the value of the rate constant and therefore the overall rate of reaction.
In the Arrhenius equation, the only two .......... are the rate constant k and the .........., while Ea, A, and R are treated as constants.
In the Arrhenius equation, the only two variables are the rate constant k and the temperature, while Ea, A, and R are treated as constants.
True or False?
In the Arrhenius equation, both the Arrhenius constant A and the gas constant R are reaction-specific constants.
False.
The gas constant R is a universal constant with the value 8.31 J mol-1 K-1 and applies to all reactions. Only A and Ea are reaction-specific constants.
Define Arrhenius plot
An Arrhenius plot is a graph of ln k plotted against 1/T that produces a straight line, allowing the activation energy to be calculated from the gradient. The equation of the line follows the form y = mx + c, equivalent to ln k = ln A − Ea/RT.
Why does a graph of ln k against 1/T produce a straight line?
Taking natural logarithms of the Arrhenius equation gives ln k = ln A − Ea/RT, which has the linear form y = mx + c. This means ln k is linearly related to 1/T with a constant gradient of −Ea/R.
True or False?
The y-intercept of a graph of ln k against 1/T equals ln A.
True.
The linearised Arrhenius equation ln k = ln A − Ea/RT shows that when 1/T = 0, ln k = ln A. This allows the Arrhenius constant A to be determined from the graph.
What does the gradient of an Arrhenius plot equal, and how is it used?
The gradient equals −Ea/R, where Ea is the activation energy and R is the gas constant. Multiplying the gradient by −R gives the value of the activation energy in J mol-1.
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