Flashcards: Chemical Kinetics — 51 cards

What does chemical kinetics study in chemistry?

Reaction rates, factors controlling them, and reaction mechanisms.

What does thermodynamics predict about a chemical reaction?

Whether the reaction is feasible.

What does chemical kinetics determine about a reaction?

How rapidly the reaction occurs and how its rate changes under conditions.

What is true about a reaction with ΔG < 0 at constant temperature and pressure?

It is thermodynamically feasible.

Why does diamond-to-graphite conversion remain imperceptibly slow?

Because the reaction is kinetically slow despite being thermodynamically feasible.

What is reaction rate in chemistry?

It is the change in concentration of a reactant or product per unit time.

What is the formula for average reaction rate at constant volume?

$r_{av}=-\frac{\Delta[R]}{\Delta t}=\frac{\Delta[P]}{\Delta t}$

What is the formula for instantaneous reaction rate?

$r_{inst}=-\frac{d[R]}{dt}=\frac{d[P]}{dt}$

What does the instantaneous reaction rate equal on a concentration–time curve?

The slope of the tangent to the curve.

What are the units of reaction rate?

Concentration per time, like mol L⁻¹ s⁻¹ or atm s⁻¹.

How is the reaction rate related to concentration changes and stoichiometric coefficients?

The rate equals each species' concentration change divided by its stoichiometric coefficient.

What is the formula for the rate of $aA+bB\rightarrow cC+dD$?

$r=-\frac{1}{a}\frac{d[A]}{dt}=-\frac{1}{b}\frac{d[B]}{dt}=\frac{1}{c}\frac{d[C]}{dt}=\frac{1}{d}\frac{d[D]}{dt}$

What is the rate expression for the reaction $2HI\rightarrow H_2+I_2$?

$r=-\frac{1}{2}\frac{d[HI]}{dt}=\frac{d[H_2]}{dt}=\frac{d[I_2]}{dt}$

What is a rate law in chemical kinetics?

An expression relating reaction rate to molar concentrations of reactants.

What is the general rate law formula for $aA+bB\rightarrow cC+dD$?

$r=k[A]^x[B]^y$ where k is the rate constant.

What is the rate law for the reaction $2NO+O_2\rightarrow2NO_2$?

$r=k[NO]^2[O_2]$ based on experimental data.

What does the rate constant $k$ represent in a rate law?

The proportionality constant at a specified temperature.

What defines the order of a reaction in its rate law?

It is the sum of the powers of reactant concentrations in the rate law.

What types of values can reaction order have?

Reaction order can be zero, an integer, or a fraction.

What characterizes a zero-order reaction's rate?

Its rate is independent of reactant concentration.

What is the overall order of the reaction with rate $r=k[A]^{1/2}[B]^{3/2}$?

The overall order is 2.

What is molecularity in an elementary reaction?

The number of reacting species colliding simultaneously.

What range of values can molecularity have?

A positive integer from one to three.

How does reaction order differ from molecularity?

It can be zero, fractional, or negative.

What controls the overall rate in a complex reaction?

The slowest elementary step.

What is an intermediate in the iodide-catalysed decomposition of hydrogen peroxide?

IO⁻

Which step is slow and rate determining in the iodide-catalysed decomposition of hydrogen peroxide?

The first elementary step.

What is the integrated rate law for a zero-order reaction $R\rightarrow P$?

$[R]=[R]_0 - kt$

How is the rate constant $k$ expressed for a zero-order reaction?

$k=\frac{[R]_0 - [R]}{t}$

What is the integrated rate law for a first-order reaction $R\rightarrow P$?

$\ln\frac{[R]_0}{[R]} = kt$ or $[R] = [R]_0 e^{-kt}$

How does a plot of [R] versus time behave for a zero-order reaction?

It is linear with slope $-k$.

How does a plot of $\ln[R]$ versus time behave for a first-order reaction?

It is linear with slope $-k$.

What is the half-life of a reaction?

The time for a reactant's concentration to halve from its initial value.

What is the half-life formula for a zero-order reaction?

$t_{1/2}=\frac{[R]_0}{2k}$

How does the half-life of a zero-order reaction depend on initial concentration?

It depends directly on the initial concentration.

What is the half-life formula for a first-order reaction?

$t_{1/2}=\frac{0.693}{k}$

Does the half-life of a first-order reaction depend on initial concentration?

No, it is independent of the initial concentration.

What defines a pseudo-first-order reaction?

A higher-order reaction behaving as first order due to one reactant in large excess.

Why does a pseudo-first-order reaction behave as first order?

Because one reactant's concentration remains nearly constant.

What is the Arrhenius equation for the rate constant k?

$k=Ae^{-E_a/(RT)}$

What does the variable A represent in the Arrhenius equation?

The frequency factor.

What is the logarithmic form of the Arrhenius equation?

$\ln k=-\frac{E_a}{RT}+\ln A$

What is the slope of a plot of ln k versus 1/T in the Arrhenius equation?

The slope is $-E_a/R$.

What happens to the rate constant when temperature increases by 10 degrees?

It approximately doubles for many reactions.

What is activation energy?

The energy required to form the activated complex from reacting molecules.

What does a catalyst do to the reaction rate?

It increases the reaction rate without permanent chemical change.

How does a catalyst affect the forward and reverse reactions?

It accelerates both to the same extent.

What is collision frequency Z?

The number of collisions per second per unit volume between reactant molecules.

What is the rate relation given by collision theory?

$r=Z_{AB}e^{-E_a/(RT)}P$

What does the steric factor P represent in the rate equation?

It accounts for proper molecular orientation.

What conditions define an effective collision?

Energy at least equal to activation threshold and suitable orientation.