# CI abcde: Kinetics: rates & orders of reactions; Arrhenius equation Flashcards

Why does increasing the concentration of a reactant not necessarily mean that the rate of reaction will increase?

The reactant may not be involved in the rate-determining step.

What influences how fast a rate-determining step is?

Its activation enthalpy.

2H_{2}O_{2} → 2H_{2}O + O_{2}

0.01 moles of oxygen are formed per second. What is the rate in terms of hydrogen peroxide?

**-** 0.02 mol s^{-1}

Rate = k[H_{2}O_{2}][catalase] at temperature T.

What is the unit of the rate constant?

k = rate / product of concentrations

mol dm^{-3} s^{-1}

(mol dm^{-3})(mol dm^{-3})

So unit of k = dm^{3} mol^{-1} s^{-1}

*Write positive indices first by convention*

Experimental evidence showed the rate of a reaction is given by the following equation:

rate = k[S_{2}O_{8}^{2-}][I^{-}]

What is the order of the reaction:

- With respect to each of the reactants?
- Overall?

- First order with respect to S
_{2}O_{8}^{2-} - First order with respect to I
^{-} - Second order overall (add the powers)

What is the unit of the rate constant for zero order reactions?

mol dm^{-3} s^{-1}

What is the unit of the rate constant for first order reactions?

s^{-1}

What is the unit of the rate constant for second order reactions?

dm^{3} mol^{-1} s^{-1}

This is the Arhenius equation:

k = Ae^{-Ea/RT}

What is represented by each symbol?

k, rate constant

A, frequency factor

e, the constant

E_{a}, activation enthalpy

R, gas constant

T, temperature in Kelvin

k = Ae^{-Ea/RT}

Prove that the Arrhenius equation can be used to plot a straight line graph. State its gradient and y-intercept.

*Take natural log of both sides (on data sheet)*

lnk = -E_{a}/RT + lnA

lnk = -E_{a}/R x 1/T + lnA

Since k and T are the only variables, this is equivalent to y = mx + c

Plot lnk against 1/T

Gradient = -E_{a}/R and y-intercept = lnA

What is the unit of A in the Arrhenius equation?

The same as that of k.

What is the magnitude of the change in rate of a reaction with activation energy 50 kJ mol^{-1 }when the temperature is increased from 20^{o}C to 30^{o}C?

In both cases, k = [X]^{m}[Y]^{n} so k is only factor affecting rate; k ∝ rate

*Convert E _{a}_{ }into J (unit of gas constant) and T into K.*

k_{1} = A e^{-50,000 / 8.314 x 293} = 1.2 x 10^{9} A

k_{2} = A e^{-50,000 / 8.314 x 303} = 2.4 x 10^{-9} A

A is a constant, so number of collisions + therefore rate is multiplied by approx. 2

Which statement about the Arhenius equation is correct?

- A plot of lnk against T gives a straight line
- When T is very large, ln k almost equals A
- E
_{a}is the gradient of a plot of ln k against 1/T - A plot of k against 1/T gives a straight line

**2**

- Should be 1/T
- lnk = -E
_{a}/R x 1/T + lnA = -E_{a}/RT + lnA so, as T → ∞, -E_{a}/RT → 0 - Should be -E
_{a}/R - Should be lnk

Suggest 5 methods of analysis which could be used to measure the rate of a reaction.

- Titration
- pH
- C
**o**lorimetry - Volume of gas evolved
- Mass change

How does quenching work?

- Sample isolated from reaction mixture
- Chemical added which stops reaction (e.g. neutralises a catalyst)
- Sample titrated to find conc. of a reactant/product