Physical 2 - rate equation

What does rate of reaction mean?

Change in concentration of a reactant/ product in a given unit of time.

What are the two ways of determining rate of reaction?

Measuring increase in product concentration over time or decrease in reactant concentration over time

What is the general rate equation?

Rate= k [A]^m '[B]^n

What does m and n represent in the rate equation?

The order of the reaction which can only be determined by experiment.

Define order:

The power to which the concentration of that reactant is raised in the rate equation, which can only be determined experimentally. It represents the dependence of the rate on the concentration of the chemical.

What does 0 order mean?

A change in the concentration of this chemical has no effect on the rate, so is not included in the rate equation.

What does first order mean?

A change in the concentration of the chemical causes a proportional change in the rate of reaction, so rate is directly proportional to the concentration of the chemical

What is second order?

A change in concentration of the chemical causes a change in the rate of reaction to the square, so rate is proportional to the square of its concentration.

What is rate constant?

The constant of proportionality in the rate equation.

What are some rules about writing rate equations?

• Solids never appear in it

• Only chemicals with first and second order appear, not 0 order

• Intermediates/ catalysts may appear if they are first or second order and are in the rate determining step

• Stoichiometry does not give information about the order of reactants

What is overall order?

The sum of the powers of all reactant concentrations in the rate equation.

What is half life?

The time taken for the concentration of a limiting reactant to become half of its initial value.

What is the initial rates sampling method?

A method involving measuring rate of reaction at very short times before any significant changes in concentration occur. This is done with known concentrations of reactants. The experiment is repeated but the concentration of one of the reactants is altered and everything else is kept constant and rate is measured again. The change in rate will be due to the change made in the concentration and nothing else.

Outline how the continuous monitoring sampling method is conducted to determine order:

• A method of following a reaction like measuring volume of gas collected is decided

• Continuous readings are collected over the course of the reaction, and one reactant should be limiting and one in excess. The limiting reactant should be the one you are determining the order for as its concentration will decrease enough to affect the rate.

• Volume of gas generated is proportional to the concentration of the reactant being used up, so a concentration v time graph can be plotted.

What does the rate against concentration graph look like for a 0 order reaction?

As the concentration changes, rate remains the same, so the graph is a straight horizontal line of constant reaction rate.

What does the gradient of a concentration v time graph represent?

The rate of reaction

What does the concentration against time graph look like for a 0 order reaction?

The gradient is constant because it represents the rate, so as the concentration changes the rate remains the same, so the line is straight with a negative slope as the reactant is being used up.

What does the rate v concentration graph look like for a first order reaction?

Reaction rate is proportional to the concentration of reactant, so the line is straight with a positive gradient that goes through the origin.

What does the graph of concentration v time look like for a first order reaction in terms of half lives?

It has a constant half life, so the time taken for the concentration to fall to half its original value remains constant. There is a slope that curves down.

What does the rate v concentration graph look like for second order reactions?

The reaction rate is proportional to the square of the concentration of the reactant, so the graph is y=x² (starts off shallow then curves steeply upwards).

What does the concentration v time graph look like for second order reactions in terms of half life?

There is an increasing half life, so two consecutive half lives will be very different. There is a curve that goes steeply downwards before becoming shallower.

How can rate of reaction be found on a concentration v time graph, including a curved line?

The gradient can be found, and for a curved line a tangent should be drawn.

How can half life be used to determine order, and what type of graph is it used on?

Used on concentration v time graphs, a line can be drawn at half the original reactant concentration and followed along to get the time taken for the concentration to halve. This is repeated a few times and if it remains consistent then it is first order.

What happens to half lives over time in a 0 order reaction?

They decrease

What is the rate determining step?

The slowest step in the reaction mechanism that determines the overall rate of the reaction.

What happens to reactants in the rate determining step in the rate equation?

Only the reactants involved in the rate determining step will be involved in the rate equation.

What effect does a reactant after the rate determining step have on overall rate of reaction?

Changing its concentration will have no effect on overall initial rate

What does SN2 mean in reaction mechanisms?

Substitution by Nucleophile with 2 species in the RDS, occurring when the hydroxide ion attacks the delta positive carbon and both the OH- ion and bromoethane are involved in the RDS so will appear in the rate equation.

What does SN1 mean?

Substitution by Nucleophile with 1 species in the RDS. Occurs in tertiary haloalkanes where there is a two stage mechanism where the first step is the RDS and the carbon-halogen bond breaks heterolytically to give a carbo-cation and halide ion, so only the haloalkane is involved in the RDS so appears in the rate equation.

Why are 0 order reactants not involved in the RDS?

Because the concentration of these reactants does not affect rate of reaction.

How can a catalyst be identified using the chemical and rate equation?

If a species appears in the rate equation but not the chemical equation then it is a catalyst.

What can be concluded if a species does not appear in the rate equation but is in the chemical equation?

It cannot be involved in the rate determining step, or any steps leading up to it so it must be after the RDS and therefore is 0 order.

How can the ratio of species in the rate equation be used to check the RDS and steps leading up to it?

The ratio will match the proportions the species are in in all the steps leading to and including the RDS. For example if the species is second order in the rate equation then 2 particles will be involved

What is the relationship between rate and rate constant?

They are proportional

Does k vary with temperature?

Yes, and a larger temperature leads to a larger constant

What does a larger value of k mean?

Greater rate of reaction

What does the graph look like of rate constant against temperature?

As temperature increases k increases exponentially because it is proportional to rate, then levels off as other limiting factors limit the rate

What does A represent in the Arrhenius equation?

The Arrhenius constant (pre-exponential factor)

What does e represent in the Arrhenius equation?

Euler’s number and is a mathematical value

What is the relationship between the units of k and A in the Arrhenius equation?

They are identical to each other

What are the units of Ea needed in the Arrhenius equation?

Jmol-1 so that it is the same as the gas constant

What does using natural logs change the Arrhenius equation into?

lnk = lnA - Ea/RT, which removes e

What is the Arrhenius equation?

k = Ae ^ -Ea/RT

Give the format of the Arrhenius equation used to plot a straight line graph in the form y = mx+ c

(y) lnk = -Ea/R (m) x 1/T (x) + lnA (C)

When plotting lnk against 1/T, what does the gradient represent?

-Ea/R