Rates of Reaction

Question 1

Define the rate of a reaction

Answer 1

The change in the concentrations of the reactants or products over time

Question 2

What does the graph of concentration against time look like for reactants?

Answer 2

Downwards curve, the gradient is zero by the end

Question 3

What does the graph of concentration against time look like for products?

Answer 3

Upwards curve, the gradient is zero by the end

Question 4

Why is the initial rate of reaction the fastest?

Answer 4

Because this is when the reactants are most concentrated so the probability of collisions is highest

Question 5

Why do both graphs for conc against time tend toward zero over time?

Answer 5

Bc the reactants eventually run out so their concs run down to zero so the reaction stops

Question 6

What factors affect the rate of a reaction?

Answer 6

Nature of reactants, physical state (SA), temperature, catalysts, concentrations of solutions and pressures of gases

Question 7

What does the stoichiometric equation show?

Answer 7

The number of moles of reactants and products

Question 8

What does the rate equation show?

Answer 8

How the rate depends on the concentrations of the reactants. It is of the form rate = k[A}^m[B]^n where m and n are usually integers

Question 9

Why can you not determine the rate equation from the stoichiometric equation?

Answer 9

Bc the stoich. equation shows moles only. The rate equation can only be determined by experiments which show how the concentration affects the rate of the reaction.

Question 10

What is an order of a reaction with respect to a reactant?

Answer 10

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

Question 11

What is the overall order of a reaction?

Answer 11

The total of all the orders in the rate equation

Question 12

What is the rate constant?

Answer 12

The constant of proportionality in the rate equation

Question 13

What is zeroth order?

Answer 13

When a reactant changes, the rate does not

Question 14

What does the graph of rate against conc for zero order look like?

Answer 14

A horizontal line - bc as the conc changes the rate does not

Question 15

What is first order?

Answer 15

When the rate is directly proportional to the concentration of A

Question 16

What does the graph of rate against conc for first order look like?

Answer 16

A straight diagonal line

Question 17

What is second order?

Answer 17

When the rate is directly proportional to the square of the concentration of A

Question 18

What does the graph of rate against conc for the second order look like?

Answer 18

An upward curve

Question 19

What does collision theory state?

Answer 19

A reaction between particles will only take place if the particles collide with the correct orientation and with energy equal to or greater than the activation energy.

Question 20

What does the Maxwell Boltzmann distribution show?

Answer 20

How the proportion of particles of a given energy varies as the energy increases

Question 21

What does the area under the curve of the MBD show?

Answer 21

The number of particles

Question 22

How does temperature effect the MBD?

Answer 22

Shifts to the right with a lower peak, more particles with energy greater than Ea, most probably energy shifts to the right, mean energy is higher

Question 23

What effect does a catalyst have on the MBD?

Answer 23

No effect at all

Question 24

What effect does concentration have on the MBD?

Answer 24

Raises the peak bc more particles

Question 25

What are catalysts?

Answer 25

Substances that speed up the rate of a reaction but are chemically unchanged themselves so they dont appear in the stoichiometric equation

Question 26

What effect do catalysts have on the activation energy of a reaction?

Answer 26

They provide an alternative reaction pathway which requires a lower activation energy (they dont lower the Ea directly) so more particles have energy greater than the activation energy

Question 27

What can you deduce about the order of a reactant if it isnt in the rate equation?

Answer 27

It’s zero order

Question 28

What is the unit of rate?

Answer 28

moldm-3s-1

Question 29

What effect does temperature have on the rate constant?

Answer 29

As the temperature increases, the rate constant increases exponentially

Question 30

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

Answer 30

An upward exponential curve

Question 31

What is the Arrhenius equation for?

Answer 31

To show the effect of temperature on the rate of a reaction

Question 32

What is the Arrhenius equation?

Answer 32

k = Ae^(Ea/RT)
k - rate constant, A - Arrhenius constant, e - constant, Ea - activation energy (J), R- gas constant, T - temperature (Kelvin)

Question 33

What do you get when you take natural logs of the Arrhenius equation?

Answer 33

lnk = lnA - (Ea/RT)

Question 34

How can you write the Arrhenius equation in a graphical format?

Answer 34

lnk = (-Ea/RT) x 1/T + lnA

Question 35

How can you experimentally determine the Ea of a reaction?

Answer 35

Carry out an experiment at different temperatures and plot the data on a graph of lnk against 1/Temp. The gradient is -Ea/R

Question 36

What does the exponential part of the Arrhenius equation show?

Answer 36

The fraction of molecules in a gas which have energies equal to or greater than the activation energy, Ea, at temperature T