Chapter 18: Rates of Reaction (18.1 - 18.5)

Question 1

define rate of reaction

Answer 1

the change in concentration of a reactant or product in a given time

Question 2

equation for rate of reaction

Answer 2

rate = change in concerntration / change in time

Question 3

what is order

Answer 3

order shows how rate is affected by the concentration

for order n: rate ∝ [A]^n

Question 4

zero order

Answer 4

rate ∝ [A]^0
rate is not affected by the concentration
horizontal line on a rate/conc graph
downward linear on a conc/time graph

Question 5

first order

Answer 5

rate ∝ [A]^1
rate changes by the same factor as concentration
upward linear relationship with line going through origin on a rate/conc graph
downward curve asymptotic to y=0 for conc/time graph

Question 6

second order

Answer 6

rate ∝ [A]^2
rate changes by the same factor as the concentration change squared
upwards steep curved line starting at origin on a rate/conc

Question 7

find reaction rate from a conc/time graph

Answer 7

take a tangent

gradient of tangent = rate of reaction

Question 8

what is overall order

Answer 8

the sum of the individual orders of the reactants.
eg for reactants A and B with orders m and n (respectively)
overall order = m + n

Question 9

the rate equation:

ie for reaction A + B –> C with orders m (for A) and n (for B) what is the rate given by

Answer 9

rate = K X [A]^m X [B]^n

k = rate constant

Question 10

units for rate

Answer 10

moldm^-3s^-1

Question 11

to work out units of K

Answer 11

• rearrange the rate equation to make K the subject
• substitute units into the rearranged rate equation
• cancel common units and write the final units on a single line

Question 12

define half-life

Answer 12

the half-life of a reactant is the time for its concentration to decrease by half

Question 13

whats special about first order reactions

Answer 13

first order reactions have a constant half-life

K = ln2 / halflife (s)

Question 14

what is a colorimeter and what is it used for

Answer 14

A colorimeter measures the intensity of light passing through a sample.
Absorbance reading is directly linked to the concentration of the solution (when either the reactant or product is coloured and the other is colourless)
eg Br2 (coloured) --> 2Br- (colourless)

Question 15

what is the rate-determining step

Answer 15

the slowest step in the reaction mechanism of a multi-step reaction.

Question 16

Why is it unlikely that the reaction would take place in one step?

Answer 16

1) Usually only two molecules would collide at once

2) The stoichiometry of the equation would not balance

Question 17

How do you predict the reaction mechanism of an equation?

Answer 17

1) The rate equation only includes reacting species

2) The orders in the rate equation match the number of species involved in the rate determining step

Question 18

What is the Arrhenius equation?

Answer 18

K = Ae^-Ea/RT

Question 19

What do the symbols in the Arrhenius equation stand for?

Answer 19

A - pre exponential factor
Ea - activation energy
R - gas constant
T - temperature in kelvin

Question 20

Logarithmic form of Arrhenius equation

Answer 20

Lnk = (-Ea/R)(1/T) + lnA

• Gradient m of -Ea/R
• intercept of c = lnA

Question 21

What effects the rate constant?

Answer 21

• Increasing temperature, increases rate constant and decreasing activation energy, increases rate constant due
• Boltzmann distribution shifting to the right when temperature increases and both cause an increase in the number of particles with energy above activation energy.

Question 22

What is the rate constant?

Answer 22

The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation

Question 23

How can you work out rate constant from first order rate-conc graph?

Answer 23

Take gradient

Question 24

How do you investigate reaction rate using the initial rates method?

Answer 24

• The time t from the start of an experiment is measured for a visual change to be observed
• Providing there is no significant change in rate the initial rate is proportional to 1/t
• Repeated for multiple concentrations and values of 1/t