Rate Equations

Question 1

what is meant by rate of reaction

Answer 1

the change in conc of a reactant or product per unit time

Question 2

whats the equation for the rate of reaction?

Answer 2

rate= change in conc / time

Question 3

what are some method for monitoring the rate of reaction?

Answer 3

• conc change = take samples from a reaction at regular intervals in time and carry out a titration to measure the conc
• gas evolving experiments = measure the volume of gas evolved, usually with a syringe, or set the system on a balance and measure the decrease in mass as the gas is released
• colour change = measure the colour change during an experiment with a colorimeter
• pH change = use a pH meter to follow the pH change during a reaction

Question 4

what does a rate order of 0 mean?

Answer 4

the conc of species doubles, the rate is unchanged

Question 5

what does a rate order of 1 mean?

Answer 5

the conc of species doubles, the rate also doubles

Question 6

what does a rate order of 2 mean?

Answer 6

the conc of species doubles, the rate is 2^2

the conc of species triples, the rate is 3^2

Question 7

how do you calculate overall order of the reaction?

Answer 7

the sum of the orders of all the reactants

Question 8

what is the effect of temperature on rate constants?

Answer 8

• temp is a measure of the amount of energy that molecules have on average -the higher the temperature, the more energy molecules have
• only particles with energies equal to or greater than the activation energy will react
• increasing the temp increases the number of particles with energy equal to or above the activation energy, leading to more frequent successful collisions
• the rate or reaction increases and therefore the rate constant increases

Question 9

what effect will increasing the temp by 10 degrees have on the rate?

Answer 9

the rate constant roughly doubles for energy 10 degree increase in temperature. Doubling the rate constant will double the rate.

Question 10

what the Arrhenius equation?

Answer 10

k = Ae^(-Ea/RT)
k= rate constant
A= Arrhenius constant
Ea= activation energy (J mol-1)
R= gas constant (8.31 JK-1 mol-1 )
T= temperature (K)

Question 11

what needs to happen for the rate constant to increase?

Answer 11

• the temp must increase - particles will move faster with more energy and are therefore more likely to collide with energy equal to or greater than the activation energy. Consequently, the rate of reaction increases.
• The activation energy must decrease- a reaction with low activation energy requires less energy for successful collisions, so more collisions will have energy equal or greater than the activation energy. Therefore, the rate of reaction increases. This requires a catalyst

Question 12

whats the rearranged Arrhenius equation?

Answer 12

ln k = -Ea/RT + ln A
y= mx + c

lnk= y
m= -Ea/R
x= 1/T
c= lnA

Question 13

how do you use the gradient to calculate activation energy?

Answer 13

Ea - -gradient * R

R = gas constant

Question 14

how do you calculate gradient from the rate graph?

Answer 14

gradient= -Ea / R

Question 15

at what angle is a conc- time graph gradient taken for initial rate?

Answer 15

the initial rate is drawn at t=0
the initial rate is represented by the steepest gradient.
this is the fastest rate at any point in the reaction

Question 16

define rate- determining step

Answer 16

the slowest step in the reaction mechanism of a multistep reaction

Question 17

what does the order tell us about the rate determining step?

Answer 17

the order indicates how many molecules of the reactant are involved in the rate- determining step (RDS), either directly or by forming an intermediate