10) Reaction rates and equilibrium

Question 1

Define rate of reaction

Answer 1

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

Question 2

rate = ?

Answer 2

change in conc. / time

Question 3

When is rate fastest?

Answer 3

at the start of a reaction as each reactant is at its highest concentration

Question 4

What is rate affected by?

Answer 4

conc. (pressure if gaseous)
temperature
use of catalysts
surface area of solid reactants

Question 5

Define collision theory

Answer 5

two reacting particles must collide for a reaction to occur, and must be in the correct orientation and have sufficient energy to overcome the activation energy of the reaction

Question 6

How does an increase in concentration affect rate?

Answer 6

increases no. of particles in the same volume
particles are closer together and collide more frequently in a given time
there will be more effective collisions and an increase in rate of reaction

Question 7

When gas is compressed into a smaller volume, and pressure is increased what happens to the concentration of gas molecules?

Answer 7

increases as the same no. occupy a smaller volume

Question 8

How can the progress of a chemical reaction by followed?

Answer 8

monitoring the removal of a reactant or the formation of a product

Question 9

Name two ways rate of reaction can be measured for reactions that produce gases?

Answer 9

monitoring volume of gas produced at regular time intervals using gas collection
monitoring the loss of mass of reactants using a balance
either of these measurements are then plotted against time and a tangent drawn for initial rate

Question 10

What apparatus is used for monitoring volume of gas produced at regular time intervals using gas collection?

Answer 10

conical flask
delivery tube
trough filled with water
measuring cylinder, filled with water and held upside down in trough with clamp
stop watch
alternative: gas syringe

Question 11

How can you tell the reaction is compete when monitoring the loss of mass of reactants using a balance?

Answer 11

no more mass is lost, remains constant

Question 12

Define catalyst

Answer 12

a substance that increases the rate of a chemical reaction without being used up in the process

Question 13

What does a catalyst do?

Answer 13

provides can alternative route for the reaction with lower activation energy
may react to form an intermediate or provide a surface on which the reaction can occur but is regenerated at the end

Question 14

Describe homogenous catalysts

Answer 14

same physical state as reactants and reacts to form an intermediate, which then breaks down to give the product and regenerate the catalyst

Question 15

Describe heterogeneous catalysts

Answer 15

different physical states from reactants
usually solids in contact w/ reactants (g/aq) which are adsorbed (weakly bonded) onto their surface where the reaction takes place. after the product molecules leave the surface by desorption

Question 16

Name 4 common catalysts and their uses in industry

Answer 16

Fe (s) - Haber process
Pt / Rh (s) - reforming
Ni (s) - hydrogenation of alkenes
V2O5 (s) - contact process (making SO3 for sulfuric acid)

Question 17

Describe catalytic converters

Answer 17

have large surface area for heterogeneous catalysts to convert harmful exhaust gases into less harmful ones which can be released into the atmosphere

Question 18

Comment on the use catalysts with regards to sustainability

Answer 18

increase the rate of reaction by lowering Ea
reduces temp. and energy requirements
less electricity / finite fossil fuels used
increases profits and less pollutants linked to global warming

Question 19

Give the x and y axis of a Boltzmann distribution

Answer 19

x = energy
y = no. of molecules with a given energy

Question 20

Describe a Boltzmann distribution showing temperature

Answer 20

at a higher temperature peak is lower and shifted right
greater proportion of molecules can overcome Ea / have energy greater than or equal to Ea
a greater proportion of collisions will lead to a reaction
increases rate of reaction
more frequent collisions as molecules are moving faster also

Question 21

Describe a Boltzmann distribution showing catalyst use

Answer 21

Ea with catalyst (left hand side) and without catalyst labelled
a catalyst provides an alt. reaction route w/ a lower Ea
greater proportion of molecules have energy greater than or equal to lower Ea
on collision more molecules will react
increases rate of reaction

Question 22

Define reversible reaction

Answer 22

a reaction that takes place in both forward and reverse directions

Question 23

Define dynamic equilibrium

Answer 23

the equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations do not change

Question 24

Define position of equilibrium

Answer 24

the relative quantities of reactants and products, indicating the extent of a reversible reaction at equilibrium

Question 25

Define Le Chatelier’s principle

Answer 25

when a system in dynamic equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of the change and to restore equilibrium

Question 26

If there are more products formed, the position of equilibrium has shifted to the _ and If there are more reactants formed, the position of equilibrium has shifted to the _?

Answer 26

right-hand side

left-hand side

Question 27

An increase in temp. results in a shift in the _ direction (ΔH is _ ) and a decrease in temp. results in a shift in the _ direction (ΔH is _)?

Answer 27

endothermic
positive
exothermic
negative

Question 28

An increase in pressure results in a shift in the direction of the _ and a decrease in pressure results in a shift in the direction of the _?

Answer 28

fewer gas molecules

more gas molecules

Question 29

A catalyst doesn’t change _ - its speeds up the rate of forwards and reverse reactions equally?

Answer 29

position of equilibrium

Question 30

Define equilibrium constant Kc

Answer 30

a measure of the position of equilibrium, the magnitude of an equilibrium constant indicates whether there are more reactants or more products in an equilibrium system

Question 31

Kc = ?

Answer 31

[products] / [reactants]
concentrations at equilibrium
^no of molecules

Question 32

What does Kc = 1 indicate?

Answer 32

position of equilibrium halfway between reactants and products

Question 33

What does Kc > 1 indicate?

Answer 33

position of equilibrium towards products

Question 34

What does Kc < 1 indicate?

Answer 34

position of equilibrium towards reactants

Question 35

The larger the value of Kc, the further the equilibrium lies to the _ and the greater the concentration of _?

Answer 35

RHS

products to reactants