Rates and equilibrium

Question 1

what is the rate of reaction equation?

Answer 1

change in concentration(mol/dm cubed)/time(s)

Question 2

describe the relationship between rate of reaction and concentration?

Answer 2

directly proportional

Question 3

Describe the concentration time graphs?

Answer 3

• rate is faster at the start (steepest) as reactant is at its highest concentration
• rate decreases as concentration of reactants decrease as they are being used up
• once the reactants are completely used up the rate is zero (horizontal line)

Question 4

what factors change the rate of reaction?

Answer 4

• concentration (pressure when gases)
• catalyst
• surface area of solid reactants
• temperature

Question 5

what are the conditions needed for a reaction to occur?

Answer 5

• sufficient energy to overcome the activation energy

- collision with the correct orientation

Question 6

how does concentration affect rate?

Answer 6

increasing concentration, increases rate of reaction as there are more molecules in a given volume so molecules are closer, increasing the frequency of successful collisions.

Question 7

how does increasing pressure in a gas affect rate?

Answer 7

increasing the pressure of a gaseous reaction increases the number of gaseous molecules in a given volume(smaller), so molecules are closer together. This leads to more frequent successful collisions and an increase in the rate of reaction

Question 8

what methods measure the progress of a reaction?

Answer 8

• monitoring the removal (decrease in concentration) of a reactant
• following the formation (increase in formation) of a product

Question 9

if the reaction product is a gas wet methods can be used to measure the rate?

Answer 9

• monitor the volume of gas in regular time intervals using an inverted measuring cylinder of gas syringe
• monitoring the mass loss of reactants using a balance

Question 10

what is a catalyst?

Answer 10

a substance that changes the rate of chemical reaction without undergoing any permanent change itself

Question 11

how does a catalyst affect the rate of reaction?

Answer 11

provides an alternative pathway of lower activation energy increasing the frequency of successful collisions, increasing the rate

Question 12

what is a homogenous catalyst?

Answer 12

it has the same physical state as the reactants. The catalyst reacts to form an intermediate. The intermediate then breaks down to give the product and regenerates the catalyst

Question 13

what is a heterogeneous catalyst?

Answer 13

A heterogeneous catalyst has a different physical state from the reactants. They are usually solids. Reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place. After the reaction, the product molecules leave the surface of catalyst by desorption.

Question 14

what are the economic advantages using a catalyst?

Answer 14

less cost is associated with developing the process (reduced temperature and energy requirements)

Question 15

how are catalysts more sustainable?

Answer 15

high atom economy and fewer pollutants (using less fossil fuels will cut emissions of carbon dioxide linked to global warming)

Question 16

when is a chemical reaction said to have undergone autocatalyst?

Answer 16

if a reaction product acts as a catalyst for that reaction. Typically the reaction starts slowly, then seeds up as products are formed.

Question 17

what is the Boltzmann distribution?

Answer 17

shows the spread of molecular energies in gases. Some molecules will have high energy and some will have low energy depending on the speed of the molecules

Question 18

In the Boltzmann distribution, how do you calculate the total number of molecules?

Answer 18

the area under the curve

Question 19

In the Boltzmann distribution, why does the curve start at the origin?

Answer 19

no molecules have zero energy

Question 20

In the Boltzmann distribution, is there a maximum energy for a molecule?

Answer 20

no, the curve doesn’t meet the x-axis at high energy only if the energy is infinite

Question 21

In the Boltzmann distribution, what does the peak curve represent?

Answer 21

the most probable energy that a molecule will have

Question 22

In the Boltzmann distribution, how do you find the average energy?

Answer 22

It is the at the right of (slightly greater) the most probable energy

Question 23

How does the Boltzmann distribution change, when temperature increases?

Answer 23

• average energy increases
• more molecules have higher energy greater than activation energy (increased frequency of successful collisions)
• graph is stretched out
• peak of graph is lower on y-axis but further on x-axis (peak is at higher energy)
• area (total number of molecules) is the same

Question 24

What is the hater process equation for manufacturing ammonia?

Answer 24

N2(g) + 3H2(g) -reversible arrow- 2NH3(g)

Question 25

In an equilibrium system…

Answer 25

• rate of forward reaction is equal to rate of reverse direction
• concentration of reactants and products do not change

Question 26

What does DYNAMIC equilibrium mean?

Answer 26

both forward and reverse reaction is taking place at the same rate

Question 27

Why must a reaction remain in equilibrium of a closed system?

Answer 27

so temperature, pressure and concentration of reactants and products are not affected.

Question 28

What does le chatlier’s principle state?

Answer 28

when a system in equilibrium is subjected to an external change, the system counteracts it to minimise its effects

Question 29

How does concentration changes affect equilibrium?

Answer 29

• if there are more products formed, equilibrium shifts to the right
• if there are more reactants formed, equilibrium shifts to the left

Question 30

What does the position of equilibrium show?

Answer 30

compares how many reactants there are compared to products, the extent of the reaction. Position of equilibrium can be detected through colour change

Question 31

How does temperature affect equilibrium?

Answer 31

• increase in temp, shifts equilibrium in the endothermic direction
• decrease in temp, shift equilibrium in the exothermic direction

Question 32

How do you answer a question on factors affecting equilibrium?

Answer 32

• include what direction is favoured
• how position of equilibrium has shifted
• do products or reactants increase?

Question 33

what condition are needed for pressure to affect equilibrium?

Answer 33

• must be gases

- uneven number of molecules

Question 34

How does increasing pressure affect equilibrium?

Answer 34

-equilibrium shifts to the side with fewer molecules, reducing pressure. Decreasing pressure shifts equilibrium to side of more molecules

Question 35

How does a catalyst affect equilibrium?

Answer 35

doesn’t affect equilibrium position as it speed rate of both forward and backward reaction equally. Catalyst increases the rate at which equilibrium was established.

Question 36

What conditions are needed for manufacturing ammonia through the hater process?

Answer 36

350 - 500 degrees
100 - 200 atm (pressure)
iron catalyst

Question 37

Why are low temps and high pressures not used in the hater process?

Answer 37

• even though low temps increase production of product, it will cause the reaction to be too slow and equilibrium may not be established
• high temps increase yield however a strong container is needed and lots of energy and money
• however unreacted reactants are recycled and eventually convert into ammonia

Question 38

what does an equilibrium constant provide?

Answer 38

the actual position of equilibrium. The magnitude of an equilibrium constant indicates whether there are more reactants or more products in an equilibrium system.

Question 39

How do you calculate exact position of equilibrium?

Answer 39

using equilibrium law

Question 40

What is the equilibrium law which defines equilibrium constant (Kc) in terms of concentrations? Using
aA + bB (reversible arrows) cC +dD

Answer 40

Kc= [C] to the power of c [D] to the power of d / [A] to the power of a [B] to the power of b

products/ reactants

Question 41

what does the value of Kc tell us?

Answer 41

• Kc value of 1 indicates equilibrium position halfway between reactants and products
• Kc value> 1 indicates position of equilibrium towards products
• Kc value<1 indicates equilibrium position towards reactants