Reactions rates and equilibrium chp 10

Question 1

whats the formula for the rate of reaction and what units are used

Answer 1

Question 2

when is the rate of reaction fastest and when is it slowest

Answer 2

• fastest at the start of the reaction, as each reactant is at its highest concentration
• slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease
• Once one of the reactants has been completely used up, the concentrations stops changing and the rate of reaction is 0

Question 3

what 4 factors effect the rate of reaction

Answer 3

-Concentration (pressure when reactants are gases)
-temperature
-use of a catalyst
-surface are of solid reactants

Question 4

what is collision theory

Answer 4

collision theory states that 2 reacting particles must collide for a reaction to occur.

in most collisions, the molecules collide but then bounce off each other and remain chemically unchanged

Question 5

Does every partical that collides react

Answer 5

Usually only a small proportion of collisions result in a chemical reaction
in most collisions, the molecules collide but then bounce off each other and remain chemically unchanged

Question 6

why are some collisions effective and why are some ineffective

Answer 6

An effective collision is one that leads to a chemical reaction
A effective collision needs 2 conditions to be met:
-the particles collide with the correct orientation
-the particles have sufficient energy to overcome the activation energy barrier of the reaction

Question 7

How does increasing the concentration affect the rate of reaction

Answer 7

• higher conc higher number of particles in same space
• particles collide more frequently
  ^more effective collisions, increase rate of reaction

Question 8

How does increasing the pressure of a gas affect the rate of reaction

Answer 8

• ↑ pressure = ↑ rate of reaction
• The concentration of the gas molecules increases as the same amount inhabits a smaller volume.
• The gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time period.

Question 9

how can the progession of a reaction be followed

Answer 9

-Monitoring the removal (decrease in concentration) of a reactant
-following the formation (increase in concentration) of a product
-other measurable properties that might change as the reaction proceed include gas volume and colour.

Question 10

if a reaction produces a gas, which 2 methods can be used to determine the rate of the reaction

Answer 10

• monitoring volume of gas produced at regular time intervals using gas syringes
• monitoring loss of mass of reactants using a balance

Question 11

what do catalysts do

Answer 11

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

Question 12

what are the characteristics of catalysts

Answer 12

-the catalyst is not used up in the chemical reaction
-the catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
-at the end of the reaction that catalyst is regenerated

Question 13

How do catalysts work on a basic level

Answer 13

A catalyst increases the rate of a chemical reaction by providing an alternative pathway or lower activation energy

Question 14

what is a exothermic reaction

Answer 14

A reaction is exothermic when the products have less energy than the reactants and so the enthaply change is negative

Question 15

what is an endothermic reaction

Answer 15

a reaction is defined as endothermic when it absorb energy and so the enthalpy change is positive

Question 16

what does a catalyst look like on a enthaply change diagram

Answer 16

Question 17

what is a homogenous catalyst

Answer 17

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

Question 18

whats an example of a reaction that involves the use of a homogenous catalyst

Answer 18

-Making esters with sulphuric acid as a catalyst
-The reactants (ethanol and ethanoic acid) and the catalyst (sulphuric acid) are all liquids

Question 19

what is a hetrogeneous catalyst

Answer 19

• different physical state than reactants

Question 20

How do hetrogeneous catalysts interact with reactant molecules.

Answer 20

Reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst, where the reactions take place.
After reaction, the product molecules leave the surface of the catalyst by desorption.

Question 21

what are some examples of reactions that need catalysts and what are those catalysts

Answer 21

Question 22

why are catalyst important for sustainability and the economy

Answer 22

90% of all chemical materials are produced using a catalyst
if a chemical process requires less energy, then less electricity or fossil fuels are used, making the product faster and using less energy can cut costs and increase profitability

Question 23

what is autocatalysis

Answer 23

a chemical reaction is said to have undergone autocatalysis if a reaction product acts as a catalyst for that reaction. typically the reaction starts slowly and then speeds up as the products are formed

Question 24

if a collison is described as elastic what does it mean

Answer 24

the molecules do not slow down as a result of a collision and no energy is lost

Question 25

what is the spread of molecular energy between gaseous molecules called

Answer 25

the boltzmann distibution

Question 26

what does a boltzmann distribution look like and what does it represent

Answer 26

it represents the spread of energy between all the molecules in a given substance and also shows the amount of molecules with activation energy

Question 27

what are some important features of the boltzmann distribution

Answer 27

• no molecules have 0 energy
• the curve starts at the origin
• area under curve is equal to total number of molecules
• there is no maximum energy for a molecule

Question 28

• what is the effect of increasing temperature on the Boltzmann distribution curve
• explain the effect

Answer 28

• As temp increases, average energy molecules increases
  ^more molecules have Ea
• small proportion of molecules will have below Ea
• increased rate of reactions
  ^more molecules have Ea and collisions occur more due to rise in kinetic energy

vice versa

Question 29

what is the effect of catalysts on the boltzmann distribution

Answer 29

• catalyst provides alternative reaction route with lower activation energy
• greater proportion of molecules would have activation energy
  ^more success full collisions

Question 30

what are reversible reactions

Answer 30

reactions that take place in both forward and reverse directions (an example is the haber process)

Question 31

what is a equilibrium system and what are its features

Answer 31

in an equilibrium system:

• rate of forward reaction is equal to rate of reverse reaction
• concentrations of reactants and products do not change
• they are dynamic

Question 32

what needs to occur for a reaction to remain in equilibrium

Answer 32

for a reaction to remain in equilibrium, the system must be closed

Question 33

what is meant by a closed system

Answer 33

A closed system is isolated from its surroundings, so the temperature, pressure and concentrations of reactants and products are unaffected by outside influences

Question 34

what does le chatelier’s principle state

Answer 34

states that when a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of that change

Question 35

what does the position of equilibrium indicate

Answer 35

a measure of the concentrations of the reacting substances at equilibrium .

Question 36

what is the effect of changing concentration on the position of equilibrium

Answer 36

• The equilibrium would shift towards the side of the reaction that hasn’t experienced the increase in concentration

Question 37

How does an increase or decrease in temperature effect the position of equilibrium

Answer 37

• ↑ temperature shifts equilibrium towards endothermic reaction
• ↓ temperature shifts equilibrium towards exothermic reaction

Question 38

what is an example of a reaction that can be used to illustate changing concentration

Answer 38

Question 39

what is the effect of increasing and decreasing pressure

Answer 39

-increasing the pressure of the system will shift the position of the equilibrium to the side with the fewer molecules, reducing the pressure of the system
-Decreasing the pressure shifts the position of equilibrium to the side with more gaseous moles

the pressure of a gas is proportional to its concentration

Question 40

what is the effect of a catalyst on the equilibrium

Answer 40

-a catalyst does not change the position of equilibrium; it merely speeds up the rates of the forward and reverse reactions equally
-it will increase the rate at which an equilibrium is established

Question 41

what are the conditions used for the haber process

Answer 41

-temperature of 350-500°C
-pressure of 100-200 atm
-an iron catalyst

Question 42

what is the formula for the the equilibrium constant

Answer 42

small letters = balancing numbers
big letters = equilibrium concentrations of substance

Question 43

what does the equilibrium constant (Kc) represent

Answer 43

-a Kc value of 1 indicates a position of equilibrium that is halfway between reactants and products
-a Kc value >1 indicates a position of equilibrium that is towards the products
-a Kc value <1 indicates a position of equilibrium that is towards the reactants
- the larger the value of Kc the further the position of equilibrium lies to the right-hand side and the greater the concentration of the products compared to the reactants