RATES OF REACTION

Question 1

collision theory

Answer 1

• for a reaction to occur, reactant particles must:
  
  • collide with each other
  • collide with sufficient energy to break bonds within the reactants (activation energy)
  • with the correct orientation to break the bonds within the reactants and allow the formation of new products
    
    • allows particular bonds to break and new bonds to form
    • if the orientation is incorrect, particles bounce off each other and no reaction occurs
• most collisions don’t result in a chemical reaction

Question 2

activation energy

Answer 2

• minimum amount of energy required for a reaction to occur
• reactions only occur when the energy of the collision is equal to or greater than the activation energy
• magnitude of activation energy determines how easy it is for a reaction to occur (known as reaction rate)

Question 3

transition state

Answer 3

• when the activation energy is absorbed, atoms enter a transition state
• transition state occurs at the stage of maximum potential energy
• bonds are breaking and forming at this stage
• arrangements of atoms during this state is highly unstable as they rearrange into products
• atoms in transition state rearrange in products as reaction progresses

Question 4

activation energy → reaction rate

Answer 4

• reactions w/ low activation energy will occur more easily than those with a high activation energy
• reactions w/ low activation energy will have a higher proportion of collisions that result in reaction
  
  • reaction rate is dependent on activation energy

Question 5

measuring rate of reaction

Answer 5

• is the change in concentration of reactants/products per unit of time - positive value
  
  • Ms^-1 or mol L^-1s^-1
• need to measure directly or indirectly how much reactant is being used up or how much product is formed in a given time period
• the gradient of slope shows rate of reaction
• calculation of rate must include consideration of time

Question 6

factors that affect reaction rate

Answer 6

• catalysts

changes frequency of collisions

• surface area of solid reactant
• concentration of reactants in solution
• pressure of any gaseous reactant

changes energy of collisions

• temperature of reaction

Question 7

surface area

Answer 7

• only the particles at the surface of the solid participate in the reaction
• greater surface area → more particles available for reaction
  
  • frequency of collisions between reactant increases
  • increased reaction rate

Question 8

concentration and pressure on rate of reaction

Answer 8

• higher concentration of reactants
  
  • increased chance of collisions
  • frequency of collisions between reactants increases
  • increased rate of reaction

Question 9

changing concentration or pressure

ways to do so

Answer 9

in solution

• adding or removing reactant
• concentration/dilution (adding or removing solvent)

in gas

• increasing / decreasing volume (of container)
• adding / removing reactant (adding more reactant gas to a fixed volume container)

Question 10

temperature -
collision theory

Answer 10

at higher temps

• particles have more kinetic energy → move faster
• increased frequency of collisions and increased energy of collisions between particles
• increased rate of reaction

summary

• temp affects reaction rate by increasing the frequency + energy of collisions
• the increase in reaction rate due to temp increase is mostly due to increased energy of collisions and greater proportion of collisions that OVERCOME ACTIVATION ENERGY BARRIER

Question 11

kinetic energy related to speed formula

Answer 11

• as temp increases
  
  • kinetic energy (KE) increases
  • average speed of particles also increase
• KE = 1/2 mv^2
  
  • m= mass
  • v= velocity

Question 12

energy of particles

Answer 12

• at any given temperqature, the particles in a substance will have a range of kinetic energy
• although most will be similar, there are always some particles with very high and very low energy

Question 13

Maxwell Boltzmann distribution curve

Answer 13

• kinetic energy distribution curve
• displays the distribution of energies of particles in a sample at a particular temperature
• only a small proportion of reactant particles overcome activation energy barrier
• area under every curve = total no. particles in the sample
• as temp increases, curve shifts to the right, as average kinetic energy of particles increases
• activation energy DOES NOT change at different temperatures
  
  • it is fixed for each chemical reaction
  • at higher temps, more particles have energy greater than or equal to Ea
  • every temp increase of 10 degrees celsius doubles the rate of reaction

Question 14

catalysts

Answer 14

• a substance added to a reaction to increase reaction rate without getting consumed in the process
• do not appear as reactants or products in a chemical equation
• increases rate of reaction by providing a more energetically favourable pathway and in some cases assist in orientation

Question 15

how catalysts work

Answer 15

• they provide an alternative reaction pathway with a reduced activation energy barrier
• the presence of a catalyst does not change the ΔH for a reaction

Question 16

catalysts and activation energy

Answer 16

• lower activation energy
• colliding particles are more likely to have energies that equal or exceed the activation energy
• bonds in reactants are likely to be broken more frequently
• higher proportion of successful collisions lead to the formation of products
• increased rate of reaction

Question 17

types of catalyst

Answer 17

• homogenous catalysts: same physical state as reactants and products
  
  • gaseous chlorine atoms act as catalyst in decomposition of ozone gas into oxygen
• heterogenous catalysts: different physical state from reactants and products
  
  • black powder MnO2 catalyses the decomposition of hydrogen peroxide solution

Question 18

catalysts in industry

Answer 18

• heterogenous catalysts are favoured because they are:
  
  • more easily separated from the products of a reaction
  • much easier to reuse
  • able to be used at high temperatures

Question 19

adsorption - how some catalysts work

Answer 19

• particles at the surface of some solids of high surface area tend to adsorb (bond with) gas molecules that strike the surface
• adsorption distorts bonds in the gas molecules or may even break them completely → allowing reaction to proceed more easily
• these solid surfaces provide a new pathway for the reaction to occur
• solid catalysts in the form of powder or sponger are often used to provide the greatest possible surface area

Question 20

answering qs on rate of reaction

Answer 20

• when using collision theory to explain effects of factors on rat eof chemical reaction
  
  • discuss effect on collision frequency
  • the number of successful collisions per unit of time