Unit 2.2 Rate of reaction

Question 1

How is the rate of reaction usually determined/calculated?

Answer 1

Measure the amount of reactant used up per unit time
Or
Measuring the amount of product made per unit time

Question 2

What are the mains ways to increase rate/speed of a chemical reaction?
(A level 6 ways)

Answer 2

1. Temperature
2. SA or particle size
3. Catalysts
4. Concentrations
5. Pressure
6. Light
   (last 2 are the new ones)

Question 3

What is the collision theory?

Answer 3

• 2 reactant particles must collide successfully
• To form new product
• An effective collision
• To increase rate of reaction, u must increase number of effective collisions

Question 4

How may many collisions between reactant particles not form new product?

Answer 4

• Reactant particles didn’t have sufficient energy
  to overcome activation energy
• Particles collide in wrong orientation

Question 5

State how increase in concentration will affect the rate of reaction

Answer 5

Increasing the conc. of a solution, or the pressure of a gas, increases the rate of reaction. This is because more reactant particles present in the same volume ∴ there is an increase in number of collision per unit time. There will ∴ be a greater chance of successful collisions

Question 6

State how increase in temperature will affect the rate of reaction

Answer 6

Increasing temperature increases rate of reaction. This is because the reactant particles have greater KE at higher temps., so they move a lot faster. More of the particles have energy that is greater than the activation energy, so more successful collisions occur per unit time.

Question 7

How do they increase surface area of a solid reaction?

Answer 7

Cut up or ground into power

Question 8

State how increase in surface area will affect the rate of reaction

Answer 8

• More of the reactant is exposed
• Larger area for the reaction to take place
• Increases rate of reaction
• More chance of an effective collision

Question 9

What is a catalyst?

Answer 9

Provides an alternate route with a lower activation energy
Without undergoing permanent change (can be re-used)

Question 10

State the 4 catalysts used for these 4 reactions:
- Haber process
- Catalytic convertors in cars
- Contact process
- Decomposition of hydrogen peroxide

Answer 10

1. Iron
2. Platinum
3. Vanadium oxide
4. Manganese oxide

All transition metals

Question 11

State how increase in pressure will affect the rate of reaction

Answer 11

• Decreases volume of gas
• While maintaining number of particles
• As they are closer together
• More likely collisions

Question 12

State how increase in light will affect the rate of reaction

Answer 12

E.g. chlorination of methane or photosynthesis
- Some reactions driven by light
- Light providing energy to either break bonds or produce more of a product
- It’s all the same tbh

Question 13

Do u know how to draw the changes of product if u change any of the factors within the reaction?

Answer 13

… yes

Question 14

How do u calculate the rate of a reaction?

Answer 14

Change in concentration/time
moldm^-3/s = moldm^-32^-1

Question 15

How to calculate rate of reaction in terms of mass?

Answer 15

g/s = gs^-1

Question 16

How to calculate rate of reaction in terms of volume?

Answer 16

cm³/s = cm³s^-1

Question 17

How to establish relationship between reactant concentrations and rate?
(graph)

Answer 17

Well on a real:
At steepest - rate is fastest at start of reaction cuz concentration of reactants are the greatest (not much has been used up)
Line become less steep - rate slows down as reaction proceeds since the concentration of reactants decreases
Horizontal - rate becomes zero when reaction stops (reactants used up)

Question 18

Define activation energy E_a

Answer 18

Minimum energy required to start a reaction by the breaking of bonds

Question 19

Explain exothermic reaction

Answer 19

Reaction gives out heat and therefore the energy of the products is less than that of reactants - they have lost energy

Question 20

Explain endothermic reaction

Answer 20

Reaction takes in heat and therefore the energy of the products is higher than that of the reactants - they have gained energy

Question 21

Describe exothermic energy profile

Answer 21

Reactants = high
Curve (E_a) = high to low
Therefore △H = negative
Products = low

Ya gotta have known how to visualise this

Question 22

Describe endothermic energy profile

Answer 22

Reactants = low
Curve (E_a) = low to high
Therefore △H = positive
Products = high

Question 23

Describe reversible reaction energy profile

Answer 23

If one side is exo, the other side is endo
It’s a reversible reaction after all

Reactants is high (Vertical arrow with E_a forward)
Curve is high to low
Products is low (Vertical arrow on whole thing with E_a reverse)

Question 24

How to calculate enthalpy change of a reversible reaction energy profile?

Answer 24

△H = E_af - E_ab
(forward reaction - backward reaction)
Exothermic reaction △H = negative
Endothermic reaction △H = positive

Question 25

How does temperature affect the amount of energy the reactants have?

Answer 25

1. Increasing temp = reactants more energy, move faster, ∴ more collisions, statistically more product, produced in a set time, ∴ rate of reaction increases
2. More particles have activation energy

Question 26

Explain the Boltzmann distribution

Answer 26

• It’s a graph.
• Y-axis = number of particles with energy, E
• X-axis = kinetic energy
• Near the ends there’s a boundary:
  E_a aka the activation energy
  Only the number of particles represented under this section of the graph have sufficient energy to react

Question 27

Explain the difference of low temperature and high temperature in a Boltzmann distribution

Answer 27

At Low T:
- N° of molecules at it’s highest
- The activation energy is very small near the ends (the area)

At high T:
- The whole line looks like optimum pH level (if u know u know)
- Therefore technically n° not at its highest
- Activation energy is relatively bigger than of low T

Question 28

Describe the relation at high T within the Boltzmann distribution

Answer 28

As the temperature increases then the proportion of molecules with greater than activation energy also increase, therefore the rate increases

Broken english? wat??
and apparently 2 parts to the answer:
1. The increase in temperature, increases the energy (speed) of the molecules, therefore more collisions, therefore rate increases.
2. A greater proportion of molecules has greater than the required activation energy, therefore more product produced in the same time, therefore rate increases

Question 29

What are the 2 types of catalysts?

Answer 29

• Homogeneous catalyst
• Heterogeneous catalyst

Question 30

Explain homogeneous catalysts

Answer 30

The catalyst and reactants are in the same phase:
usually liquid mixtures or substances in solution

Question 31

An example of a homogenous catalyst?

Answer 31

Enzymes

Question 32

Benefits of enzymes?

Answer 32

• Lower temp and pressure so lower costs
• Operate in mild conditions so will not harm food or fabrics
• Biodegradable - easy to dispose of
• Often the products are pure with no side-products so no need to separate the unwanted products

Question 33

1 downside of enzymes?

Answer 33

• Homogeneous catalyst = same phase
• Therefore difficult to separate from the product
  (Though in some and in biological reactions, not necessary to remove em)

Question 34

Difference between enzymes and chemical catalysts?

Answer 34

Enzymes = react under extremely mild conditions (eg body temperature)
Chemical catalysts = often need severe conditions (high temp & pressure)

errr extremely effective at increasing the reaction rate - much more so than industrial catalysts

Question 35

Explain heterogeneous catalysts?

Answer 35

The catalysts and reactants are in different phases
(the trick = reactants are liquid/gas/aq but the catalyst is a solid soo voila)

• Many are d-block transition metals
• Gases absorbed on surface
• React and then desorb from the surface
• Larger surface area = better catalyst works

Question 36

Why do industries rely on catalysts?

Answer 36

• Reduces costs
• Lowers activation energy
• Less energy required for molecules to react
• Less electricity needed
• LOWER COSTS

Question 37

How can colorimetry be used in studies of some reaction rates?

Answer 37

Wait…. I never know I was missing one more topic D:

Question 38

I’d like to hope that’s it. No way this topic gets any complicated

Answer 38

…. right?