topic 9

Question 1

activation energy

Answer 1

the minimum energy which particles need to collide to start a reaction

Question 2

Maxwell-Boltzmann distribution curve

Answer 2

• Emp : most probable energy (peak)
• mean energy : below the peak of the curve
• energy distribution must go through the origin as there are no molecules with no energy
• area under the curve is the total number of particles present

Question 3

effect of temperature increase on maxwell curve

Answer 3

distribution will shift towards having more molecules with higher energies.
at higher energies both the Emp and mean energy shift to higher energy values - number of molecules with those energies decrease.

Question 4

rate of reaction

Answer 4

change in concentration of a substance in unit time

Question 5

measuring rate of reaction

Answer 5

the gradient of a concentration time graph - draw a tangent to the curve

Question 6

effect of increasing concentration and pressure

Answer 6

at higher conc there are more particles per unit volume hence the particles collide with greater frequency

Question 7

doubling concentration and rate?

Answer 7

if conc/rate is doubled then the number of particles is doubled per unit volume and the frequency of effective collisions is doubled

Question 8

increased concentration - effect on graph

Answer 8

shape of energy distribution curve doesn’t change - peak is at the same energy - curve is higher and area under curve is greater because there are more particles

Question 9

effect of increasing temperature

Answer 9

the energy of the particles increases - they collide more frequently and more often with energy greater than the activation energy

peak is lower and curve is more to the right

Question 10

effect of increasing surface area

Answer 10

results in successful collisions to occur more frequently between the reactant particles and hence increases the rate of the reaction

Question 11

catalyst

Answer 11

catalysts increase the rate of reaction without getting used up

Question 12

effect on Maxwell curve of using a catalyst

Answer 12

lowers the activation energy therefore more particles will have energy that is greater than the activation energy so there will be a higher frequency of effective collisions

Question 13

heterogeneous catalyst

Answer 13

usually solids and they are in a different phase from the reactants

Question 14

effect of pressure on heterogeneous catalysis

Answer 14

increasing pressure has no effect

Question 15

why does pressure change have no effect on heterogeneous catalysis

Answer 15

because the reaction takes place on the surface of the catalyst so the active site on the catalyst surface is already saturated with reactant molecules

Question 16

adsorption

Answer 16

the process by which a solid holds molecules of a gas or liquid or solute as a thin film

Question 17

how does adsorption at the active site result is catalytic action

Answer 17

the active site is where reactants adsorb on to the surface of the catalyst. This results in the bonds within the reactant molecules to become weaker. There will also be a higher concentration of reactants at the solid surface - leading to higher collision frequency

Question 18

environmental benefits of catalysts

Answer 18

• catalysed reactions can occur at lower temperature so less fuel needed and fewer emissions from fuels
• enables use of an alternative process with higher atom economy hence fewer raw materials needed and less waste products produced

Question 19

how can a reaction go to completion if few particles have energy that is greater than activation energy

Answer 19

particles can gain energy through collisions

Question 20

effect of industrial catalysts

Answer 20

• speed up the rate
• hence allow lower temperatures to be used hence lower costs
• no effect on equilibrium