Rates 5A

Question 1

rate of reaction

Answer 1

The change in concentration of a reactant or product per unit time

Question 2

rate of reaction is measured in ….. (units)

Answer 2

mol dm-3 s-1.

Question 3

what are the four things which affect r.o.r

Answer 3

temperature
pressure/conc
catalyst
surface area if solid

Question 4

the ……………….. of the ……………… between particles determines the rate of reaction

Answer 4

the frequency of the collisions between particles determines the rate of reaction

Question 5

how do you calculate a reaction rate from a graph?

Answer 5

find the gradient:

change in quantity/ change in time

Question 6

list some ways in which the progress of a chemical reaction might be monitored.

Answer 6

• temp change
• pH change
• precipitate (turbidity)
• colour change
• volume of gas produced
• mass change
• conductivity

Question 7

what does collision theory state

Answer 7

the two reacting particles must collide for a reaction to occur

Question 8

what are the two conditions required to ensure a collision is successful?

Answer 8

• particles collide with correct orientation

- particles have sufficient energy to overcome activation energy barrier of reaction

Question 9

homogeneous catalyst

Answer 9

has the same physical state as the reactants

• reacts with reactants to form an intermediate
• intermediate breaks down into product and regenerates the catalyst

Question 10

heterogeneous catalyst

Answer 10

has different physical state to the reactants

• usually solids in contact with gaseous reactants or reactants in solution
• reactant molecules are ABSORBED (weakly bonded) to the surface of the catalyst, where a reaction takes place
• after the reaction, product molecules leave the surface of the catalyst by DESORPTION

Question 11

role of a catalyst

Answer 11

A substance that increases the rate of a chemical reaction without undergoing any permanent change itself. Allows the reaction to proceed by a different route with a lower activation energy

• may react with reactant to form intermediate before breaking up into product
• is always regenerated at the end

Question 12

why is rate of reaction fastest at the start and then slows down as the reaction proceeds until rate is 0?

Answer 12

at start: each reactant is at its highest concentration so there are more frequent successful collisions
rate decreases as conc. of reactants decreases until one reactant is completely used up and reaction stops

Question 13

how does increasing the concentration of reactant affect rate

Answer 13

rate increases
increased no. reactant particles in same vol
particles are closer together
collide more frequently
more frequent successful collisions (correct orientation and sufficient energy)

Question 14

how does increasing the pressure of a gas (reactant) affect rate

Answer 14

rate increases
same no. reactant particles in smaller vol
particles are closer together
collide more frequently
more frequent successful collisions (correct orientation and sufficient energy)

Question 15

economic and sustainable importance of catalysts

Answer 15

by increasing the ror, catalysts reduce
temp needed for process,
energy requirements, 
combustion of fossil fuels, 
CO2 emissons,
cost

Question 16

4 key features of a boltzmann distribution

Answer 16

1. no molecules have 0 energy (curve starts at origin)
2. area under curve=total no. molecules
3. no max energy for a molecule (curve doesn’t meet x axis at end, asymptote)
4. only a small proportion of molecules have more energy than Ea

Question 17

affect of incr in temp on boltzmann distribution

Answer 17

peak of graph is lower and shifted to the right
same no. molecules so area under curve is same
greater % of molecules exceed Ea
- greater proportion of collisions are successful
- higher ror
- collisions are also more frequent as the molecules are moving faster (but incr. energy of molecules is far more important)

Question 18

affect of catalyst on boltzmann distribution

Answer 18

Activation energy w catalyst (Ec) is lower than Ea, so greater % of molecules exceed the required Ec.

• more successful collisions
• higher ror