reactivity 2.2

Question 1

rate of reaction unit

Answer 1

mol dm⁻³ s⁻¹

Question 2

as the reaction proceeds, the concentration of reactants ______ and the concentration of products ______

Answer 2

decreases, increases

Question 3

equations (x2) for rate of reaction, what do the variables mean?

Answer 3

Δ[P] / Δt or – Δ[R] / Δt

[P] = concentration of product
[R] = concentration of reactant

Question 4

why may a graph with the same reactants & concentrations have different curves?

Answer 4

different mole ratios

Question 5

how to determine the rate at a particular value of time

Answer 5

draw a tangent line to the curve at the specific time point and determine its slope

Question 6

by convention, rate is always expresses as what?

Answer 6

a POSITIVE value

Question 7

two example scenarios of measuring rate of reaction (reaction situation & way of measurement) representing the statement “in most cases, the concentration is not measured directly but is measured by means of a signal that is related to the changing concentration”

Answer 7

1) for reactions that use/produce a coloured solution = measure change in colour of solution

2) for reactions that give off gas = measure change in volume or mass

Question 8

two ways to measuring change in volume of gas produced (steps & limitations if applicable)

Answer 8

a gas syringe to collect produced gas (measure change in volume at regular time intervals)

inverted burette filled with water, produced gas will displace the water (only works for gas with low solubility in water) (needs to be less dense than water so gas would float into burette)

Question 9

can H₂(g) and CO₂(g) be tested for in an inverted burette system for measuring change in volume of gas produced? why/why no?

Answer 9

H₂(g) = will work well because low solubility in water & less dense so gas will float into the burette

CO₂(g) = will not work because it will dissolve in water

Question 10

how does measuring a change in mass help determine amount of gas produced?

Answer 10

reaction mixture placed on a digital scale, cotton wool placed at the top/neck of the flask to allow gas to escape

Question 11

what types of gases will NOT be well measured by observing change in mass?

Answer 11

gases that are too light (e.g. H₂(g))

Question 12

how can change in transmission of light be measured?

Answer 12

a colourimeter OR spectrophotometer works by passing light of a selected wavelength through the solution and measuring the intensity of the light transmitted by the reaction components

Question 13

in measuring change in transmission of light, higher concentration of coloured compound ⟹ what? ⟹ what?

Answer 13

more light is being absorbed ⟹ less light is being transmitted

Question 14

how does a spectrophotometer work?

Answer 14

light of a specific wavelength is selected from an internal source and passed thru the solution places in the cuvette (test tube like thing)

transmitted light passes thru a diffraction grating, then defracted light is collected and changed into a digital signal by the charged-coupled device (CCD) detector

Question 15

how can concentration be measured through titration? how must it be performed?

Answer 15

by titrating a product/reactant against a known standard

samples are taken at regular time intervals then analysed, where quenching is used to ‘pause’ the reaction of the samples

Question 16

why can’t titration be used continuously throughout the reaction to measure change in concentration?

Answer 16

it makes chemical changes to the reaction mixture

Question 17

define quenching

Answer 17

where a substance is introduced which functions to stop the reaction in a sample at the moment it is withdrawn

Question 18

2 ways to measure change in concentration using conductivity. how are they performed?

Answer 18

using a conductivity meter (performed by immersing the inert electrodes into the solution)

using colorimetry (apparatus can be calibrated using solutions of known concentrations so readings can show concentration of present ions)

Question 19

how are ‘clock reactions’ performed

Answer 19

by measuring the time it takes for a reaction to reach a selected & fixed ‘end point’ (the point where the clock is stopped)

Question 20

what is the limitation of ‘clock reactions’?

Answer 20

data can only give an average rate over the time interval

Question 21

how are ‘clock reactions’ different from the 5 other ways to measure changes in reactions?

Answer 21

it uses time as the DV, while other conditions become the IVs

Question 22

what does kinetic molecular theory of matter state? x2

Answer 22

particles in a substance move randomly as a result of kinetic energy

not all particles at any one time have the same values of kinetic energy, but will instead have a range of values (taken average of values is directly related to absolute temp (temp measured in kelvin))

Question 23

if we supply a substance with extra energy by heating it ⟹ what? ⟹ what?

Answer 23

average KE of particles increases ⟹ temperature increases

Question 24

in the maxwell-boltzmann energy distribution curves, what does the area under the curve represent?

Answer 24

the total # of particles in the sample

Question 25

what does the maxwell-boltzmann energy distribution curves represent?

Answer 25

shows that particles in gas at a specific temp have a range in their values of KE

Question 26

what does collision theory state? x2

Answer 26

energy of collisions may result in some bonds being broken & some new bonds forming ⟹ forming products

the rate of reaction depends on # of collisions between particles that will lead to the formation of prouducts

Question 27

what 2 main factors impact the formation of products in collision theory?

Answer 27

energy of collision (how hard theyre bumping into each other)

geometry of collision (bumping into each other at a certain angle)

Question 28

in order for a collision to lead to a reaction, the particles must have a certain minimum KE, this is known as what?

Answer 28

activation energy

Question 29

what is activation energy?

Answer 29

the energy necessary for overcoming the repulsion between molecules and often for breaking some bonds in the reactants before they can react

Question 30

particles with KE < Eₐ will ______________

Answer 30

not have a successful collision (note: they may still collide, but not cause a reaction)

Question 31

define transition state

Answer 31

molecular entity with short lifetime due to high potential energy (unstable)

Question 32

in general, reactions with high Eₐ proceed slower than those with low Eₐ. why?

Answer 32

because there are fewer particles that will have the required energy for a successful collision

Question 33

in maxwell-boltzmann energy distribution curves, is Eₐ to the left higher or lower than one to its right?

Answer 33

lower

Question 34

what does the blue area (area to the right of the Eₐ line) show on a maxwell-boltzmann energy distribution curve

Answer 34

the proportion of particles in the sample that have enough energy to react

Question 35

in some reactions, orientation of the particles determines whether or not a collision will lead to a reaction. what is a good example showing how particles can be forcibly oriented?

Answer 35

dipoles or dipole moments

Question 36

temp increases ⟹ average KE of particles increases ⟹ collision frequency increases and _________________ ⟹ rate of reaction increases

Answer 36

of collisions involving particles with KE > Eₐ increases

Question 37

why does the graph of concentration over time go on a downwards slope as the reaction progresses?

Answer 37

reactants are being used up, so concentration falls

Question 38

impact on pressure of rate of reaction: ____ ⟹ ____, ____ = ____

Answer 38

increased pressure ⟹ compresses gas, more compressed gas ⟹ increased frequency in collisions

Question 39

impact on surface area of rate of reaction: ____ ⟹ ____ ⟹ ____

Answer 39

increased surface area ⟹ allows for more contact between reactants ⟹ increased probability of a reaction

Question 40

example of how SA impacts rate of reaction

Answer 40

a log is less likely to hold a flam than chopped wood

Question 41

what does a catalyst do to a reaction? what does this result in?

Answer 41

provides a different route for reaction with a lower Eₐ

increased # of particles now have values of KE > Eₐ