reactivity 2.2 Flashcards
rate of reaction unit
mol dm⁻³ s⁻¹
as the reaction proceeds, the concentration of reactants ______ and the concentration of products ______
decreases, increases
equations (x2) for rate of reaction, what do the variables mean?
Δ[P] / Δt or – Δ[R] / Δt
[P] = concentration of product
[R] = concentration of reactant
why may a graph with the same reactants & concentrations have different curves?
different mole ratios
how to determine the rate at a particular value of time
draw a tangent line to the curve at the specific time point and determine its slope
by convention, rate is always expresses as what?
a POSITIVE value
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”
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
two ways to measuring change in volume of gas produced (steps & limitations if applicable)
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)
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?
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
how does measuring a change in mass help determine amount of gas produced?
reaction mixture placed on a digital scale, cotton wool placed at the top/neck of the flask to allow gas to escape
what types of gases will NOT be well measured by observing change in mass?
gases that are too light (e.g. H₂(g))
how can change in transmission of light be measured?
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
in measuring change in transmission of light, higher concentration of coloured compound ⟹ what? ⟹ what?
more light is being absorbed ⟹ less light is being transmitted
how does a spectrophotometer work?
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
how can concentration be measured through titration? how must it be performed?
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
why can’t titration be used continuously throughout the reaction to measure change in concentration?
it makes chemical changes to the reaction mixture
define quenching
where a substance is introduced which functions to stop the reaction in a sample at the moment it is withdrawn
2 ways to measure change in concentration using conductivity. how are they performed?
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)
how are ‘clock reactions’ performed
by measuring the time it takes for a reaction to reach a selected & fixed ‘end point’ (the point where the clock is stopped)
what is the limitation of ‘clock reactions’?
data can only give an average rate over the time interval
how are ‘clock reactions’ different from the 5 other ways to measure changes in reactions?
it uses time as the DV, while other conditions become the IVs
what does kinetic molecular theory of matter state? x2
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))
if we supply a substance with extra energy by heating it ⟹ what? ⟹ what?
average KE of particles increases ⟹ temperature increases
in the maxwell-boltzmann energy distribution curves, what does the area under the curve represent?
the total # of particles in the sample
what does the maxwell-boltzmann energy distribution curves represent?
shows that particles in gas at a specific temp have a range in their values of KE
what does collision theory state? x2
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
what 2 main factors impact the formation of products in collision theory?
energy of collision (how hard theyre bumping into each other)
geometry of collision (bumping into each other at a certain angle)
in order for a collision to lead to a reaction, the particles must have a certain minimum KE, this is known as what?
activation energy
what is activation energy?
the energy necessary for overcoming the repulsion between molecules and often for breaking some bonds in the reactants before they can react
particles with KE < Eₐ will ______________
not have a successful collision (note: they may still collide, but not cause a reaction)
define transition state
molecular entity with short lifetime due to high potential energy (unstable)
in general, reactions with high Eₐ proceed slower than those with low Eₐ. why?
because there are fewer particles that will have the required energy for a successful collision
in maxwell-boltzmann energy distribution curves, is Eₐ to the left higher or lower than one to its right?
lower
what does the blue area (area to the right of the Eₐ line) show on a maxwell-boltzmann energy distribution curve
the proportion of particles in the sample that have enough energy to react
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?
dipoles or dipole moments
temp increases ⟹ average KE of particles increases ⟹ collision frequency increases and _________________ ⟹ rate of reaction increases
of collisions involving particles with KE > Eₐ increases
why does the graph of concentration over time go on a downwards slope as the reaction progresses?
reactants are being used up, so concentration falls
impact on pressure of rate of reaction: ____ ⟹ ____, ____ = ____
increased pressure ⟹ compresses gas, more compressed gas ⟹ increased frequency in collisions
impact on surface area of rate of reaction: ____ ⟹ ____ ⟹ ____
increased surface area ⟹ allows for more contact between reactants ⟹ increased probability of a reaction
example of how SA impacts rate of reaction
a log is less likely to hold a flam than chopped wood
what does a catalyst do to a reaction? what does this result in?
provides a different route for reaction with a lower Eₐ
increased # of particles now have values of KE > Eₐ
