R2.2 How fast? The rate of chemical change

How can rate of reaction be measured?

Mass

Gas production

Colorimetry

Conductivity

What is rate of reaction?

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

What is the unit for rate of reaction?

moldm^-3

How can instantaneous reaction rate be determined graphically at a specific moment in time?

Gradient of tangent of curve plotted on concentration/volume/mass vs time graph

=change in y/change in x

Steeper gradient=faster rate of reaction

How to calculate average rate of range graphically?

Gradient of secant line (touches curve at 2 points that represent the beginning and end of time period)

Note: this is NOT the same as instantaneous rate (gradient of tangent)

How does a colorimeter measure concentration

More concentrated☕→darker→blocks light→lower light transmission

Less concentrated🍵→clearer→allows more light transmission

What is a limitation of measuring rate of change with mass?

Gas must be dense enough or mass won’t be measurable

How are reactions stopped before performing titrations to find conc?

Quenching ‘freezes’ the reaction at a specific point in time

How is rate of reaction measured using conductivity?

Ions may be used up or formed during a reaction, changing conductivity

Conductivity probe used at regular time intervals

Rate of change using colorimeter graph

?

Rate of changing using change in mass

Rate of changing using gas volume

Which 2 features do particle collisions need for a successful reaction?

1. Energy of collision must exceed minimum Ea required

2. Collision must have the correct geometry

What r the factors affecting rate of reaction

1. Concentration

2. Pressure(gas only)

3. Temperature

4. Surface area

5. Catalysts

True or false: increasing pressure increases reaction rate of a solid

False. It only increases rate for gases which are compressible.

What is activation energy?

Minimum amount of energy for a reaction to occur

How to calculate activation energy of a reverse reaction?

Exo: delta H + Ea (fwd)

Endo: Ea (fwd) - delta H

What is the activated complex? (Transition state)

High-energy, unstable arrangement of atoms that exists momentarily as reactants turn into products. It represents the ‘peak’ of the energy barrier in a reaction.

It’s the state of molecules at peak energy (activation energy is the energy it takes to reach this peak)

What is the maxwell-boltzmann energy distribution curve and what are this axes?

It provides a visual representation of the distribution of kinetic energies among particles in a system.

X-axis: kinetic energy of particles

Y-axis: fraction of particles with a given kinetic energy

Area under curve= total number of particles in the system

What happens to maxwell-boltzmann curve when temperature is increased?

It flattens and spreads out, increasing proportion of particles with energy higher than Ea. End of curve will be above that of initial curve.

True or false: the peak of maxwell-boltzmann represents Ea

False. It represents the most probable energy

Temperature effect of maxwell-boltzmann curve

True or false: a catalyst alters equilibrium

False. It accelerates both fwd and bwd reactions

What are 2 types of catalysts?

Homogenous: same phase as reactants (eg: both liquid)

Heterogeneous: diff phase from reactants (eg: in born haber process, solid iron catalyst used betw rxn of gaseous nitrogen and hydrogen to produce ammonia)

What are enzymes?

Specialised proteins that function as biological catalysts, enabling essential biochemical reactions to occur efficiently at body temp.

What happens to maxwell-boltzmann curve when a catalyst is added?

Curve shape stays the same

Ea line moves to the left, allowing higher proportion of particles with energy higher than Ea

What is an elementary step?

A single molecular event in a reaction mechanism where reactants are converted into products or intermediates.

What is an intermediate?

A species produced in an elementary step and consumed in another. It doesn’t appear in the overall equation

What is molecularity of an elementary step?

The number of reacting particles that must collide simultaneously to drive a chemical change.

What is molecularity of an elementary step?

The number of reacting particles that must collide simultaneously to drive a chemical change.

What is the rate-determining step?

The slowest step in the mechanism

Why does the rate determine step matter?

It has the highest Ea=highest energy to overcome for reaction to take place

What is a transition state?

The highest-energy arrangement of atoms during an elementary step.

It’s the point where bonds are partially broken and formed, and the system is at the peak of its energy profile.

In energy profile diagram, it’s the ‘peak’ and intermediate is the ‘valley’

What are the proposed reaction mechanisms that determine whether it’s plausible or not?

1. The sum of all elementary steps must match the stoichiometry of overall reaction

2. The rate law derived from the mechanism must match the experimentally determined rate law

What’s the difference between a catalyst and intermediate?

Catalyst: consumed then re produced

Intermediate: produced then consumed

How to identify the catalyst in the elementary steps of a mechanism?

They appear in both the reactant and product sides (it’s consumed and re produced)

They do not appear in the overall equation, like intermediates

What is the equation of rate of reaction

Rate of reaction=k [A]^m [B]^n

m & n=orders of the reaction(power 0→2)

A,B=concentrations of reactants

What is the order of reaction?

It shows how the rate of rxn depends on the concentration of a reactant

Power to which concentration is raised in rate equation(+,- or fraction)

What is zero order

Order of rxn=0

Changing concentration has no effect on rate

Reactant doesn’t appear in rate equation

What is zero order

Order of rxn=0

Changing concentration has no effect on rate

Reactant doesn’t appear in rate equation

What is first order

Order of rxn=1

Rate is directly proportional to concentration (eg: doubling conc doubles rate)

No power shown on reactant in rate equation

What is second order

Order of rxn=2

Rate is proportional to square of concentration (eg: doubling conc quadruples rate)

Reactant is squared in the rate equation

What is the overall order

The sum of powers of all reactants in rate equation(m+n)

How is rate constant, k related to reaction speed

Directly proportional

High k→high speed reaction and viceversa

True or false: volume can be used to measure the rate of every reaction

False, it’s only relevant in reactions where gases r produced

How is temperature and average kinetic energy of particles related to each other

Directly proportional

True or false: reactants are more stable than products in an exothermic reaction

False

Products have lower energy therefore more stable

When do u get to use moles alone when finding Kc

When there is equal number of moles in products and reactants (eg:N2+O2=2NO)