Chapter 18 Flashcards
How are reaction rates measured?
Observing the changes in the quantities of reactants/products over time
rate =
quantity reacted or produced/time
change in concentration/change in time
What units is concentration measured in?
mol dm-3
What do square brackets represent?
Concentration
What does [A] mean?
Concentration of A
What units does [A] have?
mol dm⁻³
What does changing concentration change?
Rate of a reaction
What is rate of reaction proportional to?
The concentration of a particular reactant raised to a power
rate ∝
[A]ⁿ
What is n in rate ∝ [A]ⁿ?
Order of reaction
What are some of the common orders?
1) zero order (0)
2) first order (1)
3) second order (2)
What is zero order?
1) concentration of a reactant has no effect on the rate
2) rate ∝ [A]⁰
What is first order?
1) rate depends on concentration raised to the power of 1
2) rate ∝ [A]¹
What is second order?
1) rate depends on concentration raised to the power of 2
2) rate ∝ [A]²
What is the rate equation?
The mathematical relationship between the concentrations of the reactants and the reaction rate.
What is the rate constant, k?
1) proportionality constant
2) converts between the rate of reaction and concentration and orders
overall order =
sum of orders with respect to each reactant
What is the overall order for the rate equation rate = k[A]ᵐ[B]ⁿ
m+n
What does ∝ mean?
proportional to
Simplify the rate equation rate = k[A]⁰[B]¹[C]²
rate = k[B][C]²
What do the units of the rate constant depend on?
The number of concentration terms in the rate equation
How can you determine the units of the rate constant, k?
1) rearrange equation to make k the subject
2) substitute units into the expression for k
3) cancel common units and show the final units on a single line
How are orders of reaction determined?
Experimentally by monitoring how a physical quantity changes over time
What is continuous monitoring?
Plotting graphs from continuous measurements taken during the course of a reaction
Give some methods of continuous monitoring that produce a gas as one of the products?
1) monitoring by gas collection
2) monitoring by mass loss
What can be used to estimate a colour change?
A colorimeter
What is the gradient of a concentration-time graph?
Rate of reaction
What is the shape of a zero order concentration-time graph?
1) straight line
2) negative gradient
Does the reaction rate of a zero order reaction ever change?
No
What is the shape of a first order concentration-time graph?
1) downward curve
2) decreasing gradient
What is constant in a first order reaction?
1) half-life
3) the time for the concentration to halve
What is the shape of a second order concentration-time graph?
1) downward curve
2) decreasing gradient
What is half life?
The time taken for half of a reactant to be used up
What is exponential decay?
The concentration of a first order reaction halving every half life
How can you determine k for a first order reaction?
1) from the rate
2) from the half-life
What is the formula to calculate the rate constant from the half-life?
k = ln2 / t₁/₂
Why is it more accurate to use the formula for calculating the rate constant than drawing a tangent?
It is always difficult to judge how steep the tangent should be drawn.
What is the shape of a zero order rate-concentration graph?
1) horizontal straight-line
2) zero gradient
What is the rate equation for a zero order reaction?
rate = k[A]⁰ rate = k
What is the shape of a first order rate-concentration graph?
1) straight line graph through the origin
2) positive gradient
What is the rate equation for a first order reaction?
rate = k[A]¹ rate = k[A]
What is the shape of a second order rate-concentration graph?
1) upward curve
2) increasing gradient
What is the rate equation for a second order reaction?
rate = k[A]²
Why can you not directly obtain the rate constant of a second order rate-concentration graph?
The rate-concentration graph is a curve
How do you calculate the rate constant of a second order reaction?
By calculating the gradient of a graph of the rate against the concentration squared
What is the initial rate?
The instantaneous rate at the start of a reaction when the time t = 0
How can initial rate be found?
By measuring the gradient of a tangent drawn at t = 0 on a concentration-time graph
What is a clock reaction?
1) takes a single measurement
2) time t from the star of an experiment is measured for a visual change to be observed, often a colour or precipitate
What is the initial rate of a clock reaction?
initial rate ∝ 1/t
How is a clock reaction repeated?
With different concentrations
What colour is aqueous iodine?
Orange-brown
What happens when starch is added to iodine
It forms a complex with iodine which is an intense dark blue-black colour
Describe how the order of a reaction can be determined from the shape of a rate-concentration graph.
1) 0 order is a horizontal line
2) 1st order is a straight line. starting at the origin
3) 2nd order is an upward curve, starting at the origin
What does a colorimeter measure?
1) the intensity of light passing through a sample
2) absorbance (directly linked to the concentration of the solution)
How is the filter for a colorimeter chosen?
So that it is the complimentary colour to the colour being absorbed in the reaction
When can a reaction only take place?
When particles collide
What is a reaction mechanism?
The series of steps that make up an overall reaction
What is the rate-determining step?
The slowest step in the sequence
What does the rate equation only include?
Reacting species involved in the rate determining step
What do the orders in the rate equation match?
The number of species involved in the rate-determining step
What are haloalkanes hydrolysed by?
Hot aqueous alkali
What happens as temperature increases?
1) rate increases
2) value of the rate constant k increases
Increasing temperature shifts the Boltzmann distribution to what side?
Right, increasing the proportion of particles that exceed the activation energy Eₐ
What is the Arrhenius equation?
1) an exponential relationship between the rate constant k and temperature T
2) k - Ae⁻ᴱᵃ/ᴿᵀ
What is A in the Arrhenius equation?
Pre-exponential factor (frequency factor)
What is -Eₐ/RT in the Arrhenius equation?
Exponential factor (linked to activation energy and temperature)
What is R in the Arrhenius equation?
Gas constant - 8.314Jmol⁻¹K⁻¹
What is T in the Arrhenius equation?
Temperature in Kelvin
