TL - Rates of Reaction, Rate equations and Reaction Mechanisms Flashcards
Give 5 ways in which the rate of reaction can be followed
1) pH
2) Gas Volume
3) Loss of Mass
4) Colour change
5) Titration
Define reaction rate
The reaction rate is the change in the amount of reactants or products per unit time
If the reactants are in solution, the rate is the change in concentration per second (moldm⁻³s⁻¹)
Describe when you can measure pH to monitor a reaction and how you would do it
If one of the reactants or products is an acid or base, you could follow the reaction by monitoring the pH of the reaction mixture
The easiest way is to use a pH meter or a pH probe connected to a datalogger
Describe 2 ways in which you could monitor a reaction by the gas given off and give an example
1) Collecting it in a gas syringe and record the amount you have at regular time intervals
2) Recording the mass of a solution at regular intervals, if the reaction gives off gas (because it will lose mass)
Example: Reaction between an acid and a carbonate (CO₂ given off)
Describe how you could monitor a reaction by its colour change and give an example
Using a colorimeter measures the strength of a colour of a solution by measuring the light absorbance
Example: - Iodine and Propanone
- Bromine Clock
Describe how you could monitor a reaction by using titration
You can monitor the concentration of a reactant or product in a solution by taking small samples of the reaction mixture at regular time intervals and titrating them
Explain why you need to slow down the rate of reaction of a sample when using titration to monitor a reaction and describe a method to achieve this
So that the sample does react further whilst you are trying to measure the concentration
By adding the sample to a large known volume of distilled water so that the solution becomes very dilute
Give the equation to calculate the rate of a reaction from the loss of mass
Rate = loss of mass / time
Give the equation to calculate the rate of a reaction from the volume of gas produced
Rate = volume of gas produced / time
Give the equation to calculate the rate of a reaction from the colour of a solution
Rate = Change in absorbance / time
Give the equation to calculate the rate of a reaction from the pH of a solution
[H⁺] = 10⁻ᵖᴴ Rate = change in [H⁺] / time
Give the equation to calculate the rate of a reaction from the concentration of a product or reactant
Rate = change in concentration / time
Describe how you would find the rate of reaction at a point for a graph of mass/volume/absorbance/concentration against time
The gradient of the line (or tangent if the graph’s a curve) is proportional to the rate at that point in the reaction
Describe the relevance of the sign before the calculated rate for the change in concentration of a substance
The sign simply shows whether you are measuring the reactants or the products:
- If the sign is negative, you’re measuring the reactant concentration (because reactants decrease over time)
- If the sign is positive, you’re measuring the product concentration (because the products increase over time)
Define orders of reaction and how can they be found?
- Orders are the index to which its concentration term in the rate equation is raised (i.e. they tell you how a reactant’s concentration affects the rate)
- Orders can only be found experimentally
Give the rate equation and units for the reaction
A + B → C + D
Rate = k[A]ᵃ[B]ᵇ
Units: moldm⁻³s⁻¹
What is the overall order for the rate equation:
Rate = k[A]ᵃ[B]ᵇ
and what does k stand for?
- The overall order of reaction = m + n
- k is the rate constant (the bigger it is, the faster the reaction) which is always the same for a certain reaction at a specific temperature
Explain the increase in temperature on the rate of reaction in terms of the rate constant
1) Increasing the temp. means particles have me KE so
- move faster and collide more often
- a greater proportion of them have the activation energy to react
2) This means the rate of reaction increases
3) According to the rate equation, the rate depends on the concentration of reactants and the rate constant. Since the concentrations don’t change, the rate constant must be changing
4) The rate constant is higher at higher temperatures
How would you find the rate constant from the orders and rate of reaction?
1) Write out the rate equation
2) Insert the concentration and the rate. Rearrange the equation and calculate the value of k
3) Find the units for k by putting the other units into the rate equation
Describe the Initial Rates Method to work out the Rate Equations
The initial rate of a reaction is the rate at the very start of the reaction.
1) Repeat the experiment several times using different initial concentrations of reactants, only changing one of the reactant concentrations at a time
2) Calculate the initial rate for each experiment by finding the gradient of the tangent at time t=0 for a graph of concentration vs. time
3) See how the initial concentrations affect the initial rates and find the order for each reactant
Describe the effect on the initial rate of doubling the concentration of a:
i) 0 order reactant
ii) 1st order reactant
iii) 2nd order reactant
i) The rate stays the same
ii) The rate doubles
iii) The rate quadruples
Define half-life (concentration)
The time taken for a reactant to half in quantity
Describe the graphs of concentration vs. time for a:
i) 0 order reactant
ii) 1st order reactant
iii) 2nd order reactant
i) The rate doesn’t change as concentration falls - the graph is a straight line
ii) The graph is curved. The rate decreases as the concentration does, but the half-life (t¹′²) is constant
iii) The graph is curved. The half-life (t¹′²) increases as the reaction goes on
What is the rate-determining step in a multi-step reaction?
The slowest step (AKA the rate-limiting step)
Describe how you find which reactants from the chemical equation are involved in the rate-determining step
If a reactant appears in the rate equation, it must be affecting the rate. So this reactant, or something derived from it, must be in the rate-determining step
State whether the following statements are true or false:
i) The rate-determining step is the first step in the mechanism
ii) The reaction mechanism can’t usually be predicted from just the chemical equation
i) False, it is not always the first step in the mechanism
ii) True
How would you find the number of molecules of a reactant which are involved in the rate-determining step?
The order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in the rate-determining step
What can you predict from the rate equation?
The mechanism of the rate-determining step
Give an example of when the rate equation has predicted a different mechanism to what was first suggested theoretically
The decomposition of Nitrogen (V) Oxide (N₂O₅)
It was thought that it decomposed as follows:
2N₂O₅ → 4NO₂ + O₂
But the reaction was experimentally found to be 1st order, so the rate equation is:
Rate = k[N₂O₅]
Therefore, there is only 1 molecule of N₂O₅ in the rate-determining step, so a possible mechanism is:
N₂O₅ → NO₂ + NO₃
NO₃ + N₂O₅ → 3NO₂ + O₂
Describe how a an enzyme-catalysed reaction act differently to a chemical catalyst
1) In a very simple 1st order reaction, a substrate, S, might become a molecule of product P:
S → P
2) As the concentration of S increases, the reaction speeds up. Because it’s a 1st order reaction, doubling [S] should double the rate of reaction
3) However, when the above reaction is catalysed by an enzyme, the first doubling of [S] doubles the rate of reaction. Eventually though, the reaction reaches a stage where it can’t go any faster - no matter how you increase [S]
Describe the graphs for an un-catalysed and an enzyme-catalysed reaction, and state the order
Enzyme-catalysed reactions change order as substrate is added
1) The un-catalysed reaction is first order so the graph is just a straight line
2) The enzyme-catalysed reaction is first order when the substrate concentration is low, but it changes as more substrate is added
3) Towards the end, the line of the graph levels out until it’s horizontal. At this point, the rate is no longer affected by the concentration of S. The reaction has become 0 order
Explain why an enzyme-catalysed reaction changes order at different substrate concentrations
The order changes because the rate-determining step changes
Enzyme-catalysed reactions are split into 3 steps:
Step 1) S + E → ES
Step 2) ES → EP
Step 3) EP → E + P
1) When [S] is low, step 1 is the slowest reaction because the chance of collision is low
2) For a while, step 1 speeds up as you increase [S]
3) Eventually, you reach a stage where all the enzyme molecules have formed ES complexes (enzyme is saturated) So adding more S can’t make the reaction go any faster since there are no enzymes to attach to
4) The rate of reaction now depends on how fast ES can convert to EP so step 2 becomes the rate-determining step. The reaction is now 0 order with respect to S
