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)