Section 1: Reaction Rates Flashcards
What are reaction rates?
The reaction rate is the change in the amount of reactants or products per unit time.
What is continuous monitoring?
Measure how the amount of a product or reactant changes over the complete course of the reaction
How could you measure the formation of a gas?
If a gas is given off, you could collect it in a gas syringe and record how much you’ve got at regular time intervals
How do you measure the mass of the reaction vessel at regular intervals as the gas is lost.
Using a mass balance
How do you measure colour change?
Set the colorimeter to measure the wavelength of light that you’re interested in measuring
Calibrate the colorimeter. Place a sample of distilled water in a sample tube, know as cuvette, and place it on the colorimeter. Set the absorbance to 0.
Carry out your reaction. You should take samples of the reaction mixture at regular intervals, and measure the absorbance of each one using a colorimeter.
How do you measure a change in ph?
If either hydrogen or hydroxide ions are produced or used up in a reaction, then you can monitor the progress of the reaction by measuring the change in the ph
How do you process data of an experiment?
Concentration-time graph
How can you work out the gradient of a concentration time graph?
Gradient = change in y / change in x
If the graph is a curve how do you find the gradient?
Draw a tangent
How do you find the initial rate of the reaction through calculations?
Work out how many moles of product there are in the set amount you measure in each reaction.
Use the reaction equation to work out how many miles of the reactant you’re interested in will have been used up when the set amount of product has formed.
Divide this by the volume of the reaction mixture to calculate the change in concentration of your reactant
Divide the change in concentration of the reactant by the time taken for the set amount of product to form. This is the initial rate of your reaction.
How can you calculate initial rates from a graph?
Draw a tangent to curve at time zero, initial rate = change in y / change in x
What is a clock reaction?
The formation of a set amount of product is shown by a sudden visual change in the reaction mixture.
Add an unknown amount of substance that reacts with the product you’re interested in to the reaction vessel. The substance reacts immediately with the product as it forms. As soon as the substance is used up, the presence of unreacted product in the reaction vessel causes a sudden change, such as colour change. This is the endpoint of the reaction. The quicker the endpoint is reached, the faster the initial rate of the reaction.
What are reaction orders?
The reaction order with respect to a particular reactant tells you how the reactant concentration affects the rate.
What happens if you double the reactants concentration and the rate stays the same?
The reaction is zero order
What is the reaction order if you double the reactants concentration and the rate also doubles?
The reaction is first order
What is the reaction order if you double the reactants concentration and the rate quadruples?
The reaction is second order
What is the overall order of a reaction?
The sum of all the different reactants. E.g if there are two reactants in a reaction, and one has an order of 1 and the other has an order of 2, then the overall order is 3
What is the rate equation for A + B –> C + D?
Rate = k[A]m[B]n
In the equation rate = k[A]m[B]n what do the all stand for?
K= rate constant
[A]= concentration of A
[B]= concentration of B
M and n = orders of reaction
What is the rearrangement of the rate equation to find the rate constant?
K = rate / [A]m[B]n
How to calculate the rate constant of a first order reaction?
The rate constant is equal to the gradient of the rate concentration graph of that reactant
So.. gradient = change in y / change in x
How do you calculate rate constant from half life?
K = ln2 / t1/2
T1/2 = half life
What is the half life of a reaction?
The half life of a reaction is the time it takes for half of the reactant to be used up
What is the rate determining step?
The step with the slowest rate
How do you know what is involved in the rate determining step?
If it’s involved in the rate equation, it must affect the rate. So this reactant, or something derived from it, must be in the rate determining step
How can you predict rate equations from the rate determining step?
Find the slowest step of the reaction
Use the reactants as [A] [B]
M and n are how many molecules of the reactants are in the equation
What are the rules for suggesting a mechanism from the rate determining step / an equation and rate equation?
The rate determining step of the reaction must fit with the rate equation
All of the equations in the mechanism must balance
The different steps of the reaction must add up to the overall equation
What does k = Ae -Ea/Rt stand for in the Arrhenius equation?
K = rate constant Ea = activation energy (j mol-1) T = temperature (K) R = gas constant (8.31 J K-1 mol-1) A= the pre exponential factor (another constant) the units are the same as the rate constant
What happens to the rate constant in the Arrhenius equation when activation energy gets bigger?
K gets smaller
What does a large activation energy mean for the Arrhenius equation?
Slow rate. If a reaction has a high activation energy, then not many of the reactant particles will have enough energy to react. So only a few collisions will result in the reaction actually happening, and the rate will be slow.
What happens to the rate constant in the Arrhenius equation if temperature increases?
K increases - high temp mean reactant particles move around faster and with more energy, so they’re more likely to collide and more likely to collide with at least the activation energy, so reaction rate increases
What is the rearranged Arrhenius equation?
Ln K = -Ea/RT + lnA
How do you plot a graph using the Arrhenius equation?
Plot link against 1/T
