1-Rates of Chemical Change Flashcards
What are the factors that affect the rate of chemical reaction?
Concentration, pressure, temperature, surface area, catalysts
How do you work out the rate of reaction?
Rate of reaction=change in conc/change in time
How can you measure the rate of reaction when a gas is given off?
Measuring the volume of gas produced
Measuring the change in mass of a mixture
How do you measure the rate of reaction by measuring the volume of gas produced?
Measure the volume of gas produced at regular intervals, collecting gas in a measuring cylinder or gas syringe by water displacement
Plot concentration by time
How do you measure the change in mass of a mixture?
Measure the decrease in mass of total reacting mixture as the reaction proceeds using a balance
Many opportunities for experimental error such as splashes
What is used to measure colour change?
Colorimeter
How does a colorimeter work?
Appearance of a coloured product, or loss of coloured reactant, is used
Why is a colorimeter used?
Allows more subtle changes to be detected
How is the rate of chemical reaction measured in titrimetric analysis?
Aliquots of the reaction mixture is removed and another reagent added to quench the reaction or slowed down by putting it in an ice bath
Aliquot are then titrated
What affects a solutions conductivity?
Number and type of ions in a solution
Larger ions are slower moving
How does conductimetric analysis work?
When a reactant takes place in a solution, the ionic balance changes and the resulting changes in conductivity, showing the changes in concentration
What is a general rate equation? What does m stand for?
Rate= k [A]m [B]n
The order of reaction with respect to A
What does [ ] mean in the rate equation?
Concentration in moldm-3
What does k stand for in the rate equation?
Rate constant
How do you work out the overall order?
The sum of the indices
m+n
What is the rate determining step?
The slowest reaction when a reaction involves several steps
<p>What is the overall order of a reaction related to?</p>
<p>The molecularity of the rate determining step (number of particles involved)</p>
<p>How is the rate measured in the iodine clock reaction?</p>
<p>The time taken for the two colourless solutions to turn blue-black is recorded. The rate of reaction is proportional to 1/t</p>
<p>Why are rates of reactions studied?</p>
<p>Understand what is going on
Understand how to change the rate
Provide evidence of mechanisms</p>
<p>How do you work out the order of a reaction with respect to a product or reactant?</p>
<p>Found out by experiment by measuring the changes in concentration Plotting concentration (y) against time (x)</p>
<p>Why does the rate of reaction decrease over time?</p>
<p>The number of particles of the reactants decrease therefore there are fewer collisions</p>
<p>How do you find the rate of reaction at a particular time?</p>
<p>A tangent is drawn and the gradient calculated</p>
<p>How do you calculate the gradient?</p>
<p>Gradient= y/x</p>
<p>Why is the value of the rate of reaction negative?</p>
<p>The concentration is decreasing as time goes on</p>
<p>How can the order be determined after experiment?</p>
<p>The rate is plotted against the concentration</p>
<p>What does the concentration against time and rate against concentration graphs look like for a reaction of zero order?</p>
<p>Conc vs time- straight negative line
| Rate vs conc- horizontal line</p>
<p>What does the concentration against time and rate against concentration graphs look like for a reaction of first order?</p>
<p>I</p>
<p>What does the concentration against time and rate against concentration graphs look like for a reaction of second order order?</p>
<p>I</p>
<p>What is the rate determining step? What do you use it in?</p>
<p>The slowest step in a reaction
| The rate equation</p>
<p>How is the rate constant worked out?</p>
<p>Substituting certain values into the rate equation</p>
How can you determine the order of a reaction?
Half-lives
Comparing graphs
Comparing results
<p>What happens when a reaction is second order?</p>
<p>Doubling the concentration of a reactant quadruples the rate of reaction</p>
<p>What happens when a reaction is second order?</p>
<p>Doubling the concentration of a reactant doubles the rate of reaction</p>
<p>How do you work out the values to plot initial rate against concentration?</p>
<p>Series of experiments using different initial concentrations of the reagent, with every other factor unchanged
For each experiment, a conc vs time graph is plotted and a tangent drawn at t=0</p>
<p>What is a half-life?</p>
<p>The time taken for half a reactant to be used up during a reaction process</p>
<p>What can half-lives determine?</p>
<p>The order of the reaction</p>
<p>What are the half-lives of a first order reaction?</p>
<p>Constant
| Independent of the concentration</p>
How are half-lives worked out?
Using a graph
Initial concentration halved and find corresponding time
What can half-lives be used for?
<p>Work out the age of something, for example with radioactive decay or carbon dating</p>
What does the half-life of a second order reaction depend on?
Initial concentrations of reactants
What is the pattern of half-lives in a second order reaction?
Having the concentration doubles the half-life
Inversely proportional to the initial concentration of reactants
Why don’t some reactions occur at room temperature?
Many particles don’t collide with sufficient energy to overcome the activation energy barrier
<p>What does the profile for a catalysed reaction look like in comparison to an uncatalysed reaction?</p>
l
What is the definition of a catalyst?
A substance that alters the speed of reaction without being used up in the process
(often used up in secondary reactions)
What is a homogenous catalyst? Give an example
In the same phase as the reactants
Chlorine free-radicals in UV light
What is a heterogenous catalyst? Give an example
In a different phase from the reactants
In the Haber process solid iron catalyses gases
Why do catalysts have a large surface area?
Reacting mixtures are normally absorbed onto the surface so it allows effects to be maximised
Describe how a heterogenous catalyst works
Reactant molecules are absorbed at active sites, wearing the bonds within the molecules
Bonds between reacting atoms form, producing product molecules
Product molecules leave the surface area in desorption
What does the area under the curve on a distribution graph show?
Total number of particles
Shaded part shows number of particles with sufficient energy
What is the rate-determining step?
The slowest step in a multistep reaction
How are theoretical mechanisms supported?
The predicted rate law from the mechanism much match the rate law achieved experimentally
What is a SN1 reaction?
Substitution
Nucleophilic addition
First order
Explain the mechanism of the reaction when 2-iodo-2-methylbutane is refluxed with aqueous potassium hydroxide
The electrons in the C-I bond move toward the iodine atom (slow step), forming a carbocation
The hydroxide ion forms a bond with the carbocation
What is a SN2 reaction?
Substitution
Nucleophilic addition
Second order
Rate equation involves both the reagents
Explain the mechanism for the reaction between bromomethane and aqueous potassium hydroxide
Hydroxide ion joins the central carbon atom as the bromine is leaving
Part of the energy required to break the C-Br bond is supplied by the formation of C-OH
What sort of reaction occurs between iodine and propanone?
Substitution
How does a colorimeter measure the reaction between iodine and propanone?
Calibrated using standard iodine solutions
More lift is transmitted as the reaction mixture changes from light brown to a paler colour
How can a titration be used to determine the mechanism between iodine and propanone?
Reaction mixture quenched with sodium hydrogen carbonate then titrated against sodium thiosulphate
Results can be used to find out that it is first order with respect to propanone and hydrogen and zero order with respect to iodine
What can be determined from the titration between iodine and propanone to find out the mechanism?
Iodine is not in the rate determining step
Hydrogen ions act as a catalyst as they are regenerated during the reaction
Describe the mechanism for the reaction between iodine and propanone
1- H+ ion protonates the oxygen atom in propanone
2- Electrons in C=0 partly shift to form a carbocation
3- Slowly, the strong C-H bond is broken, the another carbon atoms stabilises it by releasing a proton to from H3O+
4- Iodine molecule acts as an electrophile and undergoes electrophilic addition reaction
5- Water molecule then removes proton in acid-base reaction
