Topic 16: Kinetics II Flashcards
What is meant by zero order
The concentration of the reactant does not affect the rate of reaction
What is meant by first order
If the concentration doubles, the rate of reaction will double rate and concentration are directly proportional to one another
What is meant by second order
If the concentration of the reactant doubles, the rate of reaction will be four times faster (2)2
What does a zero order rate vs concentration of A graph look like and explain
- Horizontal line
The rate does not depend on the concentration of A in a zero order reaction. If the concentration of A stays the same , rate stays the same.
What does a first order rate vs concentration of A graph look like and explain
- Straight line through the origin
The rate is directly proportional to the concentration of A. If the concentration of A doubles the rate doubles.
What does a first +/second order rate vs concentration of A graph look like and explain
- Curved line upwards
The rate is proportional to the square of the concentration change if the concentration of A triples the rate of reaction will be 9 times faster
What would we see in a second order rate vs concentration graph and what would we plot and what would it look like
- Straight line through the origin
we plot rate vs concentration squared
What is meant by the half-life of a reaction and state the equation
The half-life of a reaction t1/2 is the time taken for the concentration of the reactant to reach half its initial value
t0.5= Ln(2) /k
What is special about the half-life for a first order reaction
The half life is constant
What is special about the half life of second order
If half lives rapidly increase then the order is second order
What is a heterogeneous catalyst
A heterogeneous catalyst is one that is in a different phase ( state of matter) from the reactants
The reaction happens on the surface of the heterogeneous catalyst so increasing the surface area of the catalyst increases the number of molecules that can react at the same time
What is a homogeneous catalyst
A homogeneous catalyst are in the same physical state as reactants
During homogeneous catalysis the reactants combine with the catalyst to make an intermediate species which then reacts to form the products and reform the catalyst
Define rate of reaction
The change in concentration,volume, or moles of a substance in a unit time
What is the initial rate of reaction
The rate at the start of the reaction where it is the fastest
What is special about the rate constant, K
It is constant at a fixed temperature and is not affected by concentration or time
What affects the rate constant, K
If we increase the temperature we increase the rate constant
Rearrange the Arrhenius equation in log form and put it into y=mx + c form
Ink= InA - Ea/RT
y= mx + c form
Lnk= -Ea/R x 1/T + LnA
y= LnK m= -Ea/R x= 1/T c= LnA
What is the relationship between the rate constant and the rate of reaction
The rate constant is proportional to the rate of reaction
What is meant by the rate-determining step
The slowest step in a multi-step reaction
How to know which mechanism to choose when asked which is the rate determining step
The rate equation shows you what is included in the slow step, the mechanism must include what is mentioned in the rate equation. If a step has been said to be slow and is not included in the rate equation we need to include the reactants that produced it. The number of moles of reactants in the slow step will be the same as the order of the reaction of each substance.
eg= 1 mole of A in the slow step will mean A is first order
What type of halogenoalkanes tend to react via the SN2 mechanism
Primary halogenoalkanes
What type of halogenoalkanes tend to react via the SN1 mechanism
Tertiary halogenoalkanes
Why do tertiary halogenoalkanes undergo the SN1 mechanism
The tertiary carbocation is made stabilised by the electron releasing methyl groups around it and the bulky methyl groups prevent the OH- ion from attacking the halogenoalkane
Why do primary halogenoalkanes do not go through SN1 mechanism
Primary halogenoalkanes do not do the SN1 mechanism as they would only form an unstable primary carbocation
What is a clock reaction
An example of the initial rate method where the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants
Describe the steps you must take to carry out the iodine clock reaction to find the order with respect to potassium iodide
A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution. Starch is used as an indicator and turns blue-black in the presence of iodine
The sodium thiosulphate that is added to the reaction mixture reacts instantaneously with any iodine that forms
The iodine that forms in the first reaction is used up straight away in the second reaction, once all the sodium thiosulphate is used up any more iodine that is formed will stay in solution turning starch blue black - this is the end of the clock reaction
Varying the iodide concentration of hydrogen peroxide only gives different times of the color change
The time it takes for the reaction to occur and the concentration of the reactants helps us calculate the order with respect to iodine or hydrogen peroxide.
What are the assumptions made when carrying out a clock reaction
The concentration of each reactant does not change significantly over the time period of the clock reaction
The temperature stays constant
When the end point is seen the reaction has not proceeded too far
