Chapter 9 - Reaction kinetics (L)
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What is the equation for rate of reaction?
Rate of reaction = change in concentration (of product or reactant) / time for change to happen
What do we need to find out to work out rate of reaction?
Either: • how fast one of the reactants is being used up or • how fast one of the products is being formed
On a concentration-time graph, how do we calculate rate of reaction?
By using a tangent (y/x) to find the gradient of the line. This is the rate of reaction
If there was a table containing results on the time taken to collect 20cm3 of gas, how would you calculate the rate of reaction?
Volume of gas collected / time taken = rate of reaction
What are the two requirement for a reaction to occur?
• The molecules must collide in the correct orientation - The molecules must collide with the necessary activation energy
What is it called when the shapes of molecules influence reactions?
Steric factor
What is steric hinderance?
When the atoms/groups of atoms in a molecule hinder the course of a reaction
When does steric hinderance mostly occur?
When the atoms or groups of atoms are particularly large, so they can get in the way of/block an attacking species from reacting
What is the purpose of cotton wool blocking the top of flasks in changing mass tests?
So only gas can escape
How do you calculate gradient?
Change in y/change in x
What is a heterogeneous reaction?
A reaction in which the reactants are in different physical states (e.g. solid + solution)
What are the 4 factors that increase rate of reaction?
• Higher temperature • Higher pressure/concentration of reactants • Higher surface area • Presence of a catalyst
What are four important points about the Maxwell-Boltzmann curves?
• Neither curve is symmetrical • Both curves start at the origin and finish by approaching the x-axis asymptotically • The area under both curves is the same, as the number of molecules has not changed • The peak of T2 is to the right and lower than the peak of T1
What are Maxwell-Bolzmann curves?
Curves that show the distribution of energy of particles in a reaction
What is T2 compared to T1?
T2 is at a higher temperature, which means a greater distribution of particles have a higher energy, compared to T1; this will result in more particles meeting the activation energy
What is the the effect of a catalyst on a Maxwell-Bolzmann curve?
The activation energy will be lowered/moved further to the left
In gas phase, at standard conditions, how many collisions will a single particle have?
Between 10^9 and 10^10
What is the effect of increasing temperature on an endothermic reaction?
Rate of reaction increases
What is the effect of increasing temperature on an exothermic reaction?
Rate of reaction increases, unless it is a reversible reaction, in which it will benefit the backwards endothermic reaction, reducing the amount of product present at the end
How do catalysts work?
They provide an alternative route for a reaction, which has a lower activation energy
What is a catalyst?
A substance that increases the rate of a chemical reaction, but is chemically unchanged at the end of the reaction
What is a heterogeneous catalyst?
A catalyst that is a different state to that of the reactants
What are the 3 stages of how heterogenous catalysts work?
Absorption Reaction Desorption
What is the absorption stage?
Where molecules of one or both reactants form bonds with the catalyst; this weakens the bonds within the molecules