How do you calculate the average rate of a chemical reaction
Change in conc / time taken
If you plot a graph of concentration over time what does the gradient represent
The rate
For a concentration time graph how can you tell if the change in concentration is the reactant or product
If the overall conc increases it is a product
if the reactant is limiting it will eventually run out so line will touch the x axis
How do you calculate the actual rate at any given time on a graph
Draw a tangent at the point and find the gradient
How do you find the average rate over a period of time on the graph
Draw a straight line between two points on the graph and work out the gradient
The variables that affect the rate of reaction are
Surface area of solids
Concentration of solutions
Pressure of gases
Temperature of the reaction mixture
Presence of catalyst
What is collision theory
Particles must collide with enough energy in the correct orientation
What will increasing the SA do to the rate of the reaction
There will be a greater area on which collisions can occur therefore there will be an increase in frequency of successful collisions
What will increasing the conc of solutions do to the rate of the reaction
More particles per unit volume so increase in frequency of successful collisions
What will increasing the pressure of gases do to the rate of the reaction
More particles per unit volume so increase in the frequency of succeful collisions
What will increasing the temp of the reaction mixture do to the rate of the reaction
Increases kinetic energy so particles move faster and with more energy, therefore there is an increase in the frequency of collisions and an increase in the proportion of successful collisions as they have the activation energy
What will introducing a catalyst do to the rate of the reaction
Provide an alternative pathway with a lower activation energy. Therefore at a given temperature a higher frequency of collisions have the required activation energy
Key features of the maxwell Boltzmann curve graph
No molecules have zero energy (graph starts at the origin but doesn’t actually touch it)
The area under the curve is equal to the total number of molecules
The distribution is asymmetrical
The graph never touches the x axis
Y axis = number of molecules with a given energy
X axis = energy
Describe the maxwell Boltzmann curve
But curved
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What does the Ea line represent
The activation energy for the reaction
Position if the line will be different for every reaction. The higher the activation energy the further right the line
The area to the right of the Ea line represents the proportion of molecules that have an energy greater than the activation energy so could potentially react
Describe the change in graph if the reaction was at a higher temperature
The peak is lower and shifted to the right and the area to the right if the Ea line is greater
The area under the curve must be the same in both graphs
Why is the maximum lower in the maxwell Boltzmann distribution curve at a higher temperature
There must be the same area and since the particles are distributed over a wider range of energies it has to be lower
Why does the maximum move to the right in a higher temp
On average there are more particles with higher energy due to the higher temperature
What is a catalyst
A substance that speeds up the rate of a reaction but remains itself chemically unchanged at three end of the reaction
How does a catalyst work
Provides an alternative reaction pathway with a lower activation energy
How does a catalyst change the Maxwell Boltzmann distribution curve
The Ec line (activation energy with catalyst) is to the left of the Ea line
What is a heterogeneous catalyst
When the catalyst and reactants are in different phases (states)
What is a homogeneous catalyst
When catalyst and reactant are in the same phase
How do heterogeneous catalysts work
Provides an alternative pathway with a lower activation energy
Helps orientate the reactant in the right way for the reaction to occur
