3.2.2 - Reaction Rates
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What is the equation sued to calculate rate?
Rate = change in concentration / time
What is the unit for rate of reaction?
mol dm-3 s-1
What must particles do in order to react?
Collide with sufficient energy (activation energy) and the correct orientation
Do most collisions result in a reaction?
No
What are the factors that affect rate of reaction
Temperature Pressure Concentration Surface area Catalyst
What is the effect of increasing temperature on rate of reaction? Why?
Increasing the temp, increases the rate of reaction Much higher proportion of particles have energy greater than the activation energy, so many more successful collisions per second, leading to an increased rate
What is the effect of increasing concentration / pressure on rate of reaction and why?
Increased concentration / pressure = increased rate of reaction There are more particles in a given volume, more frequent successful collisions, increased rate
What are the variables in an experiment that can be monitored to calculate the rate of reaction?
Concentration of reactant or product Gas volume of products Mass of substances formed
How to calculate rate from a concentration time graph?
Draw a tangent Work out the gradient of the tangent using the equation Gradient = change in y / change in x
What is a catalyst?
A substance which increases the rate of reaction but is not used up in the reaction
How do catalysts work and how do they increase the rate of reaction?
They provide an alternate reaction pathway (with a lower activation energy) Due to lower activation energy, more particles have energy > activation energy so more frequent successful collisions, so increased rate of reaction
What does homogenous catalyst mean?
A catalyst that is in the same phase as the reactants e.g. liquid catalyst mixed with liquid reactants
What does heterogenous catalyst mean?
Catalyst used in the reaction is in different phase to the reactants e.g. gaseous reactants passed over solid catalyst
What are catalytic convertors?
They are present in vehicles to reduce toxic emission and prevent photochemical smog
Define activation energy
The minimum energy that particles must collide with for a reaction to occur
Name some important features of Boltzmann distribution (5)
Area under the curve = total number of molecules Area under the curve does not change when conditions alter The curve starts at the origin Curve down not touch or cross the energy axis Only molecules with energy greater than activation energy can react
What are the axis in a Boltzmann distribution?
X axis - energy Y axis - number of molecules with a given energy
Draw a labelled Boltzmann curve with labels of average energy, activation energy and most probable energy. Draw in a different colour the effect of increasing temperature
T1 < T2
Draw a labelled Boltzmann Curve showing the effect of a catalyst on rate of reaction
Actvation energy with a catalyst A greater proportion of molecules exceeds the lower activation energy. Reaction rate increases Activation energy without a catalyst Proportion of molecules exceeding the activation energy without a catalyst is less