2.2 - Rates of reactions Flashcards
What is the ____ of rate of reaction?
definition
formula
units
The change in conc of a reactant or product per unit time
rate = change in conc / time
moldm-3 / s
In a reaction when is the rate the fastest, slowing down, at 0?
The rate is fastest at the beginning of the reaction as each reactant is at its highest conc
Rate slows down as the reaction proceeds because the conc of the reactants decreases
Rate becomes 0 when the reaction stops, all reactant has been used up
Rates are calculated by measuring the conc of a reactant or product over a certain period of time. A conc/time graph can be drawn. How do you find the rate of reaction on a graph?
draw a tangent to the curve and find the gradient of the tangent.
M = change in y / change in x
See NC 1
State the collision theory?
reactions occur when molecules collide with a minimum amount of kinetic energy (activation energy) and at a correct orientation, (collisions must have energy greater than or equal to the AE)
What 6 factors affect the rate of reaction?
Concentration
Pressure
Temperature
Particle size (S.A)
Light
Catalysts
How does temp affect the rate of reaction?
Increase in temp = increase in KE. Molecules move faster and further. More collisions have a greater energy then A.E. Therefore, increasing temp increases the rate of reaction as more collisions of greater energy occur in a given time
How does concentration and pressure effect the rate of a reaction?
Increase in conc of reactants = more molecules in given vol. Distances between molecules are reduced –> increase in number of collision per unit time –> greater chance that number of effective collisions increases, hence the rate of reaction increases.
For a gaseous reaction, increasing the pressure is the same as increasing the conc as there is greater amount of gas molecules per unit area
How does Catalysts affect the rate of a reaction?
A catalyst increases the rate of reaction without being used up in the reaction. It provides an alternative reaction path that requires a lower A.E for the reaction to occur
How does particle size affect the rate of a reaction?
Reducing the particle size of a solid increases the S.A so the molecules are closer together and there’s an increase in the number of collisions per unit time -> increase in reaction rate
How does light affect the rate of a reaction?
Particles absorb more energy with the increase in the intensity of light thereby increasing the rate of reaction due to an increase in molecule collisions and energy
What is activation energy?
The minimum amount of energy needed to start a reaction (by breaking bonds)
In an _________ reaction, the reactants lose energy and heat is given out to the surroundings. Although energy is ultimately given out, energy must be supplied initially in order to _____ bonds and get the reaction started (e.g. lighting of match, spark)
exothermic
break
See NC 2
The molecules in a gas are moving constantly at different speeds so the energy of each molecule varies greatly.
Since the molecules collide frequently, a molecule that has been knocked can move quicker with greater energy than before while the molecule that caused the collision will slow down and have hardly any energy.
The Boltzmann energy distribution curve shows the distribution of molecular energies in gas. The energies of a few molecules are almost 0. Most molecules have an energy around an average value. Only a minority have values that equal or exceed the activation energy
At higher temps the average molecular energy will increase. Some molecules will still be almost motionless but at any one time many more molecules will have a higher energy
At 2 diff temps T1 and T2 where T2 > T1, the Boltzmann distribution curve shows why reaction rate is so dependent on temp
Both curves start at….
The areas under the 2 curves are equal and proportional to the…
At the high temp, T2, the distribution ____, the peak moves to the ____ (high energy) with a _____ height
At the higher temp, T2, the mean energy of the molecules ________. There is a wider spread of values.
Only the molecules with an energy ____ to or ______ than the activation energy are able to react
At the higher temp, T2, many more molecules have a sufficient energy to react and so the rate _________ significantly
the origin but do not touch the energy axis again
total number of molecules in the sample
flattens, right, lower
increases
equal, greater
increase
