Rates of Reaction Flashcards
Define the rate of a reaction
The change in the concentrations of the reactants or products over time
What does the graph of concentration against time look like for reactants?
Downwards curve, the gradient is zero by the end
What does the graph of concentration against time look like for products?
Upwards curve, the gradient is zero by the end
Why is the initial rate of reaction the fastest?
Because this is when the reactants are most concentrated so the probability of collisions is highest
Why do both graphs for conc against time tend toward zero over time?
Bc the reactants eventually run out so their concs run down to zero so the reaction stops
What factors affect the rate of a reaction?
Nature of reactants, physical state (SA), temperature, catalysts, concentrations of solutions and pressures of gases
What does the stoichiometric equation show?
The number of moles of reactants and products
What does the rate equation show?
How the rate depends on the concentrations of the reactants. It is of the form rate = k[A}^m[B]^n where m and n are usually integers
Why can you not determine the rate equation from the stoichiometric equation?
Bc the stoich. equation shows moles only. The rate equation can only be determined by experiments which show how the concentration affects the rate of the reaction.
What is an order of a reaction with respect to a reactant?
The power to which the concentration of a reactant is raised in the rate equation
What is the overall order of a reaction?
The total of all the orders in the rate equation
What is the rate constant?
The constant of proportionality in the rate equation
What is zeroth order?
When a reactant changes, the rate does not
What does the graph of rate against conc for zero order look like?
A horizontal line - bc as the conc changes the rate does not
What is first order?
When the rate is directly proportional to the concentration of A
What does the graph of rate against conc for first order look like?
A straight diagonal line
What is second order?
When the rate is directly proportional to the square of the concentration of A
What does the graph of rate against conc for the second order look like?
An upward curve
What does collision theory state?
A reaction between particles will only take place if the particles collide with the correct orientation and with energy equal to or greater than the activation energy.
What does the Maxwell Boltzmann distribution show?
How the proportion of particles of a given energy varies as the energy increases
What does the area under the curve of the MBD show?
The number of particles
How does temperature effect the MBD?
Shifts to the right with a lower peak, more particles with energy greater than Ea, most probably energy shifts to the right, mean energy is higher
What effect does a catalyst have on the MBD?
No effect at all
What effect does concentration have on the MBD?
Raises the peak bc more particles
What are catalysts?
Substances that speed up the rate of a reaction but are chemically unchanged themselves so they dont appear in the stoichiometric equation
What effect do catalysts have on the activation energy of a reaction?
They provide an alternative reaction pathway which requires a lower activation energy (they dont lower the Ea directly) so more particles have energy greater than the activation energy
What can you deduce about the order of a reactant if it isnt in the rate equation?
It’s zero order
What is the unit of rate?
moldm-3s-1
What effect does temperature have on the rate constant?
As the temperature increases, the rate constant increases exponentially
What does the graph of rate constant against temperature look like?
An upward exponential curve
What is the Arrhenius equation for?
To show the effect of temperature on the rate of a reaction
What is the Arrhenius equation?
k = Ae^(Ea/RT)
k - rate constant, A - Arrhenius constant, e - constant, Ea - activation energy (J), R- gas constant, T - temperature (Kelvin)
What do you get when you take natural logs of the Arrhenius equation?
lnk = lnA - (Ea/RT)
How can you write the Arrhenius equation in a graphical format?
lnk = (-Ea/RT) x 1/T + lnA
How can you experimentally determine the Ea of a reaction?
Carry out an experiment at different temperatures and plot the data on a graph of lnk against 1/Temp. The gradient is -Ea/R
What does the exponential part of the Arrhenius equation show?
The fraction of molecules in a gas which have energies equal to or greater than the activation energy, Ea, at temperature T
