CI.4 Flashcards
Orders of reactions and the use of the Arrhenius equation
What is the rate of the reaction at the start called?
Initial rate.
How can you find the initial rate?
It can be found by drawing a tangent to the point at which time is equal to 0 and measuring the gradient of the tangent.
When you have initial rates of reactions with different concentrations of a reagent but kept all other conditions the same, if the line plotted of initial rate and concentration is a straight line, what does this mean about the rate and the concentration?
Rate is directly proportional to the concentration of that reagent, in other words, rate ∝ [reagent], or rate = k[reagent].
What is k in a rate equation?
The rate constant, it is specific to temperature.
What is the general rate equation for a general reaction where A + B → products?
Rate = k[A]m[B]n, m and n are powers that [A] and [B] are to be raised to, also known as order of the reaction in respect to A and B.
How do you work out the overall order of a reaction?
Add together all the individual orders.
Can you predict the rate equation of a reaction from its balanced equation?
No, the only way to find out is by doing experiments.
If the line plotted of initial rate and concentration is a curved line going upwards, what does this mean about the rate and the concentration?
The rate is directly proportional to the square of the concentration of that reagent, in other words, rate ∝ [reagent]2, or rate = k[reagent]2.
If the line plotted of initial rate and concentration is a flat horizontal line, what does this mean about the rate and the concentration?
The rate is not related to the concentration of that reagent, it does not effect it in any way, or in other words, rate = k[reagent]0.
What unit would k have when the reaction is zero order overall?
moldm-3s-1
What unit would k have when the reaction is first order overall?
s-1
What unit would k have when the reaction is second order overall?
dm3mol-1s-1
Where are factors like temperature and catalysts included in the rate equation?
They are included in the rate constant k.
What equation is used to show the relationship between k and factors like temperature and activation energy?
The Arrhenius equation.
In the Arrhenius equation, what unit is temperature quoted at?
Kelvin.
In the Arrhenius equation, what unit is activation energy quoted at?
Joules per mole.
What can the Arrhenius equation be used to calculate?
The impact of changes in temperature and activation energy on the rate constant k and the rate of the reaction.
To find Ea and A, what values are plotted on the axis of the graph?
lnk on the y axis and 1/T on the x axis.
With a graph of lnk against 1/T, how do you find Ea?
The gradient of the graph is -Ea/R, so you would take the gradient and times by -R.
With a graph of lnk against 1/T, how do you find A?
The y intercept is lnA, use this to calculate A.
What does the rough 10 degree rule state about the effect of raising the temperature by 10 degrees have on the reaction rate?
Raising the temperature by 10 degrees roughly doubles the reaction rate.
Out of lowering activation enthalpy and increasing temperature, which has a bigger effect on the rate?
Normally lowering activation enthalpy by using a catalyst.
