Rate of reaction Flashcards
Rate of chemical reactions
Change in concentration of a reactant/product per unit time
Factors that affect rate of reaction
- Temperature
- Pressure
- Surface area
- Catalyst
- Concentration
What is a Rate Equation?
expression showing how the rate of reaction is linked
to the concentrations of the reactants
Rate equation for general reaction: A + B → C + D
rate = k[A]^m[B]^n
Where:
m = order of reaction with respect to reactant A
n = order of reaction with respect to reactant B
k = rate constant
What is the rate constant?
the proportionality constant which links the rate of reaction to the concentrations in the rate equation
What affects the rate constant?
Temperature
How can activation energy be linked to the rate constant?
by the Arrhenius equation:
k = Ae-Ea/RT
Where:
A = pre exponential factor, which is related to the number of collisions between the reactant molecules. Arrhenius constant.
e-Ea/RT = The fraction of molecules with energy greater than the activation energy
R = gas constant
T = absolute temperature
Logarithmic form of the Arrhenius equation
ln(k) = lnA - Ea/RT ln(k) = lnA - [Ea/R]1/T
Compare the Logarithmic form of the Arrhenius equation to y = c+mx
Plot lnk against 1/T you get a straight line where:
Gradient = - E/R Y-intercept = lnA
Activation energy =
Gradient x gas constant
A straight line log graph shows the reaction is
zero order
A curved log graph shows the reaction is
either first or second order
What is the rate determining step
the slowest step
Explain qualitatively why doubling the temperature has a much greater effect on the
rate of the reaction than doubling the concentration of E
Reactions occur when energy molecules have is greater than activation energy
Doubling the temperature causes many more molecules to have energy greater than the activation energy
Whereas doubling E only doubles the number of molecules with the same energy, not greater than the activation energy
Which is the rate determining step
the one where the ratio/amounts of the reactants are the same as in the rate equation
