Chapter 18: Rates of Reaction (18.1 - 18.5) Flashcards
define rate of reaction
the change in concentration of a reactant or product in a given time
equation for rate of reaction
rate = change in concerntration / change in time
what is order
order shows how rate is affected by the concentration
for order n: rate ∝ [A]^n
zero order
rate ∝ [A]^0
rate is not affected by the concentration
horizontal line on a rate/time graph
downward linear on a conc/time graph
first order
rate ∝ [A]^1
rate changes by the same factor as concentration
upward linear relationship with line going through origin on a rate/conc graph
downward curve asymptotic to y=0 for conc/time graph (constant half life)
second order
rate ∝ [A]^2
rate changes by the same factor as the concentration change squared
upwards steep curved line starting at origin on a rate/conc
find reaction rate from a conc/time graph
take a tangent
gradient of tangent = rate of reaction
what is overall order
the sum of the individual orders of the reactants.
eg for reactants A and B with orders m and n (respectively)
overall order = m + n
the rate equation:
ie for reaction A + B –> C with orders m (for A) and n (for B) what is the rate given by
rate = K X [A]^m X [B]^n
k = rate constant
units for rate
moldm^-3s^-1
to work out units of K
- rearrange the rate equation to make K the subject
- substitute units into the rearranged rate equation
- cancel common units and write the final units on a single line
define half-life
the half-life of a reactant is the time for its concentration to decrease by half
whats special about first order reactions
first order reactions have a constant half-life
K = ln2 / halflife (s)
what is a colorimeter and what is it used for
A colorimeter measures the intensity of light passing through a sample. Absorbance reading is directly linked to the concentration of the solution (when either the reactant or product is coloured and the other is colourless) eg Br2 (coloured) --> 2Br- (colourless)
what is the rate-determining step
the slowest step in the reaction mechanism of a multi-step reaction.
Why is it unlikely that the reaction would take place in one step?
1) Usually only two molecules would collide at once
2) The stoichiometry of the equation would not balance
How do you predict the reaction mechanism of an equation?
1) The rate equation only includes reacting species
2) The orders in the rate equation match the number of species involved in the rate determining step
What is the Arrhenius equation?
K = Ae^-Ea/RT
What do the symbols in the Arrhenius equation stand for?
A - pre exponential factor (frequency of collisions)
Ea - activation energy
R - gas constant
T - temperature in kelvin
Logarithmic form of Arrhenius equation
Lnk = (-Ea/RT)(1/T) + lnA
- Gradient m of -Ea/RT
- intercept of c = lnA
What effects the rate constant?
Increasing temperature, increases rate constant
Decreasing activation energy, increases rate constant
