# 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/conc 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

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.