Chapter 18 Flashcards
Rate
rate = change in conc. / change in time
order
The power to which the concentration of a reactant is raised in the rate equation.
overall order
The sum of the individual orders of reactants in the rate equation: m + n.
gives overall effect of conc. of all reactants
Zero order
When the conc. of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant.
- any number raised to the power zero =1
- conc. doesn’t influence rate
First order
When the rate depends on its conc. raised to the power of 1
- if A is doubled the reaction rate increases by the same factor.
Second order
When rate depends on the conc. raised to the power of two
- if A is doubled (x2) the reaction rate increases by the factor squared.
rate equation
For a reaction: A + B → C with orders m for A and n for B, the rate equation is given by: rate = k[A]^m[B]^n
Gives the mathematical relationship between the conc. of reactants and the reaction rate.
rate constant k
The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.
overall order=0 units
mol dm^-3 s^-1
overall order = 1 units
s^-1
Overall order = 2 units
dm^3 mol^-1 s^-1
initial rate of reaction
The change in concentration of a reactant or product per unit time at the start of the reaction: t =
Continuous monitoring
continuous measurements taken during the course of a reaction.
includes :
- monitoring by gas collection
- monitoring by mass loss
- monitoring colour change
- measuring change in conc. of H+ ions with a pH meter
Colorimeter
Measures the intensity of light passing through a sample. A filter is chosen so that it is the complementary colour to the colour being absorbed in the reaction. Absorbance is directly linked to the conc. of the solution.
Concentration - time graph gradient shows?
rate of reaction
Zero order Concentration - time graph
- straight line with constant negative gradient
- gradient = k
First order Concentration - time graph
- downward curve with decreasing gradient over time
- constant half-life
Second order Concentration - time graph
- downward curve, steeper at the start but tailing off more slowly than a first order graph
half-life
The time taken for the concentration of a reactant to decrease by half.
Exponential decay
The conc. halving every half life
Calculating k from half life
k= ln2/half-life
calculating k from rate (graph)
- calculate rate from gradient of conc. -time graph by drawing a tangent to curve
- k = rate/ conc. at point where the tangent was drawn
Rate concentration graph Zero order
- Produces a horizontal straight line
- intercept on y-axis gives k
- reaction rate does not change with increasing concentration
Rate concentration graph First order
- Produces a straight line graph through origin
- Rate is directly proportional to concentration
- rate constant k can be determined by measuring gradient
Rate concentration graph Second order
- Produces an upward curve with increasing gradient
- constant cannot be determined from graph
- plot second graph of rate against conc. squared. The gradient of this straight line is equal to k
stoichiometry
The ratio of the amount, in moles, of each substance in a chemical equation (essentially the ratio of the balancing numbers).
reaction mechanism
The sequence of bond breaking and bond-forming steps that shows the path taken by electrons during a reaction.
rate-determining step
The slowest step in the reaction mechanism of a multi-step reaction
