physical chemistry and transition elements Flashcards
(26 cards)
how are reaction rates measured?
by observing changes in quantities reactants/products over time
what is the equation for reaction rate and units + the symbol for the conc for a reactant? (3)
1) rate =change in conc÷time
2) units=mol dm-3 s-1
3) [A]=concentration A
the rate of a reaction is proportional to…
the concentration raised to a power
e.g
rate ∝ [A]^n
n= order of reaction for that reactant
what is zero order? (2)
1) concentration of the reactant has no effect on the rate
2) as any number based to the power of 0 is 1
what is first order? (3)
1) if conc doubles, rate doubles
2) if conc triples, rate triples
3) rate of reaction is directly proportional to the concentration of one reactant
what is second order? (3)
1) if the concentration is doubled, rate increases by 2^2/ x4
2) if the concentration is tripled, rate increases by 3^3/ x9
3) rate of the reaction is proportional to the square of the reactant concentration.
what is the rate equation? give units (2)
1) rate=k[A]n [ B]m
units: moldm-3s-1
k= rate constant (units will change)
[A] [B] = concentrations of reactants, moldm-3
n, m = orders
2) zero order doesn’t appear in rate equations
what does the rate equation show?
gives the mathematical relationship between the concentrations of the reactants and the rate
what is the overall order?
sum of orders with respect to each reactant
e.g
rate = k [A] [B]2
overall order = 3
how do you rearrange the rate equation to find units of the rate constant?
k= rate/ [A][B]^2
k= moldm-3s-1/ moldm-3 moldm-3 moldm-3
k= s-1/ moldm-3moldm-3
k= s-1/ mol2dm-6
k= mol-2dm6s1
how can you find orders? (2)
1) cannot find orders from chemical equation
2) order must be determined experimentally by monitoring how a physical quantity changes over time
how can you find orders from experimental results? (4)
1) for each reactant, find 2 experiments where the conc of the reactant changes as well as the rate but the other reactants remain constant
2) work out multipliers between chosen experiments for the reactant and the change in rate to work out order
3) can work out rate equation once orders are obtained
4) once rate equation is written, can calculate k by choosing concentrations + rate from 1 experiment in the table. make sure to multiply concentrations by order of reactant
how can you plot concentration-time graphs? (2)
1) concentration time graphs can be plotted from measurements taken at regular intervals during a reaction
2) this is called continuous monitoring
what other property is useful to monitor when plotting a concentration time graph in reactions that don’t produce a gas (2)
1) colour change
2) can estimate this by eye or use a colorimeter
how does colorimetry work?
1) you control the wavelength of light passing through a coloured solution using a filter
2) the amount of light that is absorbed by the solution is measured
3) using a calibration curve you can determine the concentration from light absorbed
how can you plot a concentration time graph using colorimeter experiment? (6)
1) prepare standard solutions of known concentrations
2) measure standard absorbance
3) plot calibration curve of absorbance against concentration
4) carry out reaction and take absorbance readings at timed intervals
5) use calibration curve to measure concentration
6) plot concentration time graph to determine order
what can you use concentration time graphs for? (3)
1) gradient= rate of reaction
2) shape= can determine order with respect to that reactant
3) only gives information about the order if the other reactants concentration is effectively in constant excess
what is half life and its symbol?
the time for the concentration of the reactant to half
symbol: t 1/2
what is half life like for first order reactions? (2)
1) first order reactions have a constant half-life
2) exponential decay
how can you confirm the first order relationship?
by measuring successive half lives
how can the rate constant, k, be calculated by rearranging the rate equation?
k = rate/ [A]m[B]n
how is zero order shown on a concentration time graph? (2)
1) shown on a conc time graph as a straight line with a negative gradient
2) rate= k
how is first order shown on a concentration time graph?
downward curve with decreasing gradient
how is second order shown on a concentration time graph?
downward curve which is steeper at start, tails off more slowly
