1.09 Rate Equations
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Define the term rate of reaction
The change in concentration (of any reactant or product) per unit of time
At a given instant, how could you calculate rate of reaction ?
Rate of reaction = change in concentration / change in time
How could you measure the rate of reaction experimentally (2 methods)
-Use a colorimeter at suitable intervals if there is a colour change -If gas is produced, use a gas syringe to collect volume of gas produced in a set time, or calculate the change in mass of the reaction mixture.
How would you measure reaction rate for really fast reactions ?
-Use a flash of light to break bonds, use probe flashes to record amount of light absorbed by a species -Eg used for ClO2 -> ClO. + O.
How can you determine the rate constant and rate expression for a reaction ?
Only experimentally
What affects the value of the rate constant for any given reaction ?
Only temperature
Write a generic rate expression and define the terms
*Rate = k[X]ᵃ[Y]ᵇ *k=rate constant for the reaction *[X] and [Y] are concentrations of the species *ᵃ and ᵇ are the orders of the reactants in the reaction.
Do species need to be in the chemical equation to be in the rate expression ?
No, species in the chemical equation may be excluded and species not in the chemical equation such as catalysts may be included
Define the term order of a reaction with respect to a given product
The power to which a species concentration is raised in the rate equation
Define the term overall rate
-The sum of the orders of all species in the rate expression -For k[X]ᵃ[Y]ᵇ, the total order is a+b.
How would you calculate the units of rate constant ?
-Units if rate are moldm⁻³s⁻¹ and units of concentration are mol dm⁻³ -Rearrange rate equation to get k= -Sub in units and cancel out.
How would you draw a rate concentration graph ?
Plot [A] against time, draw tangents at different values -> draw a secondary graph of rate against [A]
Draw / describe the concentration graph for a zero order reaction ?
Rate stays constant across all concentrations
Draw / describe the concentration graph for a first order reaction ?
Rate increases in linear manner as concentration incerases
Draw / describe the concentration graph for a second order reaction
Curved increases of rate against concentration second order and above are indistinguishable
How can you confirm that a curved increase rate against concentration graph was second order ?
Graph of rate against concentration squared would be a straight line through the origin
How could you find the rate expression using initial rate method ?
-Do a series of experiments, during which you vary the concentrations so the concentration of just one reactant changes each time -Plot a graph of concentration against time for each reactant and use a tangent at t=0 to find initial rate of reaction -Compare rates and concentrations between each experiment to find order of reactants and overall rate expression.
What must you add to react with I2 as it is produced in the iodine clock reaction ?
• Known moles of sodium thiosulfate and starch; reacts with I2 in a 1:2 ratio • I2 = 2S2O3 2- -> S4O6 2- + 2I-
When does the starch turn blue-black in an iodine clock reaction and why ?
When all of the NaS2O3 has been used up and so I2 is being produced, which reacts with starch and leads to a blue-black colour.
How can you determine the rate of reaction from the data in an iodine clock reaction ?
Record time taken for colour change to occur, use rate = 1/t This gives initial rate.
What is the effect of a 10K temperature increase on rate of reaction (approx.)
Doubles rate
What is the of the half life of a first order reactant (concentration against time graph)?
Half life is constant
What is the Arrhenius equation ? What does each term mean ?
*k = Ae to the power of (-Ea / RT) *k = rate constant for reaction *A = pre-exponential factor (number of collisions between reactants) *e = e *R = gas constant T = temperature in kelvin *Ea = activation energy in Joules
How can you convert the Arrhenius equation into a useful form for plotting a graph ?
*In k = -Ea/RT + In A (form of y = mx + c) *Graph of In k against 1/T is a straight line *Gradient = -Ea/R and y intercept is In A.