5.1 Rates Equilibrium And pH Flashcards
What is rate of reaction?
-is the change in concentration of a reactant or product per unit time
The ____________ with respect to a reactant is the power to which the concentration of the reactant is raised in a rate equation
ORDER
What is the rate constant?
-the rate constant (k)
-is the constant that links the rate of reaction with the concentration of the reactants raised to the powers of their orders in the rate equation
What is the rate equation?
-the rate equation for a reaction A+B—> C is given by :
Rate=k[A]^m [B]^n
-where m is the order of reaction with respect to A and n is the order of reaction with respect to B
What is the overall order of a reaction?
-the sum of the individual orders
-m+n
What is the equation for rate of reaction?
Rate of reaction(moldm^-3s^-1). = change in concentration of reactant or product
—————————————————————————
Time
What is zero order?
-if the order is 0 with respect to reactant A ,then: rate ∝[A]^0
-the rate is unaffected by changing the concentration of A
-remember any number to the power of 0 is equal to 1
What does the symbol ∝ mean?
-proportional
What is first order?
-if the order is 1 with respect to a reactant A, then: rate ∝ [A]^1
-the rate is DIRECTLY PROPORTIONAL to the concentration
->if [A] increases by 2 times the rate also increases by 2 times
-> if [A] increases by 3 times the rate also increases by 3 times
What is second order?
-if the order is 2 with respect to a reactant A then : rate ∝[A]^2
-the change in rate will be equal to the change in concentration squared
->if [A] increases by 2 times the rate increases by 2^2=4 times
->if [A] increases by 3 times the rate increases by 3^2=9 times
What is the rate equation?
Rate=k[A]^x[B]^y[C]^z
-k=rate constant
-A-concentration of reactant A
-B-concentration of reactant B
-C-concentration of reactant C
-x,y,z-orders of reaction for each reactant
What is half-life?
-the half-life of a reactant is the time taken for the concentration of the reactant to reduce by half
What order does this rate-concentration graph show?
-zero order
-changes in concentration of this reactant have no effect on the rate
-this is shown on a rate-concentration graph as a horizontal line
- rate ∝ [A]^0
What order does this rate-concentration graph show?
-first order
-the concentration of the reactant and rate are directly proportional
-doubling the concentration doubles the rate
-this graph shows that rate ∝ [B]^1
What order does this rate-concentration graph show?
-second order
-the rate is proportional to the concentration of the reactant squared
-if the concentration of the reactant is tripled the rate will be increased by 3^2
-the graph shows that the rate ∝ [C]^2
How can you determine the rate constant for a first order reaction rate-concentration graph?
-calculate the gradient
-gradient= change in y / change in x
It is a mathematical convention that ____________ ___________ are written first.
When there is more than one unit with the same sign they are written __________
-positive indices
-alphabetically
What are some experimental techniques to obtain rate data for a reaction to be plotted on a concentration - time graph?
-for reactions involving acids or bases, you can measure-
.pH changes by carrying out titrations
.pH changes by using a pH meter
-for reactions that produce gases, you can measure-
.the change in volume or pressure
.the loss in mass of reactants
-for reactions that produce visual changes (usually monitored by using a colorimeter) you can observe-
.the formation of a precipitate
.a colour change
What is colorimetry?
-Colorimetry can be used to determine the rate of reaction for a reaction that involves the formation or depletion of a coloured species.
-A colorimeter is a device that measures the amount of light that is absorbed by a solution. -The amount of light absorbed by the solution is proportional to the concentration of the coloured species.
-In a colorimetry experiment, a calibration curve is often generated.
-This involves using a colorimeter to measure the absorbance of solutions of known concentrations, from which a calibration curve is plotted.
-Then, throughout the experiment, the absorbance of samples from the reaction mixture can be measured and the calibration curve used to convert the absorbance readings into concentration values. A concentration-time graph can then be plotted.
Concentration-time graphs can be plotted from ______________ measurements taken during the course of a reaction (_______________ monitoring)
-continuous
-continuous
What is the equation used to determine the rate constant (k) from a first order concentration -time graph?
-because first order reactions have constant half-lives , the value of the half-life can be used to calculate the rate constant
What is the rate-determining step?
-the slowest step in the reaction mechanism of a multi-step reaction
What are some things to consider when determining possible steps in a reaction mechanism from the rate equation ?
The reaction mechanism must be worked out by looking at the rate equation and ensuring that :
-a rate-determining step is given that involves the number of molecules shown by the order for that reactant (e.g if it is a second order there will be 2 molecules of that reactant)
-subsequent steps are shown that eventually generate the products shown in the balanced equation
-any intermediate generated is used up in the subsequent steps
-the species involved in the reaction mechanism cancel out to leave the overall equation
What is the effect of temperature change on the rate of reaction and hence the rate constant?
-increase of temperature gives more energy to the molecules
-means more frequent collisions that exceed the activation energy (more successful collisions)
-raising the temperature speeds up the rate of most reactions by increasing the rate constant
What do the symbols of the Arrhenius equation stand for ?
-k= rate constant
-Ea =activation energy
-T= temperature (in kelvin to go from degrees Celsius to kelvin add 273)
-e=mathematical value the opposite of this is ln
-A=pre- exponential factor
-R =gas constant
What is the simplified form of the Arrhenius equation (without e)?
What does the Arrhenius equation tell us?
-temperature and rate constant are related exponentially
-as temperature increases rate constant increases
How does the Arrhenius equation relate to the straight line equation?
-an Arrhenius plot can be used to identify:
.A= the intercept on the y-axis is equal to lnA (which you can put into the exponential button on your calculator to find A )
.the activation energy Ea = the gradient is equal to -Ea/R . Where R is the known gas constant so can be rearranged to find Ea
What is a mole fraction?
-the mole fraction of a substance is a measure of how much of a given substance is present in a reaction mixture
What is partial pressure?
-the pressure an individual gaseous substance would exert if it occupied a whole reaction vessel on its own
Once an equilibrium is established Kc remains _________ unless the _____________ of the reaction changes
-constant
-temperature
What is the equation for Kc (equilibrium constant)?
How does the expression for Kc change when it is a heterogenous equilibrium?
-the concentrations of solid substances are not included in the expression
-the concentrations of liquids are not included in the expression
What is a homogenous equilibrium?
-an equilibrium in which all the species making up the reactants and products are in the same physical state
What is a heterogeneous equilibrium ?
-an equilibrium in which species making up the reactants and products are in different physical states
What is the equation for the equilibrium constant (Kp)?
-partial pressure of products divided by partial pressure of reactants
What is a conjugate acid and a conjugate base?
-a conjugate acid is the species formed when a base accepts a proton
-a conjugate base is the species formed when an acid accepts a proton
-conjugate acids and conjugate bases form conjugate acid-base pairs
What is the expression for the acid dissociation constant(Ka)?
How can you find pKa using Ka?
How can you find Ka using pKa?
What is the expression for pH?
What is the expression for the concentration of H+ ions?
How do you calculate the pH for a strong monobasic acid?
What approximations are used when calculating the pH of a weak acid?
~ means approximately
» means much greater
What is the simplified Ka expression for weak acids?
Rearrange the Ka expression to find H^+ of a weak acid
What is the expression for the ionic product of water (Kw)?
How do you calculate pKw from Kw?
How do you calculate Kw from pKw?
How do you use Kw to calculate the pH of a strong base?
How do you form an acidic buffer?
-contains a weak acid and a salt of the weak acid
What equation do you use to calculate the concentration of H+ ions in a buffer solution?
Sketch out the pH titration curve for a strong acid -strong base titration
What following points can you observe from this strong acid- strong base titration curve
• The vertical section of the graph covers a large change in pH, starting around pH 3 and ending around pH 11, with an
equivalence point at pH 7.
• Both the indicators, methyl orange and phenolphthalein, have
end points that fall within this pH range.
• Either indicator would be suitable to use in a titration between a strong acid and a strong base
Sketch out a titration curve for a strong acid-weak base titration
What observations can you make from this titration curve of a strong acid-weak base titration ?
-the vertical section of the graph covers a smaller change in pH, starting around pH 3 and ending around pH 7.5 , with the equivalence point at a more acidic value
-methyl orange has an end point that falls within this pH range , phenolphthalein does not. Only methyl orange would be a suitable indicator for titration between a strong acid and a weak base
Sketch out a titration curve for a weak acid-strong base titration
What observations can you make from this titration curve of a weak acid- strong base titration?
-the vertical section of the graph covers a smaller change in pH and occurs further up towards the higher pH values
-the equivalence point occurs at a more basic pH
-methyl orange has an end point that falls outside this pH range, phenolphthalein has an end point that falls within this pH range, only phenolphthalein would be a suitable indicator for a titration between a strong acid and a weak base
Sketch out a titration curve for a weak acid- weak base titration
What observations can be made from the titration curve of a weak acid -weak base titration
-there is no real vertical section
-neither indicator has an end point near the equivalence point-neither indicator is suitable
-an indicator would change colour gradually over a few cm^3 of base added
-no indicators are suitable for weak acid - weak base titrations
What order does this concentration-time graph show?
-zero order
the concentration of the reactant is inversely proportional to time
.This means that the reactant concentration decreases as time increases
.The graph is a straight line going down
What order does this concentration - time graph show?
-first order
-the concentration of the reactant decreases with time
-the graph is a curve going downwards and eventually plateaus
-this graph has a constant half life
