module 5 - physical chemistry and transition elements Flashcards
what is short hand for concentration of reactant ‘A’
[A]
what is the correlation between rate and concentration
directly proportional
define zero order
when the concentration has no effect on rate of reaction
in a zero order reaction:
- concentration of a reactant is to the power of 0
- any number raised to the power of 0 =1
- doesn’t appear in the rate equation
- concentration does not influence the rate of reaction
in a first order reaction:
if the concentration of A is doubled (x2) the reaction rate is increased by a factor of 2^1
(directly proportional)
in a second order reaction:
if the concentration is of A is doubled, the reaction rate increases by a factor of 2^2 = x4
what is the rate equation
rate = K [A]^m [B]^n
k = rate constant
[ ] = concentration
m&n are orders
what is the trend in concentration-time graphs for:
zero order
first order
second order
zero: straight line with negative gradient
first order: downward curve with a decreasing gradient over time - half life is constant
second order: also a downward curve but steeper at the start and tails off more slowly
define exponential decay
first order reactions have a constant half life (time taken for half a reactant to be used up)
what are the trends in rate-concentration graphs for:
zero order
first order
second order
zero: straight horizontal line with zero gradient
first: straight line directly proportional
second: upward curve with increasing gradient
how can you find the rate constant (k) from rate-concentration graphs for:
zero order
first order
second order
zero - y intercept
first: gradient
second: cannot be determined
how do you work out the initial rate of reaction from a concentration-time graph
draw a tangent at T=0 and calculate the gradient
what does the rate equation only include?
reacting species involved in the rate-determining step.
what is the Arrhenius equation? And what do the letters stand for?
k = A e^(-Ea/RT)
k = rate constant
A = frequency factor
Ea = activation energy
R = gas constant (8.314)
T = temperature (Kelvin)
what is the logarithmic form of the Arrhenius equation
Ln k = - Ea/R (1/T) + Ln A
why is the logarithmic form of the Arrhenius equation useful?
so we can determine both Ea and A to be determined graphically
a plot of Ln k against 1/T gives a straight line graph in the form of y=mx+c
gradient m = -Ea/R
intercept c of Ln A on y axis
Bromine reacting with an excess of methanoic acid
Suggest how the concentration of the bromine could be monitored
Measure the reduction of colour of bromine (turns from brown-orange to colourless)
define closed system
a system in which reactants and products cannot be added or removed once the reaction begins
define dynamic equilibrium
the rate of the forward reaction is the same as the rate of the backwards reaction
define the haber process
the industrial process of making ammonia from nitrogen and hydrogen (N2g + 3H2 <–> 2NH3
list 5 reversible reactions
- haber process (N2 + 3H2 <–> 2NH3)
- disassociation of acids (HA <–> H+ + A-)
- combustion of glucose (C6H12O6 +6O2 <–> 6CO2 +6H2O)
- haemoglobin + oxygen (Hb + O2 <–> HbO2)
- ammonium chloride (NH4 + Cl <–> NH4Cl)
when can dynamic equilibrium happen?
in a closed system in a reversible reaction
how do catalysts alter equilibrium position
allows reaction to reach equilibrium faster, economically valuable
what is Le Chatlier’s principle?
when a system at equilibrium is subject to change, the system moves to minimise the effects of change
in terms of equilibrium how do reversible reactions react to changes in concentration
increased concentration: shifts equilibrium to the direction that produces less of that substance
in terms of equilibrium how do reversible reactions react to changes in temperature
increased temperature, shifts equilibrium in the direction that takes in heat (endothermic)
in terms of equilibrium how do reversible reactions react to changes in pressure (for gases)
increase in pressure shifts equilibrium in the direction that produces fewer moles of gas
What goes at the top of the equilibrium constant expression calculation?
The products
What does a small Kc value indicate
The concentration of the reactants is larger than the products
What substances should not be included in the expression for Kc
Solids
What factor could permanently change the value for Kc and why?
Temperature because a temperature change will favour one direction increasing the concentration of either products or reactants
What does a large Kc value indicate
There are more products than reactants
Define heterogeneous in rate equations
Containing species in different states/ phases
What does a Kc value of exactly 1 suggest
The equilibrium lies directly between products and reactants
Why are aqueous species considered homogenous in liquid water
Because they are in a single uniform phase
Define Kp
Equilibrium constant when all products and reactants are gases
What are the 4 steps to calculate Kp
- Total moles of gas (products and reactants)
- Mole fraction of each gas
- Partial pressure of each gas
- Kp - (products/ reactants)
What is different about the equilibrium equations for Kc and Kp
Kc contains square brackets to show concentration [ ]
Kp contains round brackets ( )
define partial pressure and how do you work it out?
Partial pressure (p) A measure of a gas’s contribution to the total pressure exerted(P)
p(A) = x(A) x P
(Mole fraction x total pressure)
Why is there a direct relationship between Kp and Kc
Because gas concentration and pressure are proportional to each other
What does a value of Kp smaller than 1x10^ -2 suggest?
Equilibrium lies well within the reactants
Reactants are greater than the products
What is the expression for determining mole fraction?
Mole fraction x(A) = moles of A/ total moles of gas
What is meant by continuous monitoring
Data is collected throughout the reaction taking place
What is used to monitor the change of a coloured solution during a reaction
Colorimeter
How do you work out if half life is constant from a graph
Work out how long it takes for concentration to half
Then work out how long it takes for concentration to half again
How do zero order and first order concentration-time graphs differ
Zero order are a straight line with negative correlation whereas first order are downward curves
For a first order reaction what is the relationship between rate constant and half-life
k= ln2/ (t 1/2)
How do zero and first order rate-concentration graphs differ?
Zero order are horizontal but first order are directly proportional
Explain why, when hydrogen peroxide, acid and iodine are mixed together in a thiosulfate and starch solutions, the tubes go blue after a period of time rather than gradually from the start
Thiosulfate reacts with iodine until thiosulfate used up
Only then will the iodine react with starch and produce the blue-black colour
A student says chemical reactions go faster at higher temperatures because the molecules have more frequent collisions
Evaluate this statement
Student is partially correct as higher temperatures increase the kinetic energy of the molecules resulting in more frequent collisions
However, collisions are not the only factor
How does the Bronsted-Lowry model define acids and bases
By their role in proton transfer
- acids are proton donors
- bases are proton acceptors
What is a hydroxonium ion
The active part of an acid
H3O+
Define conjugate acid-base pair
Two species that can be interconverted by proton transfer
What is the expression that links pH to H+ concentration
pH = -log[H+]
What is the relationship between Ka and pKa
pKa = -log (Ka)
Ka = 10 ^-pKa
What is the equation for Ka
Ka = [H+] [A-]
/ [HA]
What is Kw
Ionic production of water
What does a large value of Ka mean in terms of equilibrium and acid strength
Equilibrium lies to the right/ products
Stronger acid
Why is Ka converted into pKa
Ka values are very small, so pKa values are easier to compare
What is the expression for determining [H+] for a weak acid
[H+] = sqrt (Ka x [HA])
What is the expression for Kw
Kw = [H+] [OH-]
What is a diacidic base
A base that releases 2 moles of OH- for one mole of base
What are buffer solutions
Systems that minimise pH changes on the addition of small amounts of an acid or base
They are composed of weak acids and their conjugate bases
What is needed to determine standard electrode potential
Electrodes with wires
Half-cells with a salt bridge to allow ion movement
What do the electrode potential values show
- more negative values tend to lose electrons (oxidation)
- more positive values tend to gain electrons (reduction)
What are the standard electrode potential conditions
1 mol dm^-3
298K
100kPa
Platinum electrode
How is cell potential calculated
Electrochemical potential of cell =
E(positive electrode) - E(negative electrode)
How is standard electrode potential determined
From hydrogen gas and a solution of H+ ions as a reference
How do you predict feasibility of an electrochemical cell reaction
Reaction occurs if the oxidising agent has a more positive E* than the redox system of the reducing agent
What elements are the strongest reducing and oxidising agent
Strongest reducing = Fe
Strongest oxidising = Cl2
In electrochemical cells, if the E*value is more negative, which direction will equilibrium shift
Left (oxidation)
In electrochemical cells, if the E* value is more positive, what direction will equilibrium shift
Right (reduction)
What are the limitations with electrochemical cell reactions
If the difference in E* values is less than 0.4V, reaction is unlikely
Non-standard conditions alter the value for E*
Half equations are equilibria so changes in concentration will shift the position, which alters electron transfer
Reaction rates may be slow due to high activation energies
Not all reactions occur in aqueous solutions
What is an electrode potential
A measure of electrons that are transferred which corresponds to energy transfer
What are most half-cells composed of
A metal and a solution of its ions
How can a half-cell be composed of two ions of a metal
By using a platinum electrode - unreactive
