Module 2: Energetics, Rates and Driving Forces of Chemical Reactions Flashcards
Which factor, when increased, will increase the rate of any reaction?
Temperature
In the rate law, k is constant for a given ___?
T temperature
Three factors which affect rate (collision model)
Collision rate
Collision effectiveness
Collision orientation
Why are the terms [A] and [B] in the rate law?
Higher concentrations increase collision rate, and therefore reaction rate
Activation energy
The minimum energy required for breaking and forming bonds in a collision for successful reaction
When Gibbs energy is negative, is energy still required for an effective collision?
Yes
Boltzmann distribution (energy of molecules)
Distribution of kinetic energy is exponential
Which quality of a molecule will increase the number of collisions with correct orientations?
Symmetry
Which properties will molecules with low chances of correct collision orientations possess?
Lots of different functional groups (no symmetry) hard to get correct orientation
Molecularity
Number of molecules that have to collide for a successful reaction
A way to classify elementary reactions
Three classes of molecularity
Unimolecular
Bimolecular
Termolecular
Which molecularity is very unlikely to cause successful collision?
Termolecular
Transition state
Loosely bound complex containing party broken and partly formed bonds
Only exists for a short time and cannot be isolated
‡
Transition state denotation
k = Ae^(-Ea/RT)
Arrhenius equation
k = Ae^(-Ea/RT)
What do these symbols stand for?
k is rate constant A is frequency factor e is an exponential Ea is activation energy R is gas constant (8.314 J K^-1 mol^-1) T is temperature
In the Arrhenius equation
k = Ae^(-Ea/RT)
Which factors will increase k, and which will decrease k?
Increase: A (frequency factor) and T (temperature)
Decrease: Ea (activation energy)
Describe the two approaches of determining Ea or A
- Use natural log version of Arrhenius equation. Take T and k at two different points and substitute
- Measure T and k at lots of points and plot a graph of lnk vs 1/T
Redox reaction
Reduction oxidation reaction in which electron transfer occurs
Ionic compound
Compound made of two or more ions
Oxidation
Loss of electrons- resulting in a positive change in oxidation number
Reduction
Gain of electrons- resulting in a negative change in oxidation number
Oxidation number
A way of identifying oxidation and reduction
When atoms exist as elements, what is their oxidation number?
Zero
What is the oxidation number of a monatomic ion?
The same as the charge on the ion
Oxidation number of hydrogen in compounds
+1
What is the oxidation number of oxygen in compounds? What is the exception?
+2
Exception is H2O2 hydrogen peroxide: -1
What can we infer about the oxidation numbers of atoms within a molecule?
The sum is zero
What can we infer about the oxidation numbers of atoms in a poly atomic ion?
The sun is equal to the charge on the ion
Half equation
Equation of one species in a redox reaction
They add together to give total reaction equation
What must both half reactions have in common?
Same number of electrons
Oxidising agent
Species which is reduced by gaining electrons- oxidises other species
Reducing agent
The species which is oxidised by losing electrons- reducing the other species
What must exist for a redox reaction to occur?
A potential difference
Units of potential difference
J C^-1
Joules per coloumb
Energy per unit charge
Reduction potential
A substance’s potential to attract electrons
What decides which species will gain electrons in a redox reaction?
The reduction potential. The species with highest (most positive) reduction potential will gain electrons.
Equation for overall potential difference
E reaction = E reduction process - E oxidisation process
What will the overall potential difference be for a spontaneous reaction?
Positive
Purpose of galvanic cells
To measure potential difference in a redox reaction
How do galvanic cells measure potential difference?
They separate the half reactions into two half cells and measure the electron transfer through the wire with a voltmeter
Two electrodes
Cathode
Anode
What does a salt bridge in a galvanic cell do?
Allows migration of ions between the cathode and anode
What occurs at the cathode?
Reduction
What occurs at the anode?
Oxidation
What indicates which electrode of a galvanic cell contains the reduction process?
The sign of the overall cell potential
Equation for overall cell potential
E cell = E RHS - E LHS
SHE
Standard hydrogen electrode
A reference electrode used to find the sign and magnitude of individual reduction potentials in a galvanic cell
What is the reduction potential of a SHE?
0.00V
Is SHE the anode or cathode?
Can act as either
E(standard) reaction = RT/nF lnQ
What does n stand for?
Number of electrons transferred
How do we find which species is being reduced in a redox reaction?
See which reduction potential is more positive
How do we balance electrons in half equations?
Multiply equations to get same number of electrons
How to find E(standard) reaction?
Subtract oxidation reduction potential from reduction reduction potential
When do we use E reaction instead of E(standard) reaction?
When conditions aren’t standard e.g. concentrations not 1 mol L^-1
Which reduction potentials depend on pH?
Those involving complex ions like MnO4^-1
Four steps for balancing redox equations
Balance for non oxygen and hydrogen ions
Balance oxygens by adding H2O molecules
Balance hydrogens by adding H+ ions
Balance charges by adding electrons
If H+ ions are present in a solution, what happens to E reaction if pH is increased?
Concentration of H+ ions will increase, therefore Q will change, and so will E reaction
What do the concentration of H+ ions ins a solution if pH=4?
1 x 10^-4
Biological standard state
All species at 1 mol L^-1 concentrations
Except H+ which is at 1 x 10^-7 mol L^-1
If we know E cell and reduction potentials, what can we find?
Concentrations
Ion selective electrode function
Can be used to measure concentration of specific ion in a solution
Concentration cells
Galvanic cell that contains the same species in each half cell at different concentrations
Do electrons flow when the half cells contain the same ions?
Yes- concentrations influence reduction potential of total cell
When writing half equations for a concentration cell, what must we ensure?
Electrons and reduced species cancel on both sides
We are just left with oxidised species on each side
When calculating E cell of concentration cells, what is E(standard) cell?
0, because at standard conditions, both concentrations at each half cell are 1 mol L^-1
Both cells contain same species, and at same concentrations, there is no difference- electrons won’t flow
Value of E cell for spontaneous reaction?
Positive
Why are ‘chemical’ oxidants not good for biological systems? (3)
Toxic
Require low pH solutions
Unselective (will oxidise anything
Which groups are oxidised in FAD?
N=C oxidised to N-H
Which group is oxidised in NADH?
C=H oxidised to H-C-H
Basic overview of metabolism (food molecules)
Food molecules are oxidised in a series of steps and the change in energy is used to perform work.
The electrons from these reactions are passed from oxidant to oxidant until they are given to O2.
Transition metal oxidants
Biological oxidants based on transition metal ions
Cytochromes
Iron-containing molecules in which the iron atom can undergo the oxidation
Enzymes that contain transition metal ions do what?
Catalyse redox reactions
Ligand
A molecule or ion in which one or more donor atoms have a lone pair of electrons
A ligand is a Lewis ____?
Base (electron pair donor)
The transition metal ion is a Lewis _____?
Acid (electron pair acceptor)
What do ligands bond to and reduce?
Transition metal cations
Most common donor atoms of a ligand (2)
Nitrogen and oxygen
Metalloproteins
Proteins in which donor atoms bond to transition metals in R groups of amino acids
In a metalloprotein, the ligand is the ____?
Amino acid
How does a metal ion get close to the R groups of amino acids?
A chain of proteins can fold itself to bring the side chains of its amino acids closer to the ion
Heme-based ligand
Other name?
Four nitrogen atoms arranged in a cyclic shape
Heme
What does (L) mean in an equation?
Ligand
What do ligands change about the metal ion they bond to?
It’s reduction potential
What determines the change in reduction potential of the metal ion by the ligand?
Their relative electron densities
List the cytochromes in order of reduction potentials
b
a
c
Which cytochrome attracts electrons the most?
Cytochrome c
