Module 2: Energetics, Rates and Driving Forces of Chemical Reactions Flashcards by Juliette Ward

Which factor, when increased, will increase the rate of any reaction?

Temperature

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In the rate law, k is constant for a given ___?

T temperature

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Three factors which affect rate (collision model)

Collision rate
Collision effectiveness
Collision orientation

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Why are the terms [A] and [B] in the rate law?

Higher concentrations increase collision rate, and therefore reaction rate

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Activation energy

The minimum energy required for breaking and forming bonds in a collision for successful reaction

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When Gibbs energy is negative, is energy still required for an effective collision?

Yes

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Boltzmann distribution (energy of molecules)

Distribution of kinetic energy is exponential

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Which quality of a molecule will increase the number of collisions with correct orientations?

Symmetry

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Which properties will molecules with low chances of correct collision orientations possess?

Lots of different functional groups (no symmetry) hard to get correct orientation

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Molecularity

Number of molecules that have to collide for a successful reaction

A way to classify elementary reactions

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Three classes of molecularity

Unimolecular
Bimolecular
Termolecular

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Which molecularity is very unlikely to cause successful collision?

Termolecular

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Transition state

Loosely bound complex containing party broken and partly formed bonds

Only exists for a short time and cannot be isolated

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‡

Transition state denotation

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k = Ae^(-Ea/RT)

Arrhenius equation

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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

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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)

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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

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Redox reaction

Reduction oxidation reaction in which electron transfer occurs

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Ionic compound

Compound made of two or more ions

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Oxidation

Loss of electrons- resulting in a positive change in oxidation number

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Reduction

Gain of electrons- resulting in a negative change in oxidation number

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Oxidation number

A way of identifying oxidation and reduction

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When atoms exist as elements, what is their oxidation number?

Zero

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What is the oxidation number of a monatomic ion?

The same as the charge on the ion

Oxidation number of hydrogen in compounds

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

Module 2: Energetics, Rates and Driving Forces of Chemical Reactions Flashcards

(88 cards)