Chemical reactions Flashcards
Bond breaking/making, nucleophiles and electrophiles, mechanisms, chemical energy, thermodynamics, equilibrium, kinetics.
Why are some chemical reactions considered to be irreversible?
For example, burning fuels.
The products cannot be easily changed back into reactants.
Many chemical reactions are reversible especially in biochemistry, organic chemistry and isomerism.
What do chemical reactions involve?
The breaking and making of bonds.
Reactions of organic compounds often involve the breaking and making of covalent bonds, therefore the movement of…
electrons.
What shows the movement of a pair of electrons?
a double-headed curly arrow
(used to draw chemical mechanisms - a series of steps to show the pathway of a chemical reaction)
How many ways can bond breaking occur? What does it depend on?
2 - how many electrons are transferred.
What bond breaking way do we focus on in BIO1332?
Heterolytic fission.
What is heterolytic fission?
A type of bond-breaking in chemistry where a covalent bond splits in such a way that both electrons from the bond go to one of the two atoms involved, rather than being split equally.
Where do the arrows start and end for heterolytic bond formation?
(If confused, look at summary sheet)
Curly arrows must start where the electrons are and end where the bond is formed.
What are nucleophiles?
Electron rich ions/molecules with a lone pair of electrons they can use to form a new dative covalent bond.
List 3 anions that are nucleophiles.
Cl-, OH -, CN-
List 4 neutral molecules that are nucleophiles.
(If confused, look at RECAP for PowerPoint).
H2O, NH3, serine (lone pair on OH group), histidine (lone pair on N).
Nucleophiles react with…
electrophiles.
What are electrophiles?
Electron deficient ions/molecules which can accept a lone pair of electrons to form a new covalent bond.
List 2 cations that are electrophiles.
Carbocations, H+.
List 4 neutral molecules that are electrophiles. Their bonds are…
(If confused, look at RECAP for PowerPoint).
polarised.
H-Cl, H3C-Cl, C=O bond in propanal, histidine (slightly positive H attached to N).
What is the OH radical?
One of the most reactive biological species.
Which direction do curly arrows always flow in?
The direction of electron travel (from the nucleophile to the electrophile).
There are three main acid-base theories, which one do we look at in BIO1332?
Bronsted-Lowry theory.
What is an acid?
A proton donor (lower pKa values).
What is pH?
A measure of the concentration of hydrogen ions.
What is a base?
A proton acceptor (higher pKa values).
Lysine and arginine are weaker bases than…
histidine.
Acid-base enzyme catalysed reactions utilise amino acid side chains to facilitate…
proton transfer.
What is oxidation?
A reactant loses an electron (or sometimes gains oxygen).
What is reduction?
A reactant gains an electron (or sometimes loses oxygen/gains hydrogen.
What are oxidoreductases?
Enzymes that catalyse electron transfer during redox reactions.
What does the question ‘how much product will form’ relate to?
Thermodynamic factors (relating heat, work, temperature and energy).
What is the question ‘ how much product will form’ determined by?
The equilibrium constant (K).
What does the question ‘how fast will the product form’ relate to?
Kinetic factors (factors affecting reaction rate).
What is the question ‘how fast will product form’ determined by?
The rate constant (k).
What is chemical energy?
The potential energy stored in chemical bonds.
What is thermodynamics?
The study of the laws that govern the conversion of energy from one form to another.
Energy changes are a characteristic feature of chemical reactions.
Many chemical reactions release energy. Give examples.
Some chemical reactions absorb energy.
What is energy measured in?
Heat/light/sound/chemical.
Joules (J).
What kind of process is bond breaking?
Why?
Endothermic - it requires an input of energy (e.g. heat/kinetic).
What kind of process is bond making?
Why?
Exothermic - energy is released (e.g. light/heat/sound).
Reaction profiles follow a reaction’s thermodynamics and…
(look at the summary sheet for a diagram)
kinetics.
What is Gibb’s free energy change in simple terms?
Difference in chemical energy between reactants and products.
What does a negative Gibb’s free energy change mean for a reaction?
It’s thermodynamically favourable and spontaneous (once started, the reaction occurs by itself without any external energy).
What does a positive Gibb’s free energy change mean for a reaction?
It’s thermodynamically unfavourable and non-spontaneous (reaction requires application of constant external energy.
HCl + (CH3)3 ⇌ (CH3)3CCl + H2O
What is present at the ‘end’ of the reaction?
Both starting materials and products.
Define equilibrium.
The rate of the forward reaction is equal to the rate of the reverse reaction.
The concentrations of the reactants and products do not change.
How is the equilibrium constant K calculated?
[products]/[reactants]
What do values for K>1 imply?
A high concentration of products at equilibrium.
What do values for K<1 imply?
A low concentration of products at equilibrium.
For the particular reaction looked at, as the temperature increases, K increases.
What does this imply?
More product is will be made at higher temperatures.
How does Gibbs free energy link to the equilibrium constant?
∆G = -RTlnK
If ∆G is negative K is…
> 1.
What is kinetics?
The study of rates of reaction.
If ∆G is positive K is…
< 1.
Reactions with large equilibrium constants do not always occur rapidly.
What needs to happen for a reaction to occur between two molecules?
- The molecules must collide with each
other. - Molecules must collide in an orientation
that facilitates bond making/breaking. - Have activation energy (sufficient energy
to start the reaction.
The activation energy determines how fast a reaction will go that is…
thermodynamically favourable.
Slower reactions have higher activation energies than…
faster ones.
What is the rate constant, k?
A measure of the speed of a chemical reaction (independent of concentration).
A higher rate constant means a…
faster reaction.
Write the Arrhenius equation.
k = Ae^(-Ea/RT)
A lower activation energy means a larger value for k so a…
faster reaction.
A higher temperature means a larger value for k so a…
faster reaction.
What are catalysts?
Compounds that speed up a chemical reaction without undergoing any permanent chemical change.
What are enzymes known as?
Biological catalysts.
How do catalysts increase the rate of chemical reactions?
Provide an alternative reaction pathway with a lower activation energy.
