Chemical reactions

Question 1

Why are some chemical reactions considered to be irreversible?
For example, burning fuels.

Answer 1

The products cannot be easily changed back into reactants.

Question 2

Many chemical reactions are reversible especially in biochemistry, organic chemistry and isomerism.

What do chemical reactions involve?

Answer 2

The breaking and making of bonds.

Question 3

Reactions of organic compounds often involve the breaking and making of covalent bonds, therefore the movement of…

Answer 3

electrons.

Question 4

What shows the movement of a pair of electrons?

Answer 4

a double-headed curly arrow

(used to draw chemical mechanisms - a series of steps to show the pathway of a chemical reaction)

Question 5

How many ways can bond breaking occur? What does it depend on?

Answer 5

2 - how many electrons are transferred.

Question 6

What bond breaking way do we focus on in BIO1332?

Answer 6

Heterolytic fission.

Question 7

What is heterolytic fission?

Answer 7

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.

Question 8

Where do the arrows start and end for heterolytic bond formation?

(If confused, look at summary sheet)

Answer 8

Curly arrows must start where the electrons are and end where the bond is formed.

Question 9

What are nucleophiles?

Answer 9

Electron rich ions/molecules with a lone pair of electrons they can use to form a new dative covalent bond.

Question 10

List 3 anions that are nucleophiles.

Answer 10

Cl-, OH -, CN-

Question 11

List 4 neutral molecules that are nucleophiles.

(If confused, look at RECAP for PowerPoint).

Answer 11

H2O, NH3, serine (lone pair on OH group), histidine (lone pair on N).

Question 12

Nucleophiles react with…

Answer 12

electrophiles.

Question 13

What are electrophiles?

Answer 13

Electron deficient ions/molecules which can accept a lone pair of electrons to form a new covalent bond.

Question 14

List 2 cations that are electrophiles.

Answer 14

Carbocations, H+.

Question 15

List 4 neutral molecules that are electrophiles. Their bonds are…

(If confused, look at RECAP for PowerPoint).

Answer 15

polarised.

H-Cl, H3C-Cl, C=O bond in propanal, histidine (slightly positive H attached to N).

Question 16

What is the OH radical?

Answer 16

One of the most reactive biological species.

Question 17

Which direction do curly arrows always flow in?

Answer 17

The direction of electron travel (from the nucleophile to the electrophile).

Question 18

There are three main acid-base theories, which one do we look at in BIO1332?

Answer 18

Bronsted-Lowry theory.

Question 19

What is an acid?

Answer 19

A proton donor (lower pKa values).

Question 20

What is pH?

Answer 20

A measure of the concentration of hydrogen ions.

Question 21

What is a base?

Answer 21

A proton acceptor (higher pKa values).

Question 22

Lysine and arginine are weaker bases than…

Answer 22

histidine.

Question 23

Acid-base enzyme catalysed reactions utilise amino acid side chains to facilitate…

Answer 23

proton transfer.

Question 24

What is oxidation?

Answer 24

A reactant loses an electron (or sometimes gains oxygen).

Question 25

What is reduction?

Answer 25

A reactant gains an electron (or sometimes loses oxygen/gains hydrogen.

Question 26

What are oxidoreductases?

Answer 26

Enzymes that catalyse electron transfer during redox reactions.

Question 27

What does the question 'how much product will form' relate to?

Answer 27

Thermodynamic factors (relating heat, work, temperature and energy).

Question 28

What is the question ' how much product will form' determined by?

Answer 28

The equilibrium constant (K).

Question 29

What does the question 'how fast will the product form' relate to?

Answer 29

Kinetic factors (factors affecting reaction rate).

Question 30

What is the question 'how fast will product form' determined by?

Answer 30

The rate constant (k).

Question 31

What is chemical energy?

Answer 31

The potential energy stored in chemical bonds.

Question 32

What is thermodynamics?

Answer 32

The study of the laws that govern the conversion of energy from one form to another.

Question 33

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?

Answer 33

Heat/light/sound/chemical. Joules (J).

Question 34

What kind of process is bond breaking? Why?

Answer 34

Endothermic - it requires an input of energy (e.g. heat/kinetic).

Question 35

What kind of process is bond making? Why?

Answer 35

Exothermic - energy is released (e.g. light/heat/sound).

Question 36

Reaction profiles follow a reaction's thermodynamics and... (look at the summary sheet for a diagram)

Answer 36

kinetics.

Question 37

What is Gibb's free energy change in simple terms?

Answer 37

Difference in chemical energy between reactants and products.

Question 38

What does a negative Gibb's free energy change mean for a reaction?

Answer 38

It's thermodynamically favourable and spontaneous (once started, the reaction occurs by itself without any external energy).

Question 39

What does a positive Gibb's free energy change mean for a reaction?

Answer 39

It's thermodynamically unfavourable and non-spontaneous (reaction requires application of constant external energy.

Question 40

HCl + (CH3)3 ⇌ (CH3)3CCl + H2O What is present at the 'end' of the reaction?

Answer 40

Both starting materials and products.

Question 41

Define equilibrium.

Answer 41

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.

Question 42

How is the equilibrium constant K calculated?

Answer 42

[products]/[reactants]

Question 43

What do values for K>1 imply?

Answer 43

A high concentration of products at equilibrium.

Question 44

What do values for K<1 imply?

Answer 44

A low concentration of products at equilibrium.

Question 45

For the particular reaction looked at, as the temperature increases, K increases. What does this imply?

Answer 45

More product is will be made at higher temperatures.

Question 46

How does Gibbs free energy link to the equilibrium constant?

Answer 46

∆G = -RTlnK

Question 47

If ∆G is negative K is...

Answer 47

> 1.

Question 48

What is kinetics?

Answer 48

The study of rates of reaction.

Question 48

If ∆G is positive K is...

Answer 48

< 1.

Question 49

Reactions with large equilibrium constants do not always occur rapidly. What needs to happen for a reaction to occur between two molecules?

Answer 49

- 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.

Question 50

The activation energy determines how fast a reaction will go that is...

Answer 50

thermodynamically favourable.

Question 51

Slower reactions have higher activation energies than...

Answer 51

faster ones.

Question 52

What is the rate constant, k?

Answer 52

A measure of the speed of a chemical reaction (independent of concentration).

Question 53

A higher rate constant means a...

Answer 53

faster reaction.

Question 54

Write the Arrhenius equation.

Answer 54

k = Ae^(-Ea/RT)

Question 55

A lower activation energy means a larger value for k so a...

Answer 55

faster reaction.

Question 56

A higher temperature means a larger value for k so a...

Answer 56

faster reaction.

Question 57

What are catalysts?

Answer 57

Compounds that speed up a chemical reaction without undergoing any permanent chemical change.

Question 58

What are enzymes known as?

Answer 58

Biological catalysts.

Question 59

How do catalysts increase the rate of chemical reactions?

Answer 59

Provide an alternative reaction pathway with a lower activation energy.