Organic chem cont + thermodynamics Flashcards
What is the main function of hybridisation?
It is a labelling technique. Mathematical concept
What is hybridisation?
It is a label given to the geometry around a given atom in a molecule –> refers to positions around a central atom. I.e. depends on whether it is tetrahedral, trigonal, straight shape etc.
What is sp3 hybridisation?
Occurs when a 2s orbital mixes with 3 2p orbitals, forcing 4x identical orbitals to form which forms 4 single covalent bonds
How can you tell if there is sp3 hybridisation?
Look at the electron groups around an atom (bonds and lone pairs). If it gives a tetrahedral shape, it is a sp3 hybridisation
What is sp2 hybridisation?
Normally forms with alkenes. Occurs if 2s orbital mixes with only 2 2p orbitals, as a result 1 p - orbital remains unhybridised and can be used to form the ‘double bond’
How can you tell if there is sp2 hybridisation?
Look at the electron groups aroudn an atom and see if it forms a trigonal pyramidal shape around the atom (3 bonds)
What is sp1 hybridisation?
Occurs if a 2s orbital mixes with only one 2p orbital. As a result, 2 p-orbitals remain unhybridised and can be used to form the ‘triple bond’
How can you tell if there is sp1 hybridisation?
The way to identify this hybrid is to look for a linear shape around an atom (2 bonds).
Is hybridisation limited to carbon only?
No it isn’t. There are also other atoms which can hybridise as well such as heteroatoms
What are the properties of alkanes in terms of intermolecular forces and the boiling point, melting point etc. ?
Molecules are held together by Van der Waals (dispersion) forces. These forces increase as the size of the molecule increases.
Because MP and BP are determined by the strength of the intermolecular forces, as the dispersion forces increase as the carbon chain gets longer, MP and BP actually increases as well.
However, branching (e.g. methyl side chains) lowers the melting/boiling point because it forces chains away from each other a bit more
Describe the combustion of alkanes
Short chain alkanes burn efficiently –> CO2 + H2O
Bigger alkanes don’t burn as well –> sooty flame
Large amounts of energy liberated in combustion. Commonly used in fuel, e.g. natural gas, petrol, etc –> environmentla consequences
What are isomers?
They have the same molecular forma but different structures?
What are the 2 categories of isomers?
Constitutional isomers and stereoisomers
What are constitutional isomers?
Different nature/sequence of bonds - in other words its structural isomers
What are stereoisomers?
Different arrangement of groups in space –> different patterns
What are the 2 categories of stereoisomers?
Conformational and configurational isomers
What are conformational isomers?
Isomers which differ by rotation about a single bond
This process doesn’t require too much energy so they are normally separable
What are configurational isomers?
Interconversion requiring breaking of bonds
This process normally requires a lot of energy so doesn’t happen at room temp
What are the 2 categories of configurational isomers?
Diastereoisomers / diastereomer or enantiomers
What are enantiomers?
Mirrored and Non superimposable isomers
What are diastereomers?
Not mirror images and not superimposable
What are the different properties of isomers? WHat are they determiend by?
Physical and chemical properties of constitutional isomers can be very different, particularly when different functional groups are present
Each functional group present will have different properties and reactivity which come with it
WHat is a double bond constructed from?
Sigma and pi bond
What is a pi bond formed by?
p - orbital overlap and are directional (electron density concentrated above and below the plane)
What are the two labels for diastereomers?
‘Z’ or ‘E’
What are the rules involved with naming double bond isomers?
Rules involve:
Assign priority to each group on each carbon of C=C
the higher the atomic number of the atom attached the higher the priority
If identical atoms attached, work along the chain until first point of difference, then go by atomic number
When do we use a Z double bond naming?
If high priority groups are on the same sides of C=C
When do we use an E double bond naming?
If high priority groups are on the opposite sides of C = C
When do we use cis and trans?
cis = priority groups on the same side
trans = priority groups on the opposite sides
Check disubstituted cycloalkanes and conformational isomers and newman projections in book
ok
What are the barriers to rotation?
In ‘straight chain’ alkanes, rotation about C-C bonds is rapid at room temp. Differences in energy are from steric interactions
The wider apart the H’s are, it reduces repulsion –> less energy
Why do configurational isomers form?
They could arise due to absence of rotation about a double bond
Can also arise when an sp3 atom has 4 different bonding partners
What does chiral mean?
An object is chiral if it isn’t super-imposable upon its mirror image
WHat are the properties of a chiral molecule?
Contains a stereogenic centre (4 unique bonds/ atoms)
It isn’t superimposable on its mirror image
Has no plane of symmetry in the molecule
What are enantiomers?
Pair of molecules which are:
mirror images of each other
aren’t superimposable
What is a stereocentre?
Also called a stereogenic centre or chiral centre, it is a term used to describe a carbon atom with 4 different groups attached
What is a racemic mixture?
It is a 50:50 mixture of both enantiomers (R or S) or (C or D)
What is the modern and older convention for labelling possibilities of enaantiomers?
R or S
OR
L or D
Why does chirality matter?
As molecular size increases –> chiral molecules are more frequent
Almost all physical properties of a pair of enantiomers are identical
Enantiomers behave in an identical way when interacting with achiral objects or reagent
Enantiomers will interact and react differently with other chiral molecules
Almost all biological catalysis by enzymes is selective for a particular enantiomer
What are examples of chiral pharmaceuticals?
Ventolin is sold as a racemic mixture but only the R enantiomer is active, and the S enantiomer slows down elimination of drug from the body
Thalidomide was also sold as a racemic mixture. The R enantiomer is a mild sedative, however the S enantiomer was found to be a potent tetratogen which can cause birth defects –> tighter control of enantiomer specification
How can chirality be detected?
Through optical activity. In plane polarised light, waves oscillate in only one plane
Their interaction with plane polarised light is generated by passing light through a polarising filter
How is optical activity measured?
When plane polarised light is passed through a solution of one pure enantiomer of a compound, the plane of polarisation is rotated
The enantiomer is said to be optically active
The amount of rotation is characteristic of the enantiomer
There are two directions of light - L and R different enantiomers will either shift left or right in terms of light
What does a rotation in a clockwise direction mean for an enantiomer?
Labelled (+)
What does a rotation in a anticlockwise direction mean for an enantiomer?
Labelled (-)
WHat is the total rotation of racemic mixtures?
Overall rotation of zero
Is it possible to predict which enantiomer is (-) or (+)?
No it isn’t without performing the experiment
What are diasteroisomers?
Configurational isomers which aren’t enantiomers are called diasteroisomers
All achiral (non-chiral) configurationa isomers are diasteroisomers
Molecules that contain two or more stereogenic centres can be either enantiomers or diasterisomers
How do we know how many isomers can form with n stereogenic centres?
It is always 2 to the power of n isomers
2^n isomers, where n is the number of stereocentres
How many joules of energy does our daily diet require?
10^7 joules
How many joules is 1 calorie?
1 calorie = 4.184 J
This is the energy required to heat 1g of water by 1 degree celsius
How much is 1 Calorie worth?
Be careful of the capital letter
1 Calorie = 1 kcal
= 1000 calories
What is a system?
part of the universe we are focussed on (i.e. a reaction)
What is a surrounding?
Everything else other than the system
WHat is the universe?
system + surroundings
What is internal energy?
It is the total energy (potential and kinetic of the system)
What are exothermic reactions?
Reactions which release heat
Occurs when products have lower internal energy than reactants
U (products) < U (reactants)
U(products) - U (reactants) < 0
What are endothermic reactions?
Reactions which absorb heat to be sustained
Products have greater internal energy than reactants
U (products) > U (reactants)
U(Products) - U (reactants) > 0
What are examples of work(w)?
Expansion and compression of gas
Light
Electrical
What is the first law of thermodynamics?
The change in internal energy of a chemical system is the sum of heat absorbed (q) by and the work (w) done on the system
Change in U = q + w
What is calorimetry?
Measuring internal energy change at a constant volume
What is heat capacity?
the amount of heat needed to cause a unit change in temperature in a given mass of material.
What needs to be considered with heat capacity?
More of a substance –> decreased temp rise for given input of heat
Different substances will change temp by a different amount (e.g. copper vs ceramic)
q = const x change in temp
The constant must be mass or moles dependent and be different for different substances
What are the different formulas for q?
q = mcΔT
q = nCΔT
ΔT = change in temp
m = mass of substance (g)
n = amount of substance (moles)
c = specific heat capacity (Jg^-1K^-1)
C = molar heat capacity (Jmol^-1K^-1)
Specific heat varies for each variable
Explain Joule annd the conversion of work into heat?
Prior to Joule, the established idea was that heat was a mass - less invisible fluid called caloric that flowed from high temp to low temp
Joule measured how work (due to a falling vessel) could result in an increase in temp (effect associated with heat)
Explain the concept of work arising from expansion or compression?
Work done on or by a system takes many forms. One common form of work in systems in contact with atmosphere arises when the system changes its volume ‘V’
Either:
Expanding –> ΔV > 0
Compressing –> ΔV < 0
The work w done on the system due to volume change ΔV in contact with a pressure P is given by:
w = -p x ΔV
Why is there a negative sign for ‘w = -p x ΔV’
Negative sign because energy of system is increased when compressed, so that w is > 0, when ΔV < 0 (i.e. compression). Since P is positive, we have to include a minus sign to ensure that the reaction holds
What is the formula for change in internal energy?
ΔU (int) = q - PΔV
What are the q values for exothermic and endothermic reactions?
exothermic : q < 0
endothermic : q > 0
What are some examples of important reactions?
enthalpy of fusion (‘melting’)
enthalpy of combustion
enthalpy of atomisation
What is enthalpy of atomisation
Converting 1 mol of a compound into gaseous atoms:
Energy required to reduce a molecule to its atoms is the energy required to break all bonds
Average bond enthalpies are available for us to estimate this energy
Breaking bonds / enthalpy of atomisation is endothermic because we need to increase energy to break them
What is the formula for Δ(reaction) H?
Δ(reaction)H = Δ(bond breaking)H + Δ(bond forming)H
Δ(bond forming)H is negative
What is collision theory?
Molecules must collide to react, and they must do so with the correct orientation and with sufficient energy to react
What is the activation energy? What are some important things to know about activation energy (Ea)?
It is the minimum energy molecules must have to react successfully. Products are only formed if Ea is overcome.
Rate of reaction depends on how quick Ea is overcome.
Formation of transition state is the rate determining step
What is Hess’s law?
Hess’s Law is a principle in chemistry stating that the total enthalpy change in a chemical reaction is independent of the pathway taken to obtain the final products. In simpler terms, it means that the total energy change for a reaction is the same whether it occurs in one step or several steps.
This is important as it allows us to calculate thermodynamic quantities for reactions we haven’t or can’t measure.
Allows us to do that equation and cross out thing
What are standard conditions defined by?
Pressure = 1 atm
Temperature = 298 K
Concentration = 1 mol / L
What is the symbol for standard conditions?
°, which is added to the thermochemical symbol
What is enthalpy of formation?
Enthalpy change when a compound is made from the stable form of its elements at the standard condition
What are standard states?
when we talk about standard states, we mean the conditions where substances are typically found or measured.
(i.e. at 1 atm, 298K)
Why are standard states important for enthalpy of formations?
This is because if either the reactant or product has the standard state of an element (i.e. O2 gas in reactants), it wont need to use an enthalpy of formation, because it doesnt need to be formed as it exists naturally. This doesn’t apply to compounds. Only pure elements such as O2 Cl2, C (s) etc.
What is the change in reaction enthalpy equation?
Sum of enthalpy of formation of products - sum of enthalpy of formation of reactants
What are thermite reactions used for?
Used to weld railroad tracks in absence of electricity to power welders.
Variant of this reaction is used as igniter/catalyst in solid fuel for rockets
What is the change in enthalpy of formation equation?
Enthalpy change of products - enthalpy change of reactants
What are some ways of measuring fuel efficiency
Energy per mole of CO2 produced
Energy per gram or per litre of fuel
Energy per mole of fuel
Probably, energy per gram/litre of fuel is most important for tasks such as space missions
How could we rate fuels?
Consider both energy per mol of CO2 and also energy produced per gram/litre
What are some of the best fuels?
Lignite (brown coal) is the main coal type found in Aus. it has low carbon content and high water and sulfur content
when burnt, lignite generates pollutants and due to energy loss arising from diving off remaining water, brown coal produces more CO2 per unit of power than any other energy sources
By calculations, it should theoretically be methane
What is a spontaneous reaction?
Reactions that happen without any additional external energy
What is the first law of thermodynamics?
CHange of internal energy of a chemical system is the sum of heat absorbed (q) by and the work (w) done on the system
Change in internal energy = q + w
Energy can’t be created or destroyed, only converted
What is the limitation of first law of thermodynamics?
It cant tell us anything about spontaneity
For example, heat flowing from hot to cold or heat flowing from cold to hot - both satisfy the first law of thermodynamics but only heat flowing from hot to cold actually occurs spontaneously. Why?
What is entropy?
Entropy refers to degree of disorder of randomness in the system
What is Clausius’ equation for change in entropy?
Change in entropy = heat flow / temperature
What happens as heat flow increases in a system?
Increases change in entropy
What happens as there is a lower temperature into a system?
Bigger increase in entropy
Heat added to system at low temp could cause greater randomness than in comparison to when heat is added to it at a higher temp
If we assume that heat flows from a hot object to a cold object, what are the rules asscoiated with this? (This is a rule by the way)
Change in entropy hot = -q / t (hot)
Change in entropy cold = q / t (cold)
Thus, if q > 0, Change in entropy total = change in entropy hot + change in entropy cold > 0
Because q / t (cold) would be very large compared to q / t (hot)
Thus, we require that Change in total entropy > 0, for q to be q > 0 (i.e. to explain the observation of heat flow from hot to cold)
What is the rule for total change in entropy?
Total change in entropy > 0
What is the 2nd law of thermodynamics?
States that for any process to occur spontaneously, the total entropy must increase
Total change in entropy > 0
This 2nd law always predicts the outcomes we can observe in nature
What are some trends in entropy (3)?
Entropy increase in a material when temperature increases
Entropy increases when molecules go from a dense state (liquids and solid) to the gas state
Entropy decreases when an ideal gas is compressed at constant pressure
What is the 3rd law of thermodyanmics
A material at absolute zero is defined as having an entropy of zero
This provides a reference point from which all other entropies can be measured
Describe the increase in entropy as temperature increases, changing a phase from solid to gas?
There is a continuous increase in entropy as temperature is raised for a single phase
Smaller sudden increases in entropy in going from solid to liquid compared to the large increase in entropy in going from liquid to gas (check out graph)
What is an isolated system
a thermodynamic system enclosed by rigid immovable walls through which neither mass nor energy can pass
What is the total change in entropy in an isolated system?
total change in entropy = change in entropy of a system
What is the total change in entropy in an open system?
change in entropy of a system + change in entropy of surroundings = total change in entropy
What is the inequality which must be true for any spontaneous process?
change in system entropy + change in entropy of surroundings > 0
How can we calculate change in entropy of system?
We can use standard entropies (provided) to calculate the change in entropy of a reaction, in the same way as heat of formations to predict heat of reaction
I.e.
change in entropy of system = change in entropy of products - change in entropy of reactants (at standard conditions)
ow do we calculate change in entropy of surroundings?
Recognise:
q surroundings = -q system
q surroundings = - enthalpy change of system
(check books)
WHat happens if change in total entropy is -ve?
Isnt spontaneous
Explain spontaneity of exothermic reactions
In exothermic reactions, most or all of entropy increases comes from change in entropy of surroundings due to flow of heat into surroundings from reaction system
Explain spontaneity of endothermic reactions
In endothermic reactions all of entropy increases comes from change in entropy of system. This entropy increases comes from various sources such as transformation of solid –> liquid, increase in entropy of solvent arorund solutes etc.
What is Δ G sys
Δ G sys = - TΔ S
What can Δ S total be given by?
Δ S system - Δ H / T
How do we know if a system is spontaneous (using entropy or gibbs free)
Change in entropy total/universe > 0
Change in gibbs free energy < 0
