Module 5 (Remember to do questions on equilibrium graphs week 3 and write syllabus points each flashcard covers)
Examples of irreversible reactions
Combustion of fossil fuels
Reaction between Mg and O2
Define dynamic equilibrium
The rates of forward and reverse reactions are equal and simultaneous but non-zero. Called dynamic since there is always movement of particles in both f.r and r.r. This is the type of equilibrium more often seen in chemical reactions. On a macroscopic level, the concentration of reactants and products are constant but on the microscopic level, reactions are continually occurring.
F.r proportional to reactants
Define static equilibrium
The rates of forward and reverse reaction are equal and zero (or practically zero). There is no reaction in either direction once equilibrium has been reached, due to the forward and reverse reactions having extremely large activation energies.
Values of ΔH, ΔS and ΔG in forward and reverse reactions
They will be the negative of that for the forward reaction meaning that it the forward reaction is exothermic, then the reverse reaction is endothermic and vice versa
Colours of CoCl2.6H2O originally, when heated, when water is added, when evaporated
The cobalt (II) chloride is a dark pink colour. When it is heated, it turns into a purple colour, and then after a while, it turns sky blue in colour. When water is added, the solution is a dark pink. When evaporated, same sequence of colour changes repeat.
Cause of the colours of CoCl2.6H2O originally, when heated, when water is added, when evaporated
The hexahydrate is pink in colour, the dihydrate is purple and the anhydrous form is sky blue. Different forms of cobalt (II) chloride can interconvert by hydration or dehydration.
CoCl2.6H2O(s)⇌CoCl2.2H2O(s) + 4H20(l)
CoCl2.2H2O(s)⇌CoCl2 + 2H2O(l)
When the hexahydrate is heated, the waters of crystallisation are gradually vaporised, and the purple dihydrate form is observed before the anhydrous form. When cobalt (II) chloride is dissolved in water, it produces a pink solution similar to the colour of the hexahydrate solid. This is because all the waters of crystallisation, would go into solution so the appearance of cobalt (II) chloride solution is the same regardless of the hydration number of the solid that was dissolved.
If the water is then allowed to evaporate without heating, CoCl2.6H2O(s) will be reformed. However reverse and forward reactions don’t occur simultaneously.
Observations in combustion of magnesium strip and steel wool
White solid forms in crucible as magnesium strip is heated. When placed in an ice bath, no changes occurs.
A reddish-brown solid forms when steel wool is heated. When placed in an ice bath, no changes occur.
Describe an closed system
A constant number of particles in the system (no matter transfer)
Energy can be transferred, (dependent on the wall of the system, either allowing the transfer of heat energy (rigid) or work (adiabatic).
Example is saucepan with lid, lid can transfer energy to the environment via radiation and conduction. However, no matter enters or exits the saucepan.
This is required for static and dynamic equilibria
Describe an open system
System can interact with the surroundings allowing the exchange of matter and energy
Types of static equilibrium
Irreversible reaction at completion
Initially, the rate of forward reaction starts high. It reduces over time as the concentration of reactants of reactants reduces
The reverse reaction doesn’t occur, so has a rate of zero
Eventually, the rate of forward reaction reaches zero, and static equilibrium is reached
An example of this type is the dissolution of salt in an unsaturated solution
NaCl(s) –>Na+(aq)+Cl-(aq)
Irreversible reaction before initiation
If insufficient energy is inputted, then neither the forward nor reverse reactions will occur so their rates will be equal to zero
An example of this is the combustion of a fuel without an initial spark
C8H18(l) +25/2 O2(g) –>8CO2(g) + 9H2O(l)
Without an initial spark, there is insufficient energy to overcome the activation energy barrier, so the fuel doesn’t combust
Reversible reaction with insurmountable activation energy
An example of this is the diamond-graphite equilibrium
This reaction has ΔG=-2.90 kj mol^-1, so it should be spontaneous. However, it demands an almost insurmountable activation energy, especially at room temperature and hence the reaction doesn’t happen. Hence, f.r and r.r of reaction are almost 0.
Advantages of modelling dynamic eq for the experiment involving cylinder A and B where water in the straws in cylinder A exchange water to cylinder B
It models how the fr is proportional to the amount of reactants, and the rate of reverse reaction is proportional to the amount of products.
Initially, the fr rate is high since the amount of water in cylinder A is high
Over time, the rate of fr decreases and the rate of r.r increases as more water is transferred to cylinder B
Shows that dynamic eq is reached when the lvls of water in cylinder A and B reach a constant lvl, the rates of fr and r.r=
The eq can be reestablished if its disturbed where the eq can be disturbed by adding more water to either cylinder
In this scenario, the rates of the f.r and r.r will instantaneously become different. Over time, a new eq will be reached.
Disadvantages of modelling dynamic eq for the experiment involving cylinder A and B where water in the straws in cylinder A exchange water to cylinder B
System isn’t closed since water can evaporate or be spilled
This model doesn’t deal with concentrations; it only deals with volumes
This system only models one reactant and one product
How does gibbs free energy impact equilibriums
Chemical reactions proceed towards a point of lowest G i.e Gibbs free energy
The extent of a reaction is used to describe the proportion of reactants that have been converted to products
Extent of reaction at lowest G Definition Implications
0% No reaction has occurred This reaction doesn’t occur spontaneously
x% (0-100%). x% of reactants by moles This reaction reaches a dynamic equilibrium
have converted to products
100% All reactants have converted to products This reaction occurs
spontaneously to completion
Difference between ΔG° and ΔG
ΔG° is the standard free energy change which has a single value for a particular reaction at given temperature and pressure.
It is the change in Gibbs free energy for a reaction that goes to completion.
ΔG varies with the extent of reaction and used to describe reactions that can reach dynamic eq. It measures the distance in free energy terms of a particular reaction mixture from reaching dynamic eq.
Key things to remember about gibbs free energy
ΔG°<0 doesn’t mean it is spontaneous always as this only works for reactions where ΔG=0 at either 0% or 100% extent of reaction (static equilibria)
Some dynamic equilibrium reactions can have ΔG°>0 but still proceed to equilibrium since ΔG=0 somewhere between 0% and 100% extent of reaction.
If a reaction reaches ΔG=0 at any extent of reaction that isn’t 0% or 100%, then the reaction will reach dynamic equilibrium. ΔG° doesn’t have to be 0 for dynamic equilibrium to be possible.
However, if a reaction has ΔG°=0, then it has ΔG=0 at every extent of reaction (0%-100%), so the reaction will definitely reach dynamic equilibrium.
Define enthalpy
The internal heat energy of a system. measured in Jmol^-1 pr kjmol^-1. If ΔH<0, the reaction has forward enthalpy drive hence exothermic reactions
If ΔH>0, the reaction has backward/reverse enthalpy drive hence endothermic reactions
How to determine entropy drive
Measured in jmol^-1K^-1 or kjmol^-1K^-1, if ΔS>0, the reaction has forward entropy drive
If ΔS<0, the reaction has backward/reverse entropy drive
What are the law of thermodynamics
Zeroth law: if two systems are in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
1st law: Energy movement into or out of a system is in accordance with the law of conservation of energy
2nd law: The entropy of an isolated system will increase over time, approaching a maximum value at equilbrium. Another interpretation is that the entropy of the universe must be increasing i.e ΔS(universe)=ΔS(system)+ΔS(surrounding)>0
3rd law: The entropy of a system approaches a minimum as temperature approaches zero.
Entropy changes in combustion
The combustion of solid sulfur has a positive entropy change. The temperature increases as the reaction proceeds with more heat energy needing to be dispersed thus producing a greater degree of disorder. General combustion reactions have forward entropy drive for example the combustion of octane (C8H18).
C8H18(l)+25/2 O2(g) ->8CO2(g) +9H20(l)
When octane is combusted at reaction conditions with a spark or flame as the initial source of energy, the temperature is much higher than the bp of water, so gaseous water is formed as a product first
There are 25/2 moles of gaseous O2 on the reactants side, and 17 moles of gas initially on the products side. This means that the entropy drive is forward,
However, we write this equation with water in liquid state to comply with the convention of writing combustion equations at standard conditions.
Entropy changes in photosynthesis
Photosynthesis reduces the entropy of the system by creating ordered glucose molecules from free carbon dioxide and water molecules.
However, we also know that photosynthesis is endothermic, so this reaction has reverse entropy and enthalpy drive and will never occur spontaneously.
A continuous supply of energy in the form of sunlight is required to drive this reaction in plants
Does photosynthesis violate the 2nd law of thermodynamics
Endothermic reactions typically have ΔSsurrounding<0. Then theoretically, this would mean that photosynthesis decreases the entropy of the universe since ΔSuniverse=ΔSsystem +ΔSsurrounding and both seem to be less than 0. This would violate this law. However, since this reaction also uses the UV energy from the sun thus it cannot be classed as an isolated system. If we consider the Sun as part of the system, then the energy released from the Sun is actually absorbed and reflected off the plant into a state of greater disorder. Indeed, the continuous photo excitation from the Sun excites the pigment molecules in the plant including chlorophyll and increases the entropy of their electrons. This amount of disorder is greater than the order gained from organisation of the molecular energy into gluose, i.e ΔS(surrounding), is actually positive, and thus the overall entropy of the universe increases.
Relationship between ΔG° and reversibility
In general, reversible reactions tend to have competing entropy and enthalpy drives.
A chemical reaction with ΔG°>0 for all temperatures is non-spontaneous and likely to be an irreversible chemical reaction. One example is photosynthesis, where ΔH°>0 and ΔS°<0
A chemical reaction with ΔG°<0 for al temperatures is spontaneous and likely to be an irreversible chemical reaction an example being combustion where ΔH°<0 and ΔS°>0.
In general reversible reactions tend to have either:
1. ΔH°<0 (forward enthalpy drive) and ΔS°<0 (reverse entropy drive) OR
2. ΔH°>0 (reverse enthalpy drive) and ΔS°>0 (forward entropy drive)
2NH3(g)⇌N2(g) +3H2(g) ΔH=+92 kjmol^-1
Explain why this reaction is more likely to reach dynamic eq than static eq
This reaction has reverse enthalpy drive as it is endothermic. It has forward entropy drive as ΔS°>0. This is because 2 moles of gas on the LHS are being converted to 4 moles on the RHS, which is an increase in entropy. Therefore, the sign of ΔG° depends on the temperature, so it is not always spontaneous or non-spontaneous. These properties means that it is more likely to reach dynamic eq than static eq.
2NO2(g)⇌N2O4(g). This reaction reaches dynamic eq.
Explain whether the forward reaction is more likely to endothermic or exothermic.
This reaction has a reverse entropy drive i.e ΔS°<0. This is because there are 2 moles of gas on the LHS and 1 mole of gas on the RHS, so there is a net reduction in entropy. Since the reaction reaches dynamic eq, the enthalpy drive is likely forward be an irreversible non-spontaneous reaction, and instead reach static eq. Therefore, the f.r. is likely to be exothermic i.e ΔH<0.
