Thermodynamics

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Reactions

A reaction consists of the following requirements

1. Reacting particles must collide
2. Collision energy must be equal to or greater than the activation energy
3. Reacting particles must collide with a suitable orientation

A reaction consists of the following process

1. As reacting particles approach, repulsive forces between electron clouds causes them to slow down causing Ek to decrease and Ep to increase
2. As the particles collide (and conditions 2 & 3 are met) the particles form a transition state/activated complex: a state corresponding to the highest Ep where reactant bonds are breaking and new bonds are forming
3. Following the formation of bonds, the activated complex disassembles with product particles moving away from each other

Enthalpy change

• $\Delta H$ = change in enthalpy, in $kJmo{l}^{-1}$
  • Change in heat energy
• System: reactants and products
  • Reactants and products both have enthalpy
  • Stored in bonds
  • Reactant (bonds break, energy required) $\to$ Products (bonds form, energy is given out)
• Surroundings: Everything that is not in the systme
• System transfers energy/heat/enthalpy to and from surroundings
• Energy cannot be created nor destroyed
• Activation energy: minimal amount of energy required for reaction to occur
• Enthalpy: total heat energy in a system

Exothermic and endothermic reactions

Exothermic Reactions

• Exothermic reactions release energy, i.e. they release heat
  • Ex = out
  • thermic: relating to heat
• Enthalpy of system decreases
• Enthalpy of surroundings increases
• Can occur spontaneously, some are explosive

1. Combustion
2. Respiration
3. Neutralisation of acids with alkalis
4. Reactions of metals with alkalies
5. Displacement reactions of metals

Endothermic Reactions

• Endothermic reactions absorb energy, i.e. they become cold
  • en = in
• Enthalpy of system increases
• Enthalpy of surroundings decreases

1. Cellular respiration
2. Photosynthesis

Catalyst

Analogy

There are 2 villages, named Reactants and Products. In between is a mountain called “Activation Energy”. To travel over the mountain is very difficult. However. We could do it another way. We could dig a tunnel in the mountain, to get to each village faster. This is basically what a catalyst does.

• Catalysts provide alternative reaction pathways that use less activation energy
• They are not consumed in the reaction
• They speed up the rate of reaction

Internal Energy

• The molecules within a body all possess kinetic energy ($E_k$)
  • This is the energy due to their random motion
• The molecules also contain potential energy($E_p$), due to chemical bonds holding them together and the bonds within their nuclei
• The sum of all these molecular kinetic and potential energies represents the body’s internal energy (U) $U=E_k+E_p$

Temperature and Heat

• The temperature of a body is a measure of the kinetic energy of its molecules
  • Temperature indicates in which direction heat will flow when 2 bodies are placed close together
• Thermal energy is the component of a body’s internal energy due to its temperature
  • Thermal energy can be supplied to a body by heating it
• Heat is the thermal energy transferred from a body at a higher temperature to a body at a lower temperature, due solely to the temperature difference.
  • Measured in Joules, not ˚C or K
• Once bodies are at the same temperature, there is no net flow of thermal energy; the bodies are in thermal equilibrium

Enthalpy (once again!)

• Enthalpy (H) is a measure of the energy stored in (or heat content of) a system
• Cannot be measured directly
  • We can calculate enthalpy change instead
• Always quoted at constant pressure
• During reactions, the enthalpy of the reactants and the products is not the same
• This results in energy being either given out or taken in during the reaction
• This energy is the enthalpy change, $\Delta H$ $\Delta H=mc\Delta T$
• c = specific heat capacity (usually of water, unless told otherwise)
  • Specific heat capacity: amount of energy to raise the temperature of 1 g of a substance by 1 ˚C
  • Water: $4.18J{g}^{-1}{C}^{-1}$
• m = mass (grams)
• $\Delta T$ = change in temperature (kelvin)

Bond Energy

The energy required to disrupt a bond/energy released in the formation of a bond. It is reflective of the strength of a bond as stronger bonds require a greater energetic input to disrupt and thus also release a greater amount of energy when formed

Bond Breaking

• Bond breaking is an endothermic process as energy is required to overcome the electrostatic force of attraction between bonding pair electrons and the positively charged nucleus’ involved in the bond
  • When bonds break, work needs to be done as a force needs to be applied that overcomes the electrostatic forces of attraction that is present between electrons and protons in the bonds
• Heat is released in a chemical reaction if it is an exothermic reaction and the enthalpy change is negative
  • Here MORE energy is released during the formation of bonds than is absorbed during the breaking of old bonds. Thus there is a net outward flow of energy(heat)from the system to the surroundings.

Bond Formation

• Bond making is an exothermic process as the total potential energy of the system decreases after a bond has been established and thus the decrease in energy is the result of energy flowing out from the system to the surroundings
• The formation of a bond represents the final stages of the reaction and therefore the products (bonded atoms) are such that the resultant molecule is stable (due to a complete valence shell being achieved).
  • Thus as the product is more stable, it contains less potential energy.
  • Therefore the decrease in potential energy that occurs during bond formation manifests as an increase in kinetic energy
• Heat is absorbed in a chemical reaction if it is an endothermic reaction and the enthalpy change is positive
  • Here MORE energy is absorbed during the breaking of old bonds than is release during the formation of new bonds. Thus there is a net inward flow of energy(heat) from the surroundings to the system.