Chemical Changes and Energy Redistribution
Examples: food metabolism, fuel combustion, mixing acids and bases, dissolution of ammonium nitrate in water
Exothermic
Processes that release energy as heat are called
exothermic processes. Products have less energy than reactants.
Endothermic
Processes in which products have more energy than reactants
Kinetic energy
the energy of a moving object (in thermochemistry the moving objects are particles such as atoms, molecules, or ions and motion includes translation, vibration, and rotation of particles).
Potential energy
results from the position of objects (it is the energy
associated with forces of attraction or repulsion between particles).
Units of measurement
The unit used for measuring energy is the joule (J).
* One joule (1 J) is the energy required to lift an object exactly 1 m against the force of one Newton (1 N).
What is a Newton?
* One Newton is a force that will give a 1 kg object an acceleration of
1 m s-2.
* So, 1 J = 1 kg m2 s-2.
Energy Flows Between System and Surroundings: Temperature and Heat
- how hot the object is.
- a measure of the average kinetic energy of its atoms, molecules or ions.
- an intensive property (it is independent of the amount of material present)
- can be estimated by measuring any property of a substance that is temperature dependent (e.g., volume of mercury in thermometers, electrical resistance in thermistors).
The Celsius scale:
- 0°C is set as the freezing point of water.
- 100°C is set as the boiling point of water (at pressure of 1 atm).
- Range of temperatures in between is divided into 100 intervals.
The Kelvin scale:
0 K is set as the minimum possible temperature (0 K = -273.16°C, atoms or molecules have no kinetic energy of translation)
- The units on the Kelvin scale are the same size as those on the Celsius scale.
Conversion: temp (K) = temp (°C ) + 273.16
What is heat?
- the energy flowing from one object to another.
- not contained in objects (only thermal energy). Heat is energy in transit.
- Energy flows from the object with higher temperature to the object with lower temperature.
- Energy flows until the two objects reach the same temperature. The objects have then reached thermal equilibrium.
The System, Surroundings, and the Universe
The system represents the object or the collection of objects being studied.
The surroundings represent everything outside of the system.
Internal energy
- the sum of the kinetic energy and potential
energy of the atoms, molecules, and ions in the system, also the “energy content” of a system. - an extensive property. It depends on the amount of a substance present.
- a state function. It depends on the amount of a
substance and its conditions (i.e., the temperature and pressure), regardless of how the conditions were achieved.
change of internal energy, ΔU
ΔU (sys) = ΔU (sys final) - ΔU (sys initial)
Exothermic: ΔU < 0
Endothermic: ΔU > 0
a consequence of:
- Energy transferred as heat between the system and the surroundings (the symbol q is used for the amount of heat transferred).
0 Energy transferred as work between the system and the surroundings (the symbol w is used to represent the work done).
enthalpy change (ΔH)
equal to the amount of heat transferred between the system and the surroundings during a process occurring at constant pressure (qp), only if the work results entirely from expansion of the system.
ΔH(system = q(p)
Melting:
Heat must be provided to a solid at its melting point while melting occurs.
- The change of enthalpy of 1 mol of a solid substance during its conversion to a liquid is called the molar enthalpy change of fusion (ΔfusH).
Vaporization:
Heat must be provided to a liquid at its boiling point while vaporization occurs.
* The change of enthalpy of 1 mol of a liquid substance during its conversion to a gas is called the molar enthalpy change of vaporization (ΔvapH)
Enthalpy Change of Reaction
The difference between the sum of the enthalpies of the products and the sum of the enthalpies of reactants is called the enthalpy change of reaction (ΔrH)
Quantitative relationships
A thermomechanical equation links the value of ΔrH to the amounts of products and reactants in the balanced chemical equation.
- The sign and magnitude of DrH are reaction-dependent.
- For a reaction performed under the same conditions (pressure, temperature, states of reactants and products), DrH is always the same.
- The magnitude of DrH is proportional to the amount of substances that react (when 2 mol of methane reacts with 4 mol of oxygen.
DrH = 2 × (-830. 3 kJ mol-1 )= -1780.6 kJ mol-1
- If a chemical reaction is reversed, the magnitude of rH remains the same but the sign is reversed.
Measurement of DrH : Calorimetry
Based on ΔT, the mass and specific heat capacity of the solution, the heat transferred to or from the solution can be calculated:
q = c × m × ΔT
From the known amounts (moles) of species that react, it is then possible to calculate DrH.
Standard States:
- For pure substances, the standard state is the most stable form and state at 1 bar and at the temperature of interest.
- For a gas, a standard state is when it is present at 1 bar.
- For a species in aqueous solution, its standard state is at a concentration of 1 mol L-1 at a pressure of 1 bar.
The standard enthalpy change of reaction (ΔrHo) at a defined temperature is the enthalpy change of reaction when all the reactants and products are in their standard states.
Enthalpy Change of Reaction from Bond Energies
can be estimated from the energy associated with breaking and forming bonds.
ΔrH = SumD(bonds broken)- SumD(bonds formed)
The molar enthalpy change of bond dissociation, or bond energy, D, is the enthalpy change for breaking a particular bond in the molecules of 1 mol of a substance with reactants and products in gas phase.
Hess’s Law
regulates the procedures for calculating the enthalpy change of a reaction based on known enthalpy changes of other reactions:
If a reaction can be written as the sum of two or more steps, its enthalpy change of reaction is the sum of the enthalpy changes of reaction of the steps.
The law can be applied to any hypothetically proposed steps as long as their sum is the same as the overall equation.
