Physical chemistry
The study of principles governing chemical reactions, energy changes, and reaction rates.
Physical Chemistry is the branch of chemistry that applies principles and theories from physics to the study of chemical systems and reactions. It provides the quantitative foundation for understanding why and how chemical processes occur. This section covers the core concepts of stoichiometry, chemical energetics, and kinetics.
### 1. Atomic Structure and Stoichiometry
Stoichiometry is the quantitative study of reactants and products in a chemical reaction. The fundamental unit in these calculations is the mole.
* Relative Atomic/Molecular Mass (Ar/Mr): The relative atomic mass (Ar) is the weighted average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12. The relative molecular mass (Mr) is the sum of the relative atomic masses of all atoms in a molecule.
* The Mole and Avogadro's Constant: A mole is the amount of any substance containing the same number of particles (atoms, molecules, or ions) as there are atoms in exactly 12g of carbon-12. This number is known as Avogadro's constant (L or NA), which is approximately 6.02 x 10²³ particles per mole.
The relationship between moles, mass, and molar mass is given by the crucial formula:
moles (mol) = mass (g) / Mr
* Empirical and Molecular Formulae: The empirical formula is the simplest whole-number ratio of atoms of each element in a compound. The molecular formula gives the actual number of atoms of each element in one molecule of the compound. The molecular formula is always a whole number multiple of the empirical formula.
### 2. Chemical Energetics (Thermochemistry)
Energetics is the study of heat changes accompanying chemical reactions.
* Enthalpy Change (ΔH): The enthalpy change (ΔH) is the amount of heat energy taken in or given out during a chemical reaction at constant pressure. Its units are kilojoules per mole (kJ mol⁻¹).
* Exothermic and Endothermic Reactions: An exothermic reaction is one that releases heat energy into the surroundings, resulting in a temperature increase. For these reactions, ΔH is negative (ΔH < 0). A common example is combustion. An endothermic reaction is one that absorbs heat energy from the surroundings, causing a temperature decrease. For these reactions, ΔH is positive (ΔH > 0). An example is the thermal decomposition of calcium carbonate.
* Standard Enthalpy Changes: To compare enthalpy changes, they are measured under standard conditions (100 kPa pressure, 298 K temperature). Key definitions include:
* Standard enthalpy of formation (ΔHf°): The enthalpy change when one mole of a compound is formed from its elements in their standard states.
* Standard enthalpy of combustion (ΔHc°): The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions.
* Calorimetry and Hess's Law: The heat change in a reaction can be measured experimentally using a calorimeter. The heat energy (q) absorbed or released is calculated using the formula: q = mcΔT, where 'm' is the mass of the substance being heated (usually water), 'c' is its specific heat capacity, and 'ΔT' is the change in temperature.
Hess's Law states that the total enthalpy change for a chemical reaction is independent of the pathway taken from reactants to products. This law is incredibly useful for calculating enthalpy changes for reactions that cannot be measured directly, by constructing an enthalpy cycle.
### 3. Chemical Kinetics (Rates of Reaction)
Kinetics is the study of the speed, or rate, at which chemical reactions occur.
* Rate of Reaction: The rate of reaction is defined as the change in concentration of a reactant or product per unit of time. It can be measured by monitoring changes in mass, volume of gas produced, or concentration over time.
* Collision Theory: For a reaction to happen, reactant particles must collide. The Collision Theory states that for a collision to be successful (i.e., result in a reaction), two conditions must be met:
* Factors Affecting Reaction Rate: Any factor that increases the frequency of successful collisions will increase the reaction rate.
Key Points to Remember
- 1The mole is the unit for amount of substance, related to mass by the formula: moles = mass / Mr.
- 2Enthalpy change (ΔH) quantifies heat released in exothermic reactions (ΔH < 0) or absorbed in endothermic reactions (ΔH > 0).
- 3Hess's Law states that the total enthalpy change for a reaction is independent of the route taken, allowing for indirect calculations.
- 4Collision Theory requires particles to collide with sufficient energy (activation energy, Ea) and correct orientation to react.
- 5The rate of reaction is the change in concentration of a substance per unit time.
- 6Reaction rates can be increased by increasing temperature, concentration, pressure, surface area, or by using a catalyst.
- 7A catalyst provides an alternative reaction pathway with a lower activation energy (Ea).
- 8Calorimetry is the experimental technique to measure heat changes in a reaction, using the formula q = mcΔT.
Pakistan Example
Fertiliser Production at Fauji Fertiliser Company (FFC)
The production of ammonia for urea fertiliser in Pakistan, such as at plants operated by Fauji Fertiliser Company, is a prime example of applied physical chemistry. The synthesis of ammonia via the Haber-Bosch process (N₂(g) + 3H₂(g) ⇌ 2NH₃(g)) is a reversible, exothermic reaction (ΔH is negative). To maximise the rate of reaction (kinetics), engineers use high pressures (around 150-250 atm) to increase the concentration of gas molecules and a moderately high temperature (around 400-450°C) as a compromise between rate and equilibrium position. Crucially, an iron catalyst is used to provide a lower activation energy pathway, making the process economically viable. The exothermic nature of the reaction (energetics) means that the heat generated must be carefully managed and is often used to pre-heat the incoming reactant gases, improving the plant's overall energy efficiency.
Quick Revision Infographic
Chemistry — Quick Revision
Physical chemistry
Key Concepts
Formulas to Know
The mole is the unit for amount of substance, related to mass by the formula: moles = mass / Mr.Calorimetry is the experimental technique to measure heat changes in a reaction, using the formula q = mcΔT.Fertiliser Production at Fauji Fertiliser Company (FFC)
The production of ammonia for urea fertiliser in Pakistan, such as at plants operated by Fauji Fertiliser Company, is a prime example of applied physical chemistry. The synthesis of ammonia via the Haber-Bosch process (N₂(g) + 3H₂(g) ⇌ 2NH₃(g)) is a reversible, exothermic reaction (ΔH is negative). To maximise the rate of reaction (kinetics), engineers use high pressures (around 150-250 atm) to increase the concentration of gas molecules and a moderately high temperature (around 400-450°C) as a compromise between rate and equilibrium position. Crucially, an iron catalyst is used to provide a lower activation energy pathway, making the process economically viable. The exothermic nature of the reaction (energetics) means that the heat generated must be carefully managed and is often used to pre-heat the incoming reactant gases, improving the plant's overall energy efficiency.