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Spontaneous Processes A process which proceeds of its own accord, without any outside assistance, is termed a spontaneous or natural process. The reverse process which does not proceed on its own, is referred to as a nonspontaneous or unnatural process. In general, the tendency of a process to occur naturally …

1. The branch of chemistry which deals with the heat changes caused by chemical reactions is called _______ (a) thermodynamics (b) thermal chemistry (c) thermochemistry (d) none of these Answer. (c) 2. In an exothermic reaction, the internal energy of the products is _______ internal energy of the reactants. (a) …

  Measurement of The Heat of Reaction ❒The experimental measurement of the heat of reaction or enthalpy change is known as calorimetry. ❒ The name (calorimetry) evidently finds its origin in the unit of heat–the calorie. ❒ The heat given out or absorbed in a chemical reaction is measured in …

Bond energy  ❒ When a bond between two atoms is formed, there is a release of energy. The same amount of energy is absorbed when the bond is broken. ❒The bond energy is defined as: the average amount of energy required to break all bonds of a particular type in …

Hess’s Law of Constant Heat Summation ❒ We have already seen that heat changes in chemical reactions are equal to the difference in internal energy (ΔE) or heat content (ΔH) of the products and reactants, depending upon whether the reaction is studied at constant volume or constant pressure.  ❒ Since …

Energy Changes During Transitions or Phase Changes ❒ The three states of matter – solid, liquid and gas differ from one another in the arrangement of their constituent particles. ❒ The magnitudes of intermolecular forces acting between the particles in these states are also different. ❒ It is a common observation that when …

Heat of Combustion ❒ The heat of combustion of a substance is defined as: The change in enthalpy of a system when one mole of the substance is completely burnt in excess of air or oxygen. ❒ It is denoted by ΔHc. As for example, heat of combustion of methane is – 21.0 kcal …

Heat of Formation ❒ The heat of formation of a compound is defined as:The change in enthalpy that takes place when one mole of the compound is formed from its elements. ❒ It is denoted by ΔHf . For example, the heat of formation of ferrous sulphide and acetylene may be expressed as: ❒ Similarly, the …

The heat or enthalpy changes accompanying chemical reactions are expressed in different ways, depending on the nature of the reaction. These are discussed below. (1) Heat of Formation ❒ The heat of formation of a compound is defined as:The change in enthalpy that takes place when one mole of the …

Heat of Reaction or Enthalpy of Reaction ❒The heat of a reaction is simply the amount of heat absorbed or evolved in the reaction. ❒ We also know that the amount of heat absorbed or evolved at constant temperature and pressure is called enthalpy. Therefore the amount of heat change …

Enthalpy of a Reaction ❒Thermochemical measurements are made either at (a) constant volume or (b) constant pressure. The magnitudes of changes observed under the two conditions are different. ❒ The change in internal energy (ΔE): is the heat change accompanying a chemical reaction at constant volume because no external work …

1. The study of the flow of heat or any other form of energy into or out of a system undergoing physical or chemical change is called_______ (a) thermochemistry  (b) thermokinetics (c) thermodynamics (d) thermochemical studies Answer. (c) 2. Thermodynamics is applicable to_______ (a) microscopic systems only  (b) macroscopic systems …

Adiabatic Expansion of an Ideal Gas ❒ A process carried in a vessel whose walls are perfectly insulated so that no heat can pass through them, is said to be adiabatic.❒ In adiabatic process there is no heat exchange between a system and surroundings, and q = 0. ❒ According …

Molar Heat Capacities ❒ By heat capacity of a system we mean the capacity to absorb heat and store energy. As the system absorbs heat, it goes into the kinetic motion of the atoms and molecules contained in the system. This increased kinetic energy raises the temperature of the system. …

Enthalpy of A System ❒ In a process carried at constant volume (say in a sealed tube), the heat content of a system is the same as internal energy (E), as no PV work is done.  ❒ But in a constant-pressure process, the system (a gas) also expends energy in …