Energetics — Long Questions

Total amount of energy absorbed = 958.38+498
= 1456.38kJ/mol
Total amount of energy released = 2 ( − 626)
= −1252kJ/mol
Enthalpy change = 1456.38-1252
= 204.38kJ/mol

Heat | Enthalpy

Definition | Definition
This form of energy is released when a bond is formed and absorbed when bond is broken. | Enthalpy (H) or heat content, is defined as the total amount of thermal energy stored in a compound.

System | System
It is not essential part of system | It is essential part of system

Process | Process
Heat just enter and leave the object. | Enthalpy change is equal to heat evolved or absorbed, at constant pressure.

Measurement | Measurement
It is measurement of thermal energy transferred between two objects at different temperature. | It is measurement of energy in thermodynamic system.

Representation | Representation
It is represented by 'q' | It is represented by'H'.

Unit | Unit
Its unit is Joule | Its unit is kJ/mol.

Bond formation take place always with the evolution of Heat. If a reaction accompanied with the evolution of heat. It is called exothermic reaction.

For Example Total energy evolved in the formation of 4−OH bonds
2O₍ₘ₎+4H₍ₘ₎ → 2H₂O₍ₘ₎ ΔH=−1936 KJ / mol

This is the energy evolved when two moles of water are formed from 4 moles of hydrogen and 2 moles of oxygen atoms. Thus for the formation of one mole of water, the energy evolved will be 968kJ / mol⁻¹.

Role of Lipid in our Body

i. Organic Compound
Lipids are group of organic compounds which include Fats, waxes, sterol etc.

ii. Energy source
Lipids serve as an energy reserve within our body. About half of the fuel our body needs comes from lipids.

iii. Energy Bank in Animal body
If you eat more food than you need in a day, excess food is stored as lipid in adipose tissue. In between meals and during exercise, our body relies on this resource to provide energy.

iv. Water proofing
Lipids such as waxes provide water proofing for skin, hair and feathers in humans and animals.

v. Absorption of fat soluble vitamins
Lipids are necessary for the absorption of fat -soluble vitamins (A,D,E and K) in the intestine.

Activation Energy

Definition
Energy absorbed by reactant or product molecule in order to be converted into transition state is called activation energy. It is represented by (Ea.)

Transition State

Definition
When the two reactant molecules mixed together, all these molecules start colliding with each other. The collisions which result by colliding molecules having average or less than average kinetic energies may not be able to produce any result. But when the two excited molecules from both the reactants collide with each other they may be able to produce the transition state

Energy profile diagram

A + B → AB + AB
Reactants Transition state Product

After a very short period of time the transition state either returns to the reactants or to the products. The progress of the reaction can be shown in the form of the above energy profile diagram drawn between path of the reaction can be shown in the form of the following energy of the reactants and the products.

Aerobic Respiration

Definition
The process of respiration in human beings is a continuous process. During this process, we breathe in oxygen and breathe out carbon dioxide. Respiration also carries complex chemical reactions inside the human body. This process that occurs in the presence of oxygen is called aerobic respiration.

Process
Aerobic respiration is an exothermic process and involves the following reactions

Reactions

Glucose → 2 Pyruvate → 6CO₂+ 6H₂O + Energy
C₆H₁₂O₆ Glycolysis C₃H₃O₃ Mitochondria O₂
Incytoplasm Energy

Some fruits and vegetables are usually safe to eat if they are not contaminated. However, many raw foods like meat and eggs may have harmful bacteria or Parasites. Cooking kills these micro-organism, make the food safe to eat. Cooking also makes food easier to digest and enhance the flavor and texture, making it more enjoyable to eat. For example, meat and poultry need to be cook thoroughly to avoid foodborne illnesses, while some fruits and nuts are soft to eat raw because they don't carry the same risks. So cooking is digestibility important for safety and taste.

Fireworks look spectacular because people enjoy bright color enlightening sky. Fireworks are result of combustion reactions that yield heat, light and sound. Different metal powders along with oxidizing agents produce a variety of colors when burnt. For example copper (Cu) produces blue colour.

Definitions Chemical reactions in which heat energy is evolved are called exothermic reactions
Those reactions in which heat energy is absorbed are called endothermic reactions.

Explanation A physical or a chemical change is almost always accompanied with either absorption or evolution of heat. Heat, which is evolved or absorbed during a chemical reaction, is called the heat of that reaction.

Example of exothermic reactions 1. Hydrogen gas and oxygen gas react to give liquid water is an exothermic reaction.
2H₂₍ₘ₎ + O₂₍ₘ₎ → 2H₂O₍ₗ₎ ΔH=−571.6 KJ / mol
Water
571.6 kJ heat energy is evolved during this reaction. If the energy evolved is shown separately it is expressed as ΔH =−571.6KJ / mol The same amount of energy will be absorbed when the reaction will move in the backward direction i.e. water will decompose to give hydrogen and oxygen back.

2. Carbon dioxide gas is produced when solid carbon burns in oxygen gas.
C₍ₛ₎ + O₂₍ₘ₎ → CO₂₍ₘ₎ ΔH=−393.5 KJ / mol
Carbon Oxygen Carbon dioxide

It is also an exothermic reaction and 393.5 KJ heat energy is evolved during this reaction. When this reaction moves in the backward direction, the same amount of energy i.e. 393.5 kJ will be absorbed. This reaction has ΔH=−393.51 KJ / mol of CO₂.

Example of Endothermic Reaction 1. The following reactions represent endothermic changes.
H₂₍ₘ₎ + I₂₍ₛ₎ → 2HI₍ₘ₎ ΔH=+53.08KJ / mol
Hydrogen Iodine Hydrogen Iodine
The enthalpy change for the reaction is ΔH = 53.08 KJ / mol
Hydrogen gas reacts with solid iodine only at high temperature and 53.08 k of heat energy is absorbed

2. Formation of NO in air due to lightening in the clouds.
N₂₍ₘ₎ + O₂₍ₘ₎ → 2NO₍ₘ₎ ΔH=+180.6 KJ / mol
Nitrogen oxygen Nitric Oxide

The enthalpy change for the reaction is ΔH = 180.6 KJ / mol

Our present-day living conditions depend heavily on energy in its various forms. Exothermic chemical reactions are extensively used to fulfill this requirement. In such reactions, chemical energy is converted into heat energy. We burn fuels like gas, oil and coal to cook food and for other heating purposes in our homes and industry. During this burning process called combustion, compounds present in fuels react with oxygen of the air to produce a large amount of heat.

Metabolism Foods such as fats and carbohydrates are important biological fuels. During metabolism, the chemical energy present in this food is converted to heat to keep us warm.

Power Station A large portion of electricity is produced at power stations by burning fuels such as natural gas and coal. The heat which comes out from their combustion is used to produce steam at high pressure. This high pressure steam is then used to turn turbines, which in turn generate electricity.

Combustion While driving a vehicle, it is the combustion of petrol or diesel that gives off energy and drives it forward.

Firework The one example of exothermic reactions peeople seem to enjoy the most is that of fireworks. Fireworks are the result of combustion reactions that yield heat, light and sound. Different metal powders along with oxidizing agents produce a variety of colours when burnt.

A chemical reaction mainly involves the processes which involve bond breaking and bond formation. In the following reaction, the chemical bonds between the atoms present in the molecules of H₂ and O₂ first break to give their atoms.

2 H₂₍ₘ₎ + O₂₍ₘ₎ → 2 H₂O₍ₘ₎

2 H₂₍ₘ₎ → 4H₍ₘ₎ O₂₍ₘ₎ → 2O₍ₘ₎
Molecule Atoms Molecule Atoms

These atoms of hydrogen then form bonds with oxygen atoms to form two molecules of gaseous H₂O.

4 H₍ₘ₎ + 2 O₍ₘ₎ → H - H H - H
¨O: + ¨O:
‾

Breaking of bonds of H₂ and O₂ absorb energy (endothermic process) while making of bonds between H and O evolve energy (exothermic process). In this reaction, weaker bonds are broken. Hence less energy is absorbed in the system. While the bonds which are formed in water molecule are stronger and thus greater energy is evolved. Hence, the energy which is evolved is more than the energy which is absorbed. The overall reaction is thus exothermic.

2H₂₍ₘ₎ → 4H₍ₘ₎ + 435 kj / mol (Endothermic)
O₂ → 2 O + 498 kj/mol (Endothermic)
O + H → O − H −484 kJ/mol (Exothermic)

Since two moles of H₂ take part in the reaction so total energy absorbed in the reaction.
2 H₂₍ₘ₎ → 4H₍ₘ₎ + 870 kJ/mol

O₍₂₉₎ → 2 O₍ₘ₎ + 498 kJ/mol

2 H₂₍ₘ₎ + O₂₍ₘ₎ → 4H₍ₘ₎ + 2O₍ₘ₎ + 1368 kJ/mol

It means 1368 kJ energy is absorbed when 2 moles of H₂ and one mole of O₂ break their bonds to convert themselves into atoms.

Total energy evolved in the formation of 4 O − H bonds.
2O₍ₐ₎+4H₍ₘ₎ → 2H₂O₍ₘ₎−1936KJ / mol

This is the energy evolved when two moles of water are formed from 4 moles of hydrogen atoms and 2 moles of oxygen atoms. Thus for the formation of one mole of water, the energy evolved will be 968 kJ.

Hence the overall energy evolved in this reaction is = −1936 + 1368 = −568kJ for two moles of water.

2H₂₍ₘ₎ + O₂₍ₘ₎ → 2H₂O₍ₘ₎ + 568kJ or ΔH = −568kJ / mol

The enthalpy change for the formation of two moles of gaseous water is thus −658 kl. So the enthalpy change for the formation of one mole of gaseous water will be
568kJ
= −284kJ / mol
2 mol

The experimental value of formation of gaseous water is -284 kJ / mol which is quite close to this calculated value.