Under normal conditions the reaction between oxygen(O₂) and Nitrogen(N₂) do not occur because the activation energy required for the reaction is very large. lightning provides a large amount of energy, which allows the reaction to overcome this activation energy, forming nitrogen oxides (NO). Once the light stops the energy input ceases, and the reaction stops as well.

The reaction between methane (CH₄) and oxygen does not occur spontaneous because the activation energy required to start the reaction is high. The burning matchstick provides the necessary energy (heat) to initiate the reaction, overcome the activation energy barrier. Once the reaction starts, it is self sustaining until one the reactant is consumed.

To calculate the enthalpy change of the given reaction, we will use the bond dissociation and bond formation energetics. The reaction in question involves the formation of the nitrogen monoxide (NO) from nitrogen (N₂) and oxygen (O₂).

The reaction we are considering
    N₂(g) + O₂(g) --> 2NO(g)

Step 1: Bond dissociation

• Bond dissociation energy of N₂ (N triple bond N bond) = 958.38 KJ per mol
• Bond dissociation energy of O₂ (O = O bond) = 498 KJ per mol

We need to break the bond in N₂ and O₂ molecules to form two NO molecules. The total energy required to break these bonds is the sum of the bond dissociation energetics.

Energy to break bonds = Bond dissociation energy of N₂ + Bond dissociation energy of O₂
Energy to break bonds = 958.38 KJ per mol + 498 KJ per mol = 1456.38 KJ per mol

Step 2: Bond formation
Bond formation energy of NO = -626 KJ per mol            (Note: This is a negative value because energy is released when bonds are formed).

Since the reaction produces two molecules of NO, the total bond formation energy for the two NO molecules is.
Energy released in bond formation = 2 x (-626) KJ per mol = -1252 KJ per mol

Step 3 - Calculate the enthalpy change (ΔH)

The enthalpy change of the reaction is the difference between the energy required to break the bonds and the energy released when bonds are formed:
ΔH = Energy to break bond - Energy released in bond formation.
ΔH = 1456.38 KJ per mol - (-1252 KJ per mol )
ΔH = 1456.38 KJ per mol + 1252 KJ per mol = 2708.38 KJ per mol

The enthalpy change for the reaction is 2708.38 KJ per mol. This indicates that the reaction is endothermic (absorbs energy).

Heat:

Heat is form of energy that flows from hot body to a cold body because of a difference in temperature. We measure heat in joules. Heat is what we call the transfer of

thermal energy. Heat is not essential part of a system, it just comes and goes. 

Representation:

Heat is represented by "Q"

Enthalpy:

Enthalpy is defined as the total amount of thermal energy stored in a compound. Enthalpy is an essential part of a system since it depends on the number of molecules

present in that system, its chemical composition and its structure.

Representation:

Enthalpy is represented by "H".
                                      

Bond formation is always considered an exothermic process because when atoms come together to form a bond, they achieve a lower energy state compared to their isolated state, resulting in the release of energy, usually in the form of heat, to the surrounding environment, as they become more stable. essentially the system loses energy as new bonds are formed, making it an exothermic reaction.

Why the Reaction is Exothermic:

The bonds broken in H₂ are weaker, so less energy is absorbed. The bonds formed in water (H₂O) are stronger, So more energy is released. The energy released during bond formation is greater than the energy absorbed during bond breaking, making the overall reaction exothermic.

H₂(g) ----> 2H(g) + 435 KJ / mol  (Endothermic)
O₂(g) ----> 2O(g) + 498 KJ / mol  (Endothermic)
O + H ----> O - H - 484 KJ / mol   (Exothermic)

Lipids, also known as fats have many important roles within our bodies. They happen to produce energy that enables the growth of hormones. If it wasn't for lipids you would not be able digest and absorb food properly. In proper amounts lipids are a healthy part of our diet but of course over consumption could potentially lead to weight gain.

Role of lipids:

• lipids such as fat and waxes, act as energy reserves stored in adipose tissues.
• They provide 50% of the energy needs during exercise or fasting.
• Excess food is stored as lipids and used by the body when required.
• Lipids are group of organic compounds which include fats, waxes, sterols etc.
• Lipids serve as an energy reserve within our body.
• About half of the fuel our body needs comes from lipids.
• If you eat more food than you need in a day, the excess food is stored as lipids in adipose cells.

1) Activation energy:

The activation energy (Ea) is the minimum energy required for reactants to form the transition state, where bonds are progressively breaking or forming. It presents the energy barrier that must be overcome for a reaction to proceed.

For example:

In a fire, we transform carbon in wood into CO₂, which is a more stable form of carbon than wood. Therefore, the reaction continues and create to ignite the fire. Our efforts and matches are evidence of this.

2) Transition state:

A transition state is an unstable molecular configuration that occur during a chemical reaction. It is the state with the highest energy along the reaction coordinate.

Characteristics of transition state:

• It's short live intermediate state.
• It's made up of incomplete bonds.
• It's has partial charges.

3) Aerobic respiration:

Aerobic respiration is a biological process in which glucose (C₆H₁₂O₆) is broken down in the presence of oxygen (O₂) to produce carbon dioxide (CO₂), water (H₂O)  and energy in the form of ATP. It occurs in two stages:

i) Glycolysis:

Happens in the cytoplasm, where glucose is converted to pyruvate. 

ii) Krebs Cycle and Electron Transport chain:

Take place in mitochondria, where pyruvate is completely oxidized to CO₂ and H₂O

Cooking food is essential for several reasons:

- Safety:

Cooking food, especially meat, poultry, and seafood help kill harmful bacteria, viruses, and parasites that could cause foodborne illnesses.

- Digestibility: 

Cooking breaks down complex molecules in food (such as proteins and starches) into simpler forms, making them easier to digest and absorb.

- Nutritional benefits:

Cooking can make certain nutrients more available for absorption, such as lycopene in tomatoes or beta-carotene in carrots.

- Taste and texture:

 Cooking improves the flavour, aroma, and texture of food making it more palatable.
Some foods, like fruits and vegetables can be eaten raw because they don't pose significant health risks, and cooking may even destroy some of their nutrients.

Fireworks look spectacular because they produce a brilliant display of colors, light, and sound due to the chemical reactions that occur when they are ignited. The

reactions involve the rapid oxidation of certain chemical compounds, producing intense heat and light.

Color production:

The colors in fireworks are produced by metal salts. Different metal salt emit different colors when they are heated:

 i)  Strontium salts: Red    

 ii) Copper compounds: Blue

iii) Barium compound: Green  

iv) Sodium compounds: Yellow

v)  Calcium compound: Orange

Exothermic reactions:

Fireworks contain a mixture of oxidizers and fuels. The oxidizers (like potassium nitrate, potassium chlorate, or potassium perchlorate) provide oxygen and the fuels

(such as aluminum, charcoal, or sulfur) burn in the presence of oxygen producing heat, light and gases. The reaction of these compounds is highly exothermic,

meaning it releases energy in the form of light and heat.

The spectacular display is due to the combination of these reactions, which creates the bright flashes of color, sound (explosions) and the energetic release of light.