Energetics

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Ans. Bond breaking is Endothermic.

Bond breaking is an endothermic process because when two atoms combine, energy is stored in them and when we break bond, equal amount of energy is given to break that bond this energy is also known as dissociation energy. e.g dissociation energy for hydrogen is 435 kJ/mol.

H₂ + Cl₂ ⟶ 2HCl

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i. 2H₂ + O₂ ⟶ 2H₂O

ii. C + O₂ ⟶ CO₂

Ans. Glycogen is primary storage form of glucose. It is stored in liver and muscle. It help to regulate blood sugar level in our body.

Ans. Boiling water in a beaker is endothermic reaction. Heat energy is transferred in this system.

Ans. Yes, energy can be transferred in other form than heat like light, sound etc.

Ans. It is not possible to calculate the absolute enthalpy of a system because enthalpy (H) is a state function that depends on the internal energy (E), pressure (P) and volume (V) of a system, we cannot determine the absolute values of these quantities for a system in isolation, instead we calculate the change in enthalpy (ΔH).

Ans. Reaction between sodium and water is exothermic that's why sodium react violently with water.

Ans. Melting of ice is endothermic change.

Ans. Yes, exothermic reactions can be reversed. Hydrogen gas and oxygen gas react to give liquid water in an exothermic reaction.

2H_2(g) + O_2(g) ⟶ 2H₂O_(ℓ)   (Hydrogen)   (Oxygen)   (Water)
ΔH = −571.6 KJ/mol

571.6 kJ heat energy is evolved during this reaction. 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 gases back.

2H₂O_(ℓ) ⟶ 2H_2(g) + O_2(g)   ΔH = −571.6kJ/mol

Ans. Enthalpy change for the formation of one mole of liquid water is :

2H_2(g) + ¹⁄₂O_2(g) ⟶ 2H₂O_(ℓ)   ΔH = −284.6kJ/mol

ΔH = −571.6kJ/mol
ΔH = −571.6 / 2 kJ/mol
ΔH = −284.6kJ/mol

The enthalpy of elements in standard states H₂ and O₂ is zero. The enthalpy of liquid water is −284 kJ/mol under standard conditions

Ans. Energy diagrams are useful because it gives us immediate answer about energy changes happening in a reaction.

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Ans. Thermodynamics deals with how the energy changes during chemical reaction affect the properties of a chemical system.

Ans. Thomas Young was the first to use the word 'energy' to the field of physics in 1802.

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Ans. Energy evolved during a chemical reaction is used in everyday for cooking, heating, lightning, transportation, communication, entertainment and much more.

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Ans. Enthalpy is important because it tells us how much heat is present in a system. Heat is important because we can extract useful work from it.

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Ans. Standard enthalpy of reaction (ΔH⁰)

i. "Amount of heat evolved or absorbed when one mole of reactants are converted into products under standard condition (25°C and 1 atm) in a chemical reaction"

ii. It is represented by (ΔH⁰)

iii. It's sign is positive for endothermic reaction and negative for exothermic reaction.

Reactants and products should be taken in standard physical state.

C_(s) + O_2(g) ⟶ CO_2(g)   ΔH⁰ = −393.5kJ (exothermic)
H_2(g) + I_2(g) ⟶ 2HI_(g)   ΔH⁰ = +53.8kJ (endothermic)

Standard enthalpy change: Enthalpy of reaction measured at 25°C (or 298K) and one atmospheric pressure is known as standard enthalpy change.

Representation: It is denoted by ΔH⁰.

i. C_(s) + O_2(g) → CO_2(g)   ΔH⁰ = −393.5 kJ
ii. H_2(g) + I_2(g) → 2HI_(g)   ΔH⁰ = +53.8 kJ

Ans. Enthalpy of Reaction: The amount of heat or thermal energy evolved or absorbed in a chemical reaction is called enthalpy of reaction.

Sign: Its sign is negative for exothermic and positive for endothermic reactions.

Ans. Nitrogen of the atmosphere reacts with oxygen to produce nitrogen oxide (NO) only in the presence of lightening. This is because reaction is highly endothermic, so only lightening can provide enough energy for this reaction to take place.

Ans. (i) Exothermic Reaction

(ii) Endothermic Reaction

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Ans. When the two reactant molecules are 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 colloide with each other they may be able to produce the transition state as shown in the following.

Ans. The energy of the transition state is higher than that of reactants or products because the bonds between the reactant or product molecules are being cleaved or stressed progressively.

Ans. An addition of the catalyst in a reaction increases the rate of reaction because it changes the path adopted by the reactants whereby the activation energy value of the reaction is substantially decreased. As a result, more reactants are now able to be converted into product molecules and hence the rate of reaction will increase.

Ans. A substance that increase the rate of a chemical reaction without itself undergoing any permanent chemical change.

Example:

i. Nickel (Ni) acts as a catalyst in the hydrogenation of oil to give banaspati ghee.
ii. Platinum (Pt) acts as a catalyst in the production of H₂SO₄.
iii. Chlorine (Cl₂) acts as a catalyst promoting the breakdown of ozone.

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Ans. During glycolysis one molecule of glucose is split into two molecules of pyruvate. This process involves a series of reactions catalyzed by enzymes, with a net production of 2 ATP (Adenosine Triphosphate). When cells of our body require energy for performing the metabolic activities, they use this ATP and break it down to get the required energy.

Ans. The food we eat undergoes digestion in our body and the digested food molecules that are absorbed by the cells undergo oxidation to produce energy.

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N_2(g) + O_2(g) ⟶ 2NO_(g)

Ans. Nitrogen and oxygen does not react under normal conditions because it is highly endothermic reaction. Nitrogen molecule has triple covalent bond which require very high energy to break. That energy is supplied by lightening. So as long as lightening continue reaction will continue.

Ans. Activation energy of methane is very high which is supplied by burning match stick. Once reactant break and combine to form new products reaction goes on, until one or both reactant get consumed.