Unit 5: Energetics — Short Questions

Heat vs Enthalpy

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

System Heat: It is not essential part of system
Enthalpy: It is essential part of system

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

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

Representation Heat: It is represented by 'q'
Enthalpy: It is represented by 'H'.

Unit Heat: Its unit is Joule (J)
Enthalpy: Its unit is kilo joules per mol (kJ/mol).

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

H—H H.....H
Cℓ—Cℓ → Cℓ···Cℓ → HCℓ + HCℓ
Reactants Transition State Product

i. 2H₂ + O₂ → 2H₂O

Energy diagram showing

• Reactant at lower energy level
• Transition state with activation energy Ea marked
• Product at lower energy level
• ΔH shown as negative value
• Reaction coordinate on x-axis

ii. C + O₂ → CO₂

Energy diagram showing

• Reactant at lower energy level
• Transition state with activation energy Ea marked (lower than reaction i)
• Product at lower energy level
• ΔH shown as negative value
• Reaction coordinate on x-axis

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

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

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

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).

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

Melting of ice is endothermic change.

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

2H₂(g) + O₂(g) → 2H₂O(l)
ΔH = -571.6kJ / 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) → 2H₂(g) + O₂(g) ΔH = -571.6kJ/mol

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

2H₂(g) + ½O₂(g) → 2H₂O(l) ΔH = -284.6kJ / mol

ΔH = -571.6kJ / mol

ΔH = -571.6 kJ/ mol / 2

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

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

Energy diagram showing

• Transition state at peak
• Activation energy Ea marked
• Reactant and Product at same energy level
• ΔH = 0 (no net energy change)
• Reaction coordinate on x-axis

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

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

Chemical Energy This energy is stored in a molecule in which atoms are bonded to each other

Heat Energy This form of energy is released when a bond is formed and absorbed when it is broken.

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

System 1. Any physical or chemical change under study, may also be called system.
2. It includes reactants, products, catalyst, solvent, and anything else which is important to study a particular reaction.

Surrounding 1. Everything else which does not fall in the system is called surrounding.
2. It includes all other things such as beaker burner test tube etc.

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.

ΔE:

• ΔE is the change in internal energy.
• It is equal to heat absorbed by the system at constant volume.
• ΔE = qᵥ
• It is equal to sum of heat and work.
• ΔE = q + w

ΔH:

• ΔH is the enthalpy change.
• It is equal to the heat of reaction at constant pressure.
• ΔH = qₚ
• It is equal to sum of internal energy and product of pressure volume work.
• ΔH = ΔE + PΔV

Standard enthalpy of reaction (ΔH⁰)

i. "Amount of heat evolved or absorbed when one mole of reactants are converted into products under standard condition (25⁰C 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.

Standard enthalpy change Enthalpy of reaction measured at 25⁰C (or 298K) and one atmospheric pressure is known as standard enthalpy change.

Representation It is denoted by ΔH⁰.

i. C(s) + O₂(g) → CO₂(g)
ΔH⁰ = -393.5 kl

ii. H₂(g) + I₂(g) → 2HI(g)
ΔH⁰ = +53.8 kJ

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.

Reactants and products should be taken in standard physical state.
C(s) + O₂(g) → CO₂(g) ΔH⁰ = -393.5 kl(exothermic)
H₂(g) + I₂(g) → 2HI(g) ΔH⁰ = +53.8 kJ(endothermic)

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.

(i) Exothermic Reaction:
Energy diagram showing:

• Reactant at higher energy level
• Transition state at peak with activation energy Ea marked
• Product at lower energy level
• ΔH shown as negative (energy released)
• Reaction coordinate on x-axis

(ii) Endothermic Reaction:
Energy diagram showing:

• Reactant at lower energy level
• Transition state at peak with activation energy Ea marked
• Product at higher energy level
• ΔH shown as positive (energy absorbed)
• Reaction coordinate on x-axis

Exothermic Reactions A chemical reaction that proceeds with the evolution of heat is called an exothermic reaction.
i. C(s) + O₂(g) → CO₂(g)
ΔH⁰ = -393.5kl
ii. 2H₂(g) + O₂(g) → 2H₂O(l)
ΔH⁰ = -571.6kJ

Endothermic Reactions A chemical reaction that proceeds with the absorption of heat is called an endothermic reaction.
i. H₂(g) + I₂(g) → 2HI(g)
ΔH⁰ = +53.8kJ
ii. N₂(g) + O₂(g) → 2NO(g)
ΔH⁰ = +180.6kJ

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 collide with each other they may be able to produce the transition state as shown in the following.

A₂ + B₂ → Transition state → AB + AB
Reactants Products

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.

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.

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.

Energy diagram showing

• Transition state without catalyst at higher peak
• Transition state in the presence of catalyst at lower peak
• Activation energy in presence of catalyst (ΔE) marked as lower
• Reactants and Products at same energy levels
• Reaction coordinate on x-axis

Aerobic Respiration i. It occurs in the presence of oxygen.
ii. It occurs in higher animals like human body.
iii. The product of this reaction is CO₂ and H₂O.

Glucose (C₆H₁₂O₆) → 2 Pyruvate (C₃H₆O₃) → 6CO₂ + 6H₂O + Energy

Anaerobic Respiration i. It occurs in the absence of oxygen.
ii. It occurs in bacteria and algae.
iii. The product of this reaction is alcohol.

Glucose (C₆H₁₂O₆) → 2 Pyruvate (C₃H₆O₃) → C₂H₅OH + CO₂ + Energy

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.

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.

Physical change | Exothermic or Endothermic

Conversion of hydrated salt into anhydrous salt | Endothermic

Burning paper | Exothermic

Vapourizing liquid nitrogen | Endothermic

Evaporation of dry ice | Endothermic

Conduction of electricity by metals | Neither Endothermic Or Exothermic

Dissolving ammonium chloride in water | Endothermic

Formation of rain from clouds | Exothermic

Dissolving sodium carbonate in water | Endothermic

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

N₂(g) + O₂(g) → 2NO(g)

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.