Chapter 5: EnergeticsLong Questions with Answers

Heat

Heat is a form of energy that flows from a 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. It exists only while it is flowing.

Enthalpy

Enthalpy (H), or heat content, is the total amount of thermal energy stored in a compound, measured in kJ mol⁻¹. It 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.

Difference

Link between them

When heat leaves or enters a system, it results in a change of enthalpy. At constant pressure the enthalpy change is equal to the heat evolved or absorbed. Heat in: ΔH positive (endothermic); heat out: ΔH negative (exothermic).

What happens when atoms bond

When two atoms approach each other, the electrons of each come under the attraction of the other atom's nucleus. This new attraction lowers the energy of the pair. At the bond length the attractions win by the most and the energy is at its lowest: a bond has formed.

Where the energy goes

The energy the atoms lose is not destroyed: it is released to the surroundings as heat. So bond formation always evolves energy, i.e. it is exothermic.

Bond breaking is the reverse

To separate the atoms again, exactly the same amount of energy must be supplied: bond breaking is endothermic. The book calls the H₂ value (435 kJ mol⁻¹) its bond dissociation energy and the O–H value (484 kJ mol⁻¹) the bond formation energy.

Effect on reactions

In a reaction, bonds of reactants break (energy absorbed) and new bonds form (energy evolved). When weaker bonds are broken and stronger bonds are formed, the reaction is overall exothermic, as in the formation of water.

What lipids are

Lipids are a group of organic compounds which include fats, waxes, sterols, etc.

Energy reserve

Lipids serve as an energy reserve within our body. About half of the fuel our body needs comes from lipids. Fats release about 37 kJ per gram when oxidised, more than twice as much as carbohydrates, so they are the most compact way to store energy.

Storage

If you eat more food than you need in a day, the excess food is stored as lipids in adipose cells. In between meals and during exercise our body relies on this resource to provide energy.

Release of energy

Stored fats are broken down and oxidised in respiration, an exothermic process; the energy is captured as ATP and used by cells, and some is given off as heat that keeps us warm.

Glycogen, the short-term store

Glycogen is the primary storage form of glucose. It is stored in the liver and muscles. It is used first; lipids are the long-term reserve.

Activation energy

The energy absorbed by the reactant molecules in order to be converted into the transition state is called the activation energy (Eₐ) of the reaction. It is the minimum energy colliding particles must have for a successful collision. In an energy profile it is the height of the hump measured from the reactants. A catalyst decreases the activation energy by changing the reaction path.

Transition state

A reaction takes place when the reactant molecules collide with each other to give a transition state. When two excited molecules (more than average kinetic energy) collide, the old bonds begin to break while new ones begin to form. The transition state is shown at the peak of the curve. After a very short period of time it either returns to the reactants or goes on to the products. Its energy is higher than that of reactants or products because bonds are being cleaved.

Aerobic respiration

The process of respiration that occurs in the presence of oxygen is called aerobic respiration. It is an exothermic process.

During glycolysis one molecule of glucose is split into two molecules of pyruvate, with a net production of 2 ATP; pyruvate is then oxidised completely in the mitochondria. The ATP is broken down when cells need energy.

What cooking does

Cooking supplies heat energy. Many of the changes it brings about are endothermic or have a high activation energy, so they do not happen at room temperature.

Foods eaten raw

Fruits, salads, nuts and milk products are already soft, digestible and safe (when washed or pasteurised). Their sugars dissolve and are digested easily. Cooking them would destroy heat-sensitive vitamins such as vitamin C, so many are better raw.

Why spectacular

Fireworks are the result of combustion reactions that yield heat, light and sound. The reactions are fast and highly exothermic: the heat makes the metal atoms glow in bright colours and the expanding gases give the bang.

Compounds involved

Colours

Different metal salts along with oxidising agents produce a variety of colours when burnt. The heat excites the electrons of the metal atoms; as they fall back to lower energy levels they give out light of a colour typical of that metal.

Heat of reaction

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

Exothermic reactions

Chemical reactions in which heat energy is evolved are called exothermic reactions. The heat goes to the surrounding and the container in which such a reaction is carried out gets hot.

When these reactions are carried out in reverse, the same amount of energy is absorbed.

Endothermic reactions

Chemical reactions in which heat energy is absorbed are called endothermic reactions. The absorption of heat from the surrounding decreases the temperature of the container.

Energy profiles

Uses of exothermic reactions

Principle

A chemical reaction mainly involves the processes of bond breaking and bond formation. Breaking of bonds absorbs energy (endothermic) while making of bonds evolves energy (exothermic). The enthalpy change is the sum of the energies absorbed and released.

Step 1: bonds broken

In 2H₂ + O₂ → 2H₂O the bonds between the atoms in H₂ and O₂ first break to give their atoms. Bond dissociation energy of H₂ is 435 kJ mol⁻¹ and of O₂ is 498 kJ mol⁻¹:

Step 2: bonds formed

The hydrogen atoms then form bonds with the oxygen atoms to form two molecules of H₂O. Four O–H bonds form; bond formation energy of one O–H bond is 484 kJ mol⁻¹:

Step 3: overall

Why exothermic

Weaker bonds (H–H) are broken, so less energy is absorbed, while the bonds formed in water are stronger and greater energy is evolved. The energy evolved is more than the energy absorbed; the overall reaction is exothermic.

Collisions

A reaction takes place when the reactant molecules collide with each other to give a transition state. Consider A₂ + B₂ → 2AB. Before mixing, the molecules are in random motion. Their kinetic energies are not the same: the majority possess average kinetic energy, a few possess more (excited molecules) and others less.

Successful collisions

Collisions between molecules having average or less than average kinetic energy may not produce any result. But when two excited molecules collide, they may produce the transition state.

Transition state and activation energy

The energy of the transition state is higher than that of reactants or products because bonds are being cleaved progressively. The energy absorbed by the reactant molecules to be converted into the transition state is called the activation energy (Eₐ). After a very short time the transition state either returns to the reactants or goes to the products.

Exothermic profile

The difference between the energy of the reactants and that of the products comes out as heat, representing the enthalpy (ΔH) of the reaction. Products lower: exothermic, ΔH negative.

Endothermic profile

Products higher than reactants: energy absorbed, ΔH positive.

Definition

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

How it works

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

What it does not change

Examples

Respiration

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 out complex chemical reactions inside the human body.

Aerobic respiration

The process that occurs in the presence of oxygen is called aerobic respiration. It is an exothermic process.

Glycolysis

During glycolysis one molecule of glucose is split into two molecules of pyruvate. This involves a series of reactions catalysed by enzymes, with a net production of 2 ATP.

Anaerobic respiration

In certain organisms like bacteria and yeast, respiration occurs in the absence of oxygen and is called anaerobic respiration. It is also exothermic; glucose is converted into carbon dioxide and ethanol with the evolution of energy.

Energy for the body

When cells require energy for metabolic activities, they use ATP and break it down to get the required energy. The food we eat is digested, and the digested food molecules absorbed by the cells undergo oxidation to produce energy. Glycogen (liver, muscles) and lipids (adipose cells) are the stores.