Chapter 5Energetics

In a bond the two atoms are held together by the attraction of their nuclei for the shared (or transferred) electrons; this lowers their energy. To pull the atoms apart, energy must be supplied to overcome this attraction. The energy is taken from the surroundings, so bond breaking is always endothermic. Example: H₂ → 2H needs 435 kJ mol⁻¹.

When an excited H₂ molecule collides with an excited Cl₂ molecule, the H–H and Cl–Cl bonds start to break while two new H–Cl bonds start to form. This short-lived four-atom arrangement, at the top of the energy curve, is the transition state. It then breaks up into two HCl molecules (or back into H₂ and Cl₂).

Both reactions have products lower than reactants (ΔH negative). The faster reaction has the lower activation energy, so more colliding molecules have enough energy to get over the barrier.

Glycogen is the primary storage form of glucose. It is stored in the liver and muscles. When blood glucose falls (between meals, during exercise) glycogen is broken down to glucose, which is respired to release energy. Muscle glycogen fuels the muscles themselves; liver glycogen keeps the blood glucose level steady.

It is endothermic: the water (system) takes in energy from the burner (surrounding). The energy transferred is heat (thermal) energy, which is used to separate the water molecules into steam.

Yes. Energy can be transferred as light (burning magnesium, fireworks), electrical energy (in a cell or battery), sound (explosions) and work (expanding gases push a piston in an engine).

The total enthalpy H of a system cannot be measured directly, because it includes all the energy of all its particles (bond energies, motions of molecules, energy of electrons and nuclei) and there is no zero to measure it from. We can only measure the change ΔH, from the heat evolved or absorbed.

2Na + 2H₂O → 2NaOH + H₂ is highly exothermic and has a low activation energy, so it starts at once. The heat melts the sodium and can ignite the hydrogen gas, so the reaction is violent.

Endothermic. Ice takes in heat (latent heat of fusion, 6 kJ mol⁻¹) from the surroundings to break the hydrogen bonds holding its molecules in the crystal. The surroundings get cooler, which is why ice cools a drink.

Yes, if the same amount of energy is supplied. The book: "The same amount of energy will be absorbed when the reaction moves in the backward direction, i.e. water will decompose to give hydrogen and oxygen back." Water is decomposed back by electrolysis.

Yes. An energy profile shows at a glance: whether a reaction is exothermic or endothermic (products below or above reactants), the size of ΔH, the activation energy and so how easily the reaction starts, and how a catalyst lowers the barrier.

The products end at exactly the same energy level as the reactants, so ΔH = 0. There is still a hump: the reaction still needs activation energy.

Any physical or chemical change under study is called a system. Everything else which does not fall in the system is called the surrounding. When boiling water in a beaker, the water molecules are the system and the beaker and burner are the surrounding.

When energy is transferred from surrounding to the system, the change is endothermic and has a positive sign. When energy is transferred from system to surrounding, the change is exothermic and carries a negative sign.

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

The standard enthalpy of reaction (ΔH°) is the enthalpy change when reactants in their standard states undergo reaction to produce products in their standard states. It is the heat of reaction at constant pressure. Example: 2CO + O₂ → 2CO₂, ΔH° = −566.0 kJ.

When energy is absorbed during a reaction, the total enthalpy of the system increases. When energy is evolved, the total enthalpy of the system decreases.

Thermodynamics deals with the energy changes during chemical reactions and how these changes affect the properties of a chemical system. Chemical energetics is a part of it.

Chemical reactions in which heat energy is evolved are called exothermic reactions, while those in which heat energy is absorbed are called endothermic reactions.

Heat evolved during an exothermic reaction goes to the surrounding, so the container gets hot. In an endothermic reaction the absorption of heat from the surrounding decreases the temperature of the container.

Heat which is evolved or absorbed during a chemical reaction is called the heat of that reaction. At constant pressure it equals ΔH.

N₂ + O₂ → 2NO is highly endothermic (ΔH = +180.6 kJ) and has a very high activation energy because the N≡N triple bond is very strong. Only lightning provides enough energy; when lightning stops, the reaction stops.

At power stations fuels such as natural gas, oil and coal are burnt. The heat from their combustion produces steam at high pressure, which rotates turbines, which in turn generate electricity.

Fireworks are the result of combustion reactions that yield heat, light and sound. Different metal salts along with oxidising agents produce a variety of colours when burnt.

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

When two atoms come together to form a bond, the attraction between their nuclei and the bonding electrons lowers their energy. The extra energy is released to the surroundings, so bond formation always evolves heat. It is the reverse of bond breaking: the same energy that is needed to break a bond is released when it forms.

In a reaction, if weaker bonds are broken and stronger bonds are formed, the energy evolved is more than the energy absorbed: the reaction is exothermic overall. If the bonds broken are stronger than those formed, it is endothermic.

Bond dissociation energy of H₂ = 435 kJ mol⁻¹ and of O₂ = 498 kJ mol⁻¹. Energy absorbed = 2(435) + 498 = 1368 kJ: 2H₂ + O₂ → 4H + 2O.

Weaker bonds are broken (H–H) and less energy is absorbed (1368 kJ), while the O–H bonds formed in water are stronger and more energy is evolved (1936 kJ). The energy evolved is more than the energy absorbed, so ΔH = −568 kJ for 2 mol.

The energy needed to break one mole of a particular bond in gaseous molecules into separate atoms. H–H 435 kJ mol⁻¹, O=O 498 kJ mol⁻¹, C–H (average) 412 kJ mol⁻¹. The same amount is released when the bond forms.

Majority of molecules possess average kinetic energy, but a few possess more than average energy. The molecules which possess more than average kinetic energy may be called excited molecules. Only their collisions can produce the transition state.

The arrangement formed when excited reactant molecules collide, with old bonds partly broken and new ones partly formed. It is shown at the peak of the energy curve. After a very short period of time the transition state either returns to the reactants or goes to the products.

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.

Because in the transition state the bonds between the reactant or product molecules are being cleaved progressively. Partly broken bonds hold the atoms less tightly, so the arrangement has more energy.

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Ni in the hydrogenation of oil to banaspati ghee; platinum in the production of H₂SO₄; chlorine promoting the breakdown of ozone.

It changes the path adopted by the reactants, whereby the activation energy is substantially decreased. As a result more reactants are now able to be converted into products, and the rate increases.

No. The catalyst gives a lower hump but the reactants and products stay at the same energy levels, so the difference between them, ΔH, is unchanged.

Profile (b), where the products are at the same level as the reactants. The barrier is the same height from both sides, so the backward reaction is as easy as the forward one. In (a) the backward barrier is far higher, so that reaction is practically one-way.

Because they have a high activation energy. Natural gas and air, or diamond turning into graphite (ΔH = −3 kJ mol⁻¹), do not react at room temperature: too few molecules have enough energy to reach the transition state. A flame or spark supplies that energy.

The process of respiration which occurs in the presence of oxygen is called aerobic respiration. It is exothermic: glucose → 2 pyruvate (cytoplasm) → 6CO₂ + 6H₂O + energy (mitochondria).

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.

During glycolysis one molecule of glucose is split into two molecules of pyruvate. It takes place in the cytoplasm, is a series of enzyme-catalysed reactions and gives a net production of 2 ATP.

ATP (adenosine triphosphate) is the energy carrier of the cell. Energy released in respiration is stored in ATP; when cells require energy for metabolic activities, they break down ATP to get the required energy.

Lipids are a group of organic compounds which include fats, waxes and sterols. They serve as an energy reserve within our body. About half of the fuel our body needs comes from lipids. Excess food is stored as lipids in adipose cells.

Why no reaction normally

N₂ + O₂ → 2NO is highly endothermic (ΔH = +180.6 kJ). It also has a very high activation energy: the N≡N triple bond (about 945 kJ mol⁻¹) and the O=O bond must start breaking before NO can form. At ordinary temperatures almost no molecules have this much energy, so air does not react.

Why lightning works

A lightning flash heats the air to thousands of degrees. Now many molecules have energy above Eₐ, and the reaction also gets the large amount of heat it must absorb.

Why it stops

Being endothermic, the reaction needs a continuous supply of energy. It does not release heat to keep itself going, so the moment the lightning stops, the temperature falls and the reaction stops.

Why no reaction on mixing

CH₄ + 2O₂ → CO₂ + 2H₂O is exothermic, but it has a high activation energy. At room temperature too few molecules have enough energy to reach the transition state, so nothing happens.

Role of the match

The flame supplies the activation energy to the gas molecules nearby: they react.

Why it keeps going

The reaction is highly exothermic (about −890 kJ per mole of methane). The heat given out by the first molecules raises the energy of the next molecules above Eₐ, so the reaction sustains itself until the methane or the oxygen is used up.

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.