First Law of Thermodynamics: Law of Conservation of Energy Chemistry Tutorial

Key Concepts

• Thermodynamics is the study of the changes of energy, or transformations of energy or energy conversions, which accompany physical and chemical changes in matter.
• Thermodynamics describes the behaviour of macroscopic systems.
• The First Law of Thermodynamics is also known as the Law of Conservation of Energy (or the Law of Energy Conservation).
• The Law of Conservation of Energy states that energy can not be created nor destroyed, but it can be transformed (changed or converted) from one form of energy to another.

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Types of Energy

Energy can be defined as the capacity to do work.

The amount of energy stored in a substance, or in a device, is the amount of work that the substance, or device, can be made to do.

The S.I. unit of energy is the joule (J).¹

We recognise a number of different types of energy, or forms of energy, such as those listed in the table below:

Each of the above is considered a form of energy, or a type of energy, because, under the right conditions, each can be used to do work.

Energy Transformations or Energy Conversions

The First Law of Thermodynamics states that energy can neither be created nor destroyed.

For a given chemical reaction in a closed system this means that whatever energy was present as chemical energy in the reactants, the same amount of energy must be present after the reaction has occurred.

Because the chemical energy (the energy stored) in reactants will not be the same as the chemical energy of the products, some energy will be transformed (changed or converted) from one type to another.

Common energy transformations (energy conversions) that occur during chemical reactions are:

• chemical energy to heat energy (thermal energy)
  and the reverse: heat energy (thermal energy) to chemical energy
• chemical energy to electrical energy
  and the reverse: electrical energy to chemical energy
• chemical energy to light energy
  and the reverse: light energy to chemical energy

Transforming Chemical Energy into Heat Energy (Thermal Energy)

Combustion reactions release heat energy (thermal energy).
When you light a campfire, wood is combusted to produce heat energy which you can feel, light energy which you can see, and sound energy which you can hear.
The chemical energy stored in the wood is being transformed (changed or converted) into other forms of energy.

In a combustion reaction a fuel is combusted (burnt) in the presence of oxygen (an oxidiser) to produce the products of combustion and heat energy.

fuel + oxygen → combustion products + heat energy

According to the First Law of Thermodynamics, energy must be conserved, that is, the chemical energy of the reactants (fuel and oxygen) must be equal to the chemical energy of the combustion products + the heat energy released.

chemical energy (fuel and oxygen) = chemical energy (combustion products) + heat energy released

This means that some of the chemical energy of the reactants has been transformed into heat energy.

The balanced chemical equations for the combustion of some common fuels and the amount of heat energy released in kilojoules per mole of fuel are given below:

A combustion reaction involves some chemical energy of the reactants being transformed into heat energy, and some chemical energy being stored in the compounds that are produced as a result of the combustion of the fuel.
Note that if you watch a fuel combust you will not only feel the heat energy being released but you will also see some light energy produced and you might even hear some sound energy being produced.

Transforming Heat Energy Into Chemical Energy

In a thermal decomposition reaction, heat energy is supplied and transformed into stored chemical energy in the products of the decomposition reaction.

reactant + heat energy → products

According to the First Law of Thermodynamics energy must be conserved, that is, the chemical energy stored in the reactant + the heat energy supplied must equal the amount of chemical energy stored in the product molecules.
This means that the heat energy supplied is being transformed into chemical energy.

For example, the calcium carbonate (CaCO_3(s)) found in limestone undergoes thermal decomposition to produce calcium oxide (CaO_(s), known as quick lime) and carbon dioxide gas (CO_2(g)).
178 kJ of energy must be supplied for every mole of calcium carbonate that undergoes thermal decomposition:

CaCO_3(s) + 178 kJ mol^-1 → CaO_(s) + CO_2(g)

According to the First Law of Thermodynamics energy must be conserved, that is, the chemical energy of CaCO_3(s) + heat energy supplied must equal the chemical energy of the products CaO_(s) and CO_2(g).

So the heat energy supplied is being transformed into chemical energy which is stored in the chemical bonds in the product compounds.

Transforming Chemical Energy Into Electrical Energy

In a coal-fired powered station, the chemical energy in coal (largely carbon) is transformed into heat energy when the coal combusts.
This heat energy is used to heat water in a boiler in which the heat energy is transformed into mechanical energy (kinetic energy, or increased movement, of the water molecules).
The kinetic energy of the water molecules is then used to drive a turbine, that is, the kinetic energy of the water molecules is transformed into a different form of kinetic or mechanical energy.
The mechanical (kinetic) energy of the turbine is then transformed into electrical energy via generators.

The energy transformations associated with a coal-fired power plant can be summarised as:

chemical energy of coal → heat energy → mechanical energy (kinetic energy) → electrical energy

We often think of a battery as a vessel that somehow stores electrical energy, but in fact, when a battery discharges, the chemical energy of the reactants is converted directly into electrical energy.

For example, in an alkaline cell, the chemical energy stored in the reactants, solid zinc (Zn_(s)), manganese(IV) oxide (MnO_2(s)) and water, is converted into electrical energy + the chemical energy stored in the products zinc oxide (ZnO_(s)) and manganese(III) hydroxide (Mn(OH)_3(s)).
About 300 kJ of electrical energy is produced per mole of zinc (Zn_(s)) consumed by the reaction:

Zn_(s) + 2MnO_2(s) + 3H₂O_(l) → ZnO_(s) + 2Mn(OH)_3(s) + 300 kJ mol^-1

By the First Law of Thermodynamics, energy must be conserved, so, the energy stored in the reactants must equal the energy stored in the product compounds + the electrical energy produced.
This means that some of the chemical energy has been transformed into electrical energy.

A fuel cell is another device by which the chemical energy stored in the reactants (fuel and oxidiser) is transformed into electrical energy.

Transforming Electrical Energy Into Chemical Energy

Some batteries can be recharged.
When a battery is recharged, electrical energy is converted directly into chemical energy.

For example, when a car is running, the alternator supplies electricity to the lead-acid battery to recharge it.
Electrical energy is used to convert the chemical energy of lead(II) sulfate (PbSO_4(s)) and liquid water (H₂O_(l)) to chemical energy stored in the products lead (Pb_(s)), lead(IV) oxide (PbO_2(s)) and sulfuric acid (H₂SO_4(aq)):

electrical energy + 2PbSO_4(s) + 2H₂O_(l) → Pb_(s) + PbO_2(s) + 2H₂SO_4(aq)

By the First Law of Thermodynamics, energy must be conserved, so, the chemical energy of the reactants + the electrical energy supplied must equal the chemical energy of the products.
This means that some of the electrical energy must be transformed (changed or converted) into the chemical energy stored in the products.

Electrolysis is the name given to the process by which electrical energy is converted into chemical energy.

Electrical energy can also be used to decompose compounds (known as electrolytic decompostion).

For example, if you pass electrical energy through liquid water, the water will decompose into hydrogen gas and oxygen gas:²

electrical energy + H₂O_(l) → H_2(g) + ½O_2(g)

The First Law of Thermodynamics tells us that energy must be conserved so the electrical energy supplied + the chemical energy of water must equal the chemical energy of the products (oxygen gas and hydrogen gas).

Some of the electrical energy has been converted into chemical energy.

Transforming Chemical Energy Into Light Energy

As discussed above, during the combustion of a fuel, most of the chemical energy stored in the fuel and oxidiser is converted into heat energy, but a small amount is also converted into light energy which you can see as a flame.

Bright white light is produced when magnesium (Mg_(s)) combusts in oxygen gas (O_2(g)) to produce magnesium oxide (MgO_(s)), along with heat energy:

Mg_(s) + ½O_2(g) → heat energy + light energy + MgO_(s)

By the First Law of Thermodynamics, energy must be conserved, so the chemical energy stored in the magnesium and oxygen must be equal to the heat energy produced + the light energy produced + the chemical energy stored in the magnesium oxide.
Therefore, some of the chemical energy in the reactants has been converted into heat energy and into light energy.

Fireworks are also an example of chemical energy being transformed into light energy.
The different colours of fireworks are related to the amount of energy being transformed into light energy.

Transforming Light Energy Into Chemical Energy

Possibly the most important example of light energy being transformed into chemical energy on Earth is during the process of photosynthesis.

During photosynthesis, green plants use ultraviolet light energy from the sun to convert carbon dioxide (CO_2(g)) and liquid water (H₂O_(l)) into glucose (C₆H₁₂O_6(aq)) and oxygen gas (O_2(g)):

light energy + 6CO_2(g) + 6H₂O_(l) → C₆H₁₂O_6(aq) + 6O_2(g)

By the First Law of Thermodynamics, energy must be conserved.
This means that light energy must be transformed into chemical energy which is stored in the product molecules (glucose and oxygen).

Footnotes