What Are Energy Changes?

What is Energy?

Energy is defined as the capacity to do work. It refers to the ability to cause motion or create change. Energy comes in different forms such as kinetic energy, potential energy, thermal energy, chemical energy, nuclear energy and more. These forms can be grouped into two main types – stored (potential) energy and working (kinetic) energy.

Kinetic energy is the energy possessed by an object in motion. It depends on the object’s mass and velocity. A moving object has the capacity to do work by colliding with and displacing other objects. Kinetic energy can be transferred between objects during collisions.

Potential energy is stored energy possessed by an object due to its position or chemical composition. It is energy that is waiting to be released or transformed. Types of potential energy include gravitational (dependent on height), elastic (dependent on deformation of an elastic object), chemical (stored in the bonds between atoms), and nuclear (stored in the nucleus of an atom).

Other important forms of energy include thermal energy (related to the kinetic energy of atoms and molecules), electromagnetic radiation (energy carried by electromagnetic waves), electrical energy (energy from electric charge flow), sound energy (energy carried by sound waves), and radiant energy (energy emitted from nuclear reactions).

Understanding the different forms of energy and how they can be transformed from one to another is key to explaining many physical phenomena and technologies in our world.

Types of Energy Changes

Energy can change from one form to another during physical and chemical processes. Some common types of energy changes include:

Chemical to Thermal Energy
Chemical reactions, such as combustion or cellular respiration, involve chemical potential energy being converted into thermal energy or heat. For example, when wood burns, the chemical energy stored in the wood’s molecular bonds is released as thermal energy.

chemical energy being converted into thermal energy during combustion
Electrical to Light Energy
Light bulbs convert electrical energy into light energy and thermal energy. The electricity provides energy for the filament in the bulb to heat up and emit photons.

Mechanical to Sound Energy

When a guitar string vibrates, it causes the air particles around it to vibrate and transmit energy through the air in waves. This mechanical energy from the string converts into sound energy you can hear.

Nuclear to Electrical Energy
In nuclear power plants, nuclear fission converts nuclear potential energy into thermal energy, which is used to heat water into steam that drives a turbine to generate electricity.

Radiant to Chemical Energy
Plants convert radiant light energy from the sun into chemical energy in the form of glucose molecules during photosynthesis. The plant uses this stored chemical energy for growth and reproduction.

Energy changes always involve one form of energy being converted into another. The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one state to another.

Exothermic vs Endothermic Reactions

Chemical reactions involve energy changes that are categorized as either exothermic or endothermic.

Exothermic reactions release energy in the form of heat. Examples of exothermic reactions include combustion, many oxidation reactions, and some polymerization reactions. When wood burns in a fireplace, the combustion reaction releases heat energy that we can feel in the form of warmth. When iron rusts, the oxidation reaction gives off heat. The setting of epoxy glue is an exothermic polymerization reaction, which is why epoxy glues feel warm as they harden.

In contrast, endothermic reactions absorb energy. Many endothermic reactions feel cold because they are drawing heat from their surroundings. Examples of endothermic reactions include photosynthesis in plants, the reaction between baking soda and vinegar, and the hydration of anhydrous salts. Plants use the energy from sunlight to drive the endothermic photosynthesis reaction. When baking soda and vinegar react to form carbon dioxide, the solution feels cold because the reaction absorbs energy. Hydrating anhydrous compounds like calcium chloride is endothermic and makes the hydrated compound feel noticeably colder than before the reaction.

Understanding whether a chemical reaction is exothermic or endothermic is important for maximizing or minimizing the heat transfer involved. Chemists apply this knowledge to optimize reactions and make use of the energy transfer in various processes.

Measuring Energy Changes

Calorimetry is the most common method for measuring energy changes that occur during chemical reactions. This involves using a calorimeter, an insulated container filled with a known amount of water or other liquid. As a reaction occurs inside the calorimeter, the temperature change of the water is carefully monitored.

By knowing the mass of water, the specific heat capacity of water, and the temperature change, the energy change for the reaction can be calculated using the following equation:

q = mcΔT


  • q = energy change (Joules)
  • m = mass of water (grams)
  • c = specific heat capacity of water (4.184 J/g°C)
  • ΔT = change in temperature of water (°C)

This simple calculation allows scientists to quantify the energy changes for a wide variety of chemical reactions. Exothermic reactions will heat up the surrounding water, resulting in a positive temperature change. Endothermic reactions absorb heat from the water, causing it to decrease in temperature.

By carefully measuring mass, temperature change, and accounting for heat losses, calorimetry provides an accurate and reproducible way to measure energy changes for chemical processes.

Energy Diagrams

Energy diagrams, also known as reaction profiles, are graphical representations of the energy changes that occur during a chemical reaction. They provide valuable insight into the energetics of a reaction and allow us to visualize concepts like activation energy, exothermic vs. endothermic reactions, and transition states.

On an energy diagram, the x-axis represents the reaction progress from reactants to products, while the y-axis represents the energy content. The reactant and product energy levels are shown as horizontal plateaus. The difference in energy between the reactants and products indicates whether the reaction is exothermic (energy releasing) or endothermic (energy absorbing).

The most important feature of an energy diagram is the activation energy (Ea). This is the minimum amount of energy required for a chemical reaction to take place. Reactions need activation energy to overcome the energy barrier and reach the transition state. The transition state represents the highest energy, unstable intermediate configuration between reactants and products.

For exothermic reactions, the products have lower energy than the reactants. So energy is released as the reaction goes from high to low energy state. On the diagram, the product plateau is at a lower level than the reactant plateau. The opposite is true for endothermic reactions, where the products have higher energy and the reaction absorbs energy overall.

By analyzing activation energies and energy changes, energy diagrams let us predict how different factors like temperature and catalysts will affect reaction rates and yields. They provide a powerful visualization of chemical processes from an energy perspective.

First Law of Thermodynamics

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy can neither be created nor destroyed, only converted from one form to another. This means the total amount of energy in the universe remains constant.

For example, when a match is struck the chemical energy in the match’s head is converted to heat, light, and sound. The total amount of energy before and after remains the same, it is just redistributed into different forms. This concept can be applied to all energy transformations.

The First Law of Thermodynamics is a fundamental law of nature that has many important implications. It tells us that perpetual motion machines are impossible, since they would have to create energy out of nothing. It also means that we can’t create or destroy energy, only harness and redirect it. Understanding this law is key for evaluating energy changes in chemical reactions, biological processes, and other phenomena.


Entropy is a measure of the tendency of a system toward disorder or randomness. In terms of chemistry, entropy relates to the spontaneity of reactions and the dispersal of energy.

When a chemical reaction occurs, there is often a dispersal of energy and matter. For example, when a solid dissolves into a solution, the ordered structure of the solid breaks down as its molecules spread out into the surrounding solution. This increases the disorder of the system, and thus increases its entropy.

Reactions that increase the entropy of the universe, like dissolution, tend to be spontaneous and thermodynamically favorable at constant temperature and pressure. On the other hand, reactions that decrease entropy by increasing order, like crystal formation, tend to be nonspontaneous. Reactions will only proceed spontaneously if the increase in entropy is greater than the energy needed to break interactions.

Changes in entropy can be used along with enthalpy changes to determine if a process will occur spontaneously using Gibbs free energy. Understanding entropy is key to predicting the directionality and spontaneity of chemical reactions.

Gibbs Free Energy

Gibbs free energy is an important thermodynamic function that determines whether a chemical reaction will occur spontaneously under constant temperature and pressure conditions. It is represented by the symbol G and is expressed in units of energy such as Joules or calories.

Gibbs free energy takes into account two other thermodynamic properties – enthalpy (H) and entropy (S). Enthalpy accounts for the total heat content of a system, while entropy is a measure of the disorder or randomness of molecules. Gibbs free energy is defined by the following equation:

G = H – TS


  • G = Gibbs free energy
  • H = Enthalpy
  • T = Absolute temperature in Kelvin
  • S = Entropy

If Gibbs free energy is negative, the reaction will occur spontaneously. This is because the release of energy from the reaction can be used to do useful work. If Gibbs free energy is positive, an input of energy is required for the reaction to proceed, so it will not occur spontaneously.

To determine spontaneity, the standard Gibbs free energy change (ΔG°) of the reaction is calculated. If ΔG° is negative, the reaction is spontaneous. If ΔG° is positive, the reaction is non-spontaneous. Reactions with a ΔG° of zero are at equilibrium.

Gibbs free energy provides useful insights into whether a reaction will occur spontaneously based on the competing factors of enthalpy and entropy. Understanding Gibbs free energy is key for predicting the feasibility and direction of chemical reactions.

Real World Applications

Energy changes and conversions occur constantly in the world around us. Here are some examples:

Biological Systems

Plants convert light energy from the sun into chemical energy through photosynthesis. The chemical energy is stored in the bonds of glucose molecules and later released through cellular respiration to power life processes.

Animals acquire chemical energy by breaking down food through cellular respiration. The energy released is used for muscle contraction, nerve impulses, and other biological functions.

Inside cells, energy conversions drive processes like protein synthesis and cell division. Energy is also required to maintain organization through biosynthesis and the breakdown of molecules.


Batteries and fuel cells rely on electrochemical reactions to produce electrical energy from chemical reactants. This powers numerous devices and vehicles.

Solar panels convert light energy into electricity that can be used to power homes and grids. The photovoltaic effect enables this conversion.

Fossil fuel power plants burn coal, natural gas, or oil to heat water. The steam produced turns turbines connected to generators, converting heat energy into electrical energy.

Nuclear power plants split uranium atoms in nuclear fission reactions, releasing energy that heats water to produce steam. The steam spins turbines to generate electricity.

Wind turbines use the kinetic energy of wind to turn blades connected to generators that produce electricity.

Importance of Understanding Energy

Understanding energy and energy changes is crucial for advances in science, technology, and industry. The laws of thermodynamics that describe energy underpin many of the processes we use and take for granted today. For example, understanding how chemical energy can be converted into electrical energy enabled the development of batteries and fuel cells that now power our phones, laptops, cars, and more.

Knowledge of energy transformations has also enabled revolutionary technologies like internal combustion engines, steam and gas turbines, jet engines, and rocket engines that drive transportation and provide the backbone of the industrialized world. Even seemingly simple inventions like refrigerators and air conditioners rely on principles of energy transfer.

Looking ahead, mastering energy conversions will be key to tackling global challenges like climate change and transitioning to renewable energy sources. Fundamental research into novel energy materials, storage, and generation can help realize a more sustainable energy future.

In summary, comprehending energy changes is at the heart of scientific and technological progress. A deep understanding of thermodynamics and energy transformations will continue to enable innovations that transform society.

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