Chemical Energy

Chemical energy is energy stored in the bonds between atoms in molecules. Every chemical bond holds a certain amount of energy. When bonds break and reform during a chemical reaction, energy is released (or absorbed). Food, fuel, batteries and even your own muscles all rely on chemical energy. Burning a candle, running a car engine, eating breakfast and lighting up your phone screen are all examples of chemical energy in action.

  • What it isEnergy in atomic bondsIn molecules
  • Released byChemical reactionsBurning, digesting, charging
  • Food energyAbout 17 kJ/g (carbs)37 kJ/g (fats)
  • Petrol energy46 MJ/kgVery energy-dense
  • Battery storageUp to 800 kJ/kgLithium-ion best
  • Daily food intakeAround 8,400 kJ2,000 kcal

How chemical energy is stored

Every chemical bond between two atoms holds energy. Breaking a bond requires energy; forming a bond releases energy. In a chemical reaction:

  • Some bonds in the reactants break (this takes energy).
  • New bonds form in the products (this releases energy).
  • If the new bonds release more energy than was needed to break the old ones, the reaction overall RELEASES energy (exothermic).
  • If the old bonds needed more energy than the new bonds release, the reaction ABSORBS energy (endothermic).

Most useful energy sources (food, fuel, explosives) are exothermic: they release stored chemical energy as heat, light or motion.

Chemical energy in food

Food contains chemical energy stored in carbohydrates, fats and proteins. Your body breaks down these molecules in digestion, then uses the energy to:

  • Power your muscles for movement.
  • Run your brain (which uses about 20 per cent of your daily food energy).
  • Keep your body warm.
  • Build new cells, hair, nails and skin.
  • Drive every chemical process inside you.

Different foods store different amounts of energy:

  • Carbohydrates: 17 kJ per gram (4 kcal).
  • Proteins: 17 kJ per gram (4 kcal).
  • Fats: 37 kJ per gram (9 kcal). More than twice as much as carbohydrates per gram.
  • Alcohol: 29 kJ per gram (7 kcal). (Adults only.)
Fact Food labels in the UK and Europe list both kilocalories (kcal) and kilojoules (kJ). 1 kcal equals 4.184 kJ. The "calorie" you see in everyday talk is really a kilocalorie. An average adult needs about 2,000 to 2,500 kcal per day (8,400 to 10,500 kJ), with higher amounts for very active people, athletes and growing children.

Chemical energy in fuels

Fossil fuels (petrol, diesel, coal, gas) store huge amounts of chemical energy from ancient organic matter:

  • Petrol: about 46 MJ per kg (46,000 kJ).
  • Diesel: 45 MJ per kg.
  • Natural gas (methane): 55 MJ per kg.
  • Coal: 24-30 MJ per kg depending on type.
  • Hydrogen: 142 MJ per kg (very energy-dense, but very low density as gas).
  • Wood: about 16 MJ per kg.

Burning a fuel releases this stored energy as heat (and a small amount of light). Engines and boilers convert that heat into useful motion or hot water.

Batteries

A battery stores chemical energy and converts it to electrical energy when connected to a circuit. Inside, two different chemicals (one at each electrode) react with each other slowly, pushing electrons through the wire as they do so.

  • Alkaline batteries (AA, AAA, etc.): use zinc and manganese dioxide. Single-use.
  • Lead-acid batteries (car batteries): use lead, lead oxide and sulfuric acid. Rechargeable.
  • Lithium-ion batteries (phones, laptops, electric cars): use lithium compounds. Lightweight, rechargeable.
  • Lithium-polymer batteries: similar chemistry but with a polymer electrolyte. Flexible and slim.
  • Nickel-metal hydride: rechargeable, used in some hybrid cars.

The energy density of modern lithium-ion batteries (up to 800 kJ per kg) is impressive, but still much less than petrol. That is why electric vehicles need very heavy battery packs to match the driving range of a small petrol tank.

Did you know? Your bodys cells use a special molecule called ATP (adenosine triphosphate) to carry chemical energy from the food you digest to the parts of cells that need to do work. ATP is sometimes called the "energy currency" of life. The bonds in ATP store small, easily released amounts of energy. Each cell makes and uses millions of ATP molecules every second. In total, your body produces and consumes around your own body weight in ATP every day, all by recycling it constantly.

How chemical energy is released

  • Combustion (burning): fast reaction with oxygen, releases lots of heat. Used in cars, power stations, fires.
  • Respiration: slow, controlled "burning" of glucose inside cells, releases energy as ATP. Used by every living thing.
  • Electrochemistry: chemical reactions in batteries push electrons through a wire as electric current.
  • Photosynthesis (in reverse): plants store solar energy as chemical energy in sugars and other compounds.
  • Explosions: very fast exothermic reactions that release energy as heat, sound, light and shock waves.

Energy released, where it goes

The chemical energy released by a reaction can end up as:

  • Heat: most common. A fire warms everything around it.
  • Light: from a candle flame, an LED in a chemical reaction, fireflies or other bioluminescent creatures.
  • Motion: in muscles, in a car engine, in a jet.
  • Electricity: in batteries and fuel cells.
  • Sound: explosions and crackling reactions.
Try this Light a small candle (with adult permission) and watch the flame. You are witnessing chemical energy being released. The wax (a long carbon-hydrogen molecule) reacts with oxygen, breaking its bonds and forming carbon dioxide and water. The energy that was stored in the wax bonds is released as heat (which you can feel near the flame) and light (which lets you see). One small candle releases a continuous stream of energy from the chemical bonds inside.
Deeper dive: how a lithium-ion battery works

The lithium-ion battery powers your phone, laptop, headphones and (probably) some of your friends electric scooters. It is one of the most important technologies of the 21st century, and won its inventors the 2019 Nobel Prize in Chemistry.

Inside a lithium-ion battery are three main parts:

  • A positive electrode (cathode) made of a lithium compound, usually lithium cobalt oxide or similar.
  • A negative electrode (anode) made of graphite (a form of carbon).
  • An electrolyte: a liquid or gel between the electrodes that allows lithium ions (charged atoms) to flow.

When the battery is discharging (powering your phone), lithium ions move from the anode (graphite) through the electrolyte to the cathode. At the same time, electrons flow through the external circuit (your phones electronics) from anode to cathode, producing the useful electrical current. The chemical energy stored in the lithium compound is converted to electrical energy.

When the battery is charging (plugged in), the external power source pushes the electrons (and lithium ions) back the other way. Lithium returns to the anode, ready for the next cycle.

Each cycle slightly degrades the materials. A typical phone battery is good for around 500 to 1,000 full charge cycles before it noticeably loses capacity. New chemistries (solid-state batteries, sodium-ion, lithium-sulfur) are being developed to improve energy density, lifetime and safety. The next decade is likely to see batteries improving as rapidly as computer chips did in the 2000s, opening the door to longer-range electric cars and grid-scale renewable storage.

For more, see electrical energy and heat (thermal energy).