6.4: Chemical Energy

Chemical energy is a form of potential energy that is stored within the bonds of atoms and molecules. Think of it as the "hidden" energy inside a substance, waiting to be released when a chemical reaction takes place. It is important to note that chemical bonds require energy input to break the bonds. All stable chemicals have negative potential energy, that is, the bond sinks the chemical energy down an energy "well". Imagine a rock that has fallen down a literal well. That rock requires energy or work to be done to lift it out. The same is true of chemical bonds.  However, when a chemical reaction happens, these bonds are broken and new ones are formed, and this process can either release or absorb energy. If energy is released, the products have lower energy than the reactants. This is an exothermic reaction. If energy is absorbed, the products have higher energy than the reactants. This is an endothermic reaction. Both kinds of reactions can occur naturally under the right circumstances.

Here are a few ways to think about it with everyday examples.

Food: Fuel for Your Body

When you eat food, your body performs a series of chemical reactions to break down the food molecules (like sugars, fats, and proteins). The energy that was stored in the chemical bonds of those food molecules is released. Your body then uses this released energy to power your muscles, keep you warm, and allow your brain to think. This is why we say food is "fuel" for our bodies. The most commonly referred to food reaction is the combustion of glucose, where glucose (C₆H₁₂O₆) plus oxygen (O₂) react to from carbon dioxide (CO₂) and water (H₂O). The reaction is written as

\[{\rm C_6H_{12}O_6~+~6O_2~\rightarrow~6CO_2~+~6H_2O}\]

Example: Chemical Reaction Balancing

The subscripts in a molecule indicate how many atoms of that type are in the molecule. The numbers in front of each molecule indicate the number of molecules needed to balance the reaction. Notice there are sixes in front of the oxygen, carbon dioxide and water. These are to balance the chemical reaction input (reactants) with the output (products). No atoms are created or destroyed in a chemical reaction, but the molecules change. Table \(\PageIndex{1}\) shows the count of each element (carbon, oxygen, and hydrogen) in the reactants and products.

Element	Reactants	Products
Carbon, C	6	6
Oxygen, O	6 + 6(2) = 18	6(2) + 6 = 18
Hydrogen, H	12	6(2) = 12

Table  \(\PageIndex{1}\): An accounting of the balance between reactants and products for the combustion reaction of glucose.

Batteries: Stored Power

A battery is an excellent example of chemical energy in action. Inside a battery, there are chemicals that are arranged in a specific way. When you connect the battery to a device (like a flashlight), you complete a circuit that allows a chemical reaction to occur. This reaction releases electrons, and this flow of electrons is what we call electricity. The chemical energy stored in the battery is converted into electrical energy. The reaction in a lead acid battery like in your gas-powered automobile is

\[{\rm Pb~+~HSO_4^-~\rightarrow~PbSO_4~+~H^+~+~2e^-}\]

Exercise \(\PageIndex{2}\)

Verify that the equation for the lead acid reaction is balanced.

Answer

• There is one lead (Pb) atom on each side.
• There is one hydrogen (H) atom on each side.
• There is one sulfur (S) atom on each side.
• There are four oxygen (O) atoms on each side.
• We can also balance the charges. There is one negative on the reactant side. There are one positive and two negatives on the product side (1-1-1=-1).

Gasoline: Powering Cars

Gasoline is a mixture of hydrocarbons, which are molecules with a lot of energy stored in their chemical bonds. When you turn on your car's engine, a small spark ignites the gasoline, causing a rapid chemical reaction (combustion). This reaction breaks the bonds of the hydrocarbon molecules, releasing a large amount of energy very quickly. This energy pushes the pistons in the engine, which ultimately makes the car's wheels turn. The chemical energy in the gasoline is converted into mechanical energy. The combustion of octane, a key component of gasoline, is

\[{\rm 2C_8H_{18}~+~25O_2~\rightarrow~16CO_2~+~18H_2O}\]

Exercise \(\PageIndex{3}\)

Verify that the equation for the octane combustion reaction is balanced.

Answer

• There are 2x8 = 16 reactant and 16 product carbon (C) atoms.
• There are 2x18 = 36 reactant and 18 product hydrogen (H) atoms.
• There are 25x2 = 50 reactant and 16x2 + 18 = 50 product oxygen (O) atoms.

Key Concepts

• Potential Energy: A molecule has negative chemical potential energy stored in its bonds. Reactions that make this energy less negative occur naturally and release energy.

• Chemical Reaction: This is the process where bonds are broken and new ones are formed. It's the event that releases the stored chemical energy.

• Exothermic Reaction: A reaction that releases energy, often in the form of heat, light, or sound. Burning gasoline is a great example.

• Endothermic Reaction: A reaction that absorbs energy from its surroundings. An example is an instant cold pack; when you break the inner bag, the chemicals react and absorb heat from the surroundings, making the pack feel cold.

In simple terms, chemical energy is the power source hidden inside a substance, and you need to perform a chemical reaction to unlock it.

PhET Exploration: Balancing Chemical Reactions

If you would like more practice, here is a simulation that provides various chemical reactions that you can balance.