Discover what a combustion reaction is as well as what reactants are needed and what products are produced. Learn to write and balance a combustion reaction. Through the concepts of bond energies, learn how to explain why combustion reactions are largely exothermic.

## Combustion Reactions

A combustion reaction is when a substance reacts with oxygen and releases a huge amount of energy in the form of light and heat. A combustion reaction always has oxygen as one reactant. The second reactant is always a hydrocarbon, which is a compound made up of carbon and hydrogen. A combustion reaction also always produces CO2 and H2O.

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## Balancing Combustion Reactions

Balancing combustion reactions is easy.

1. First, balance the carbon and hydrogen atoms on both sides of the equation.
2. Then, balance the oxygen atoms. You balance oxygen after carbon and hydrogen because it is always standing alone as O2, and it is easier to balance than H and C. It’s usually better to start balancing the harder things first, then the easier things after.
3. Finally, balance anything that has become unbalanced.

Balance the following equation of burning propane:

C3H8 + O2 = CO2 + H2O

#### Step 1

There are eight hydrogen atoms on the left and two on the right. So, add a four in front of the water molecule.

C3H8 + O2 = CO2 + 4 H2O

Now, balance the carbons. There are three on the left and only one on the right, so add a three in front of the CO2 on the right.

C3H8 + O2 = 3 CO2 + 4 H2O

#### Step 2

Count the oxygen atoms on the product side (the right) and write that number in front of the O2 on the left. There are 10 total oxygen atoms on the right – six in the 3 CO2 and four in the 4 H2O. In order to balance the oxygen atoms, you need to put a five in front of the O2 on the left.

#### Step 3

Everything is balanced. The final equation is:

C3H8 + 5 O2 = 3 CO2 + 4 H2O

Try another one:

C2H5OH + O2 = CO2 + H2O

Count the carbons on the left. There are two. Count the carbons on the right. There is one. So, put a two in front of the CO2.

C2H5OH + O2 = 2 CO2 + H2O

Count the hydrogen atoms on the left – there are six. Count the hydrogen atoms on the right. There are two. So, put a three in front of the water molecule.

C2H5OH + O2 = 2 CO2 + 3 H2O

Now, count the oxygen atoms. There are three on the left and seven on the right. How can these most easily be balanced? If we put a three in front of the O2, then we’d have a total of seven on the left and seven on the right.

C2H5OH + 3 O2 = 2 CO2 + 3 H2O

Double-check. Is everything balanced?

Right side Left side
2 C 2 C
6 H 6 H
7 O 7 O

Yep. Everything is balanced.

## Bond Energy

Bond energy is the energy required to break a chemical bond and form a neutral atom.

Every bond has a certain energy, as you can see in the table:

Bond Energy in kJ/mol
H-H 436
H-Cl 431
C-C 347
C-O 351
C-H 414
C=O 799
O=O 495
O-H 460

There are two types of chemical reactions in which bonds are formed or broken. A chemical reaction that releases energy is an exothermic reaction. The energy that is released can be in the form of light, heat, or sound. An endothermic reaction is a reaction that takes in or absorbs heat. A combustion reaction is an exothermic reaction because it releases light and heat.

Bond-making is an exothermic process. Bond-breaking is an endothermic process. Using bond energies, you can determine the final energy profile of a combustion reaction and prove that it is exothermic.

To determine the final energy of a reaction, simply add up the energy number of the bonds broken and subtract the energy number of the bonds made. If the final number is negative, then energy was released, and the reaction was exothermic. If the final number is positive, then energy was required, and the reaction was endothermic.

## Determining Final Reaction Energy

The first step to determine the final energy of a reaction is to write out the structural formula for the compounds that are reacting.

Let’s use the equation we just balanced previously.

C2H5OH + 3 O2 = 2 CO2 + 3 H2O

Determine the number of each type of bond that is broken and made.

Type of bond Number of bonds broken Number of bonds made Energy in kJ/mol
C-H 5 0 414
C-O 1 0 351
O-H 1 6 460
O=O 3 0 495
O=C 0 4 799

Now, you just have to do the math.

Total energy for the bonds being broken on the reactant side:

(5 x 414) + (1 x 351) + (1 x 460) + (3 x 495) + (0 x 799) = 4366

Total energy for bonds being formed on the product side:

(0 x 414) + (0 x 351) + (6 x 460) + (0 x 495) + (4 x 799) = 5956

Energy of bonds broken minus energy of bonds made equals final energy.

4366 – 5956 = -1590

The final energy is negative, so the reaction is exothermic.

See how easy that is? We’ll just do one more example with the first equation that we balanced.

C3H8 + 5 O2 = 3 CO2 + 4 H2O

Determine the number of each type of bond that is broken and made.

Type of bond Number of bonds broken Number of bonds made Energy in kJ/mol
C-H 8 0 414
O-H 0 8 460
O=O 5 0 495
O=C 0 6 799
C-C 2 0 347

Total energy for the bonds being broken on the reactant side:

(8 x 414) + (0 x 460) + (5 x 495) + (0 x 799) + (2 x 347) = 6481

Total energy for bonds being formed on the product side:

(0 x 414) + (8 x 460) + (0 x 495) + (6 x 799) + (0 x 347) = 8474

Energy of bonds broken minus energy of bonds made equals final energy.

6481 – 8474 = -1993

The final energy is negative, so it is an exothermic reaction.

## Lesson Summary

A combustion reaction is when a substance reacts with oxygen and releases a huge amount of energy in the form of light and heat. A combustion reaction always includes a hydrocarbon and oxygen as the reactants and always produces carbon dioxide and water as products.

Balancing combustion reactions is similar to balancing other types of reactions. First, balance the carbon and hydrogen atoms on both sides of the equation. Next, balance the oxygen atoms, and finally, double-check to make sure everything is still balanced.

Combustion reactions are exothermic, meaning they release energy. This can be shown by calculating the final energy of the reaction by subtracting the total energy of the bonds made from the total energy of the bonds broken. If the final energy is a negative number, then energy was released, and the reaction was exothermic.

## Learning Outcomes

After studying this lesson, you may be able to:

• Define combustion reaction
• List the steps required to balance combustion reactions
• Differentiate between exothermic and endothermic reactions
• Prove a reaction is exothermic using bond energy calculations