4.1 Writing and balancing chemical equations (Page 2/14)

Chemistry Page 2 / 14

Balancing equations

The chemical equation described in section 4.1 is balanced , meaning that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. It may be confirmed by simply summing the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal. Note that the number of atoms for a given element is calculated by multiplying the coefficient of any formula containing that element by the element’s subscript in the formula. If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together. For example, both product species in the example reaction, CO ₂ and H ₂ O, contain the element oxygen, and so the number of oxygen atoms on the product side of the equation is

(1 {CO}_{2} molecule \times \frac{2 O atoms}{{CO}_{2} molecule}) + (2 H_{2} O molecule \times \frac{1 O atom}{H_{2} O molecule}) = 4 O atoms

The equation for the reaction between methane and oxygen to yield carbon dioxide and water is confirmed to be balanced per this approach, as shown here:

{CH}_{4} + 2 O_{2} ⟶ {CO}_{2} + 2 H_{2} O

Element	Reactants	Products	Balanced?
C	1 $\times$ 1 = 1	1 $\times$ 1 = 1	1 = 1, yes
H	4 $\times$ 1 = 4	2 $\times$ 2 = 4	4 = 4, yes
O	2 $\times$ 2 = 4	(1 $\times$ 2) + (2 $\times$ 1) = 4	4 = 4, yes

A balanced chemical equation often may be derived from a qualitative description of some chemical reaction by a fairly simple approach known as balancing by inspection. Consider as an example the decomposition of water to yield molecular hydrogen and oxygen. This process is represented qualitatively by an unbalanced chemical equation:

H_{2} O ⟶ H_{2} + O_{2} (unbalanced)

Comparing the number of H and O atoms on either side of this equation confirms its imbalance:

Element	Reactants	Products	Balanced?
H	1 $\times$ 2 = 2	1 $\times$ 2 = 2	2 = 2, yes
O	1 $\times$ 1 = 1	1 $\times$ 2 = 2	1 ≠ 2, no

The numbers of H atoms on the reactant and product sides of the equation are equal, but the numbers of O atoms are not. To achieve balance, the coefficients of the equation may be changed as needed. Keep in mind, of course, that the formula subscripts define, in part, the identity of the substance, and so these cannot be changed without altering the qualitative meaning of the equation. For example, changing the reactant formula from H ₂ O to H ₂ O ₂ would yield balance in the number of atoms, but doing so also changes the reactant’s identity (it’s now hydrogen peroxide and not water). The O atom balance may be achieved by changing the coefficient for H ₂ O to 2.

2 H_{2} O ⟶ H_{2} + O_{2} (unbalanced)

Element	Reactants	Products	Balanced?
H	2 $\times$ 2 = 4	1 $\times$ 2 = 2	4 ≠ 2, no
O	2 $\times$ 1 = 2	1 $\times$ 2 = 2	2 = 2, yes

The H atom balance was upset by this change, but it is easily reestablished by changing the coefficient for the H ₂ product to 2.

2 H_{2} O ⟶ 2 H_{2} + O_{2} (balanced)

Element	Reactants	Products	Balanced?
H	2 $\times$ 2 = 4	2 $\times$ 2 = 4	4 = 4, yes
O	2 $\times$ 1 = 2	1 $\times$ 2 = 2	2 = 2, yes

These coefficients yield equal numbers of both H and O atoms on the reactant and product sides, and the balanced equation is, therefore:

2 H_{2} O ⟶ 2 H_{2} + O_{2}

Balancing chemical equations

Write a balanced equation for the reaction of molecular nitrogen (N ₂ ) and oxygen (O ₂ ) to form dinitrogen pentoxide.

Solution

First, write the unbalanced equation.

N_{2} + O_{2} ⟶ N_{2} O_{5} (unbalanced)

Next, count the number of each type of atom present in the unbalanced equation.

Element	Reactants	Products	Balanced?
N	1 $\times$ 2 = 2	1 $\times$ 2 = 2	2 = 2, yes
O	1 $\times$ 2 = 2	1 $\times$ 5 = 5	2 ≠ 5, no

Though nitrogen is balanced, changes in coefficients are needed to balance the number of oxygen atoms. To balance the number of oxygen atoms, a reasonable first attempt would be to change the coefficients for the O ₂ and N ₂ O ₅ to integers that will yield 10 O atoms (the least common multiple for the O atom subscripts in these two formulas).

N_{2} + 5 O_{2} ⟶ 2 N_{2} O_{5} (unbalanced)

Element	Reactants	Products	Balanced?
N	1 $\times$ 2 = 2	2 $\times$ 2 = 4	2 ≠ 4, no
O	5 $\times$ 2 = 10	2 $\times$ 5 = 10	10 = 10, yes

The N atom balance has been upset by this change; it is restored by changing the coefficient for the reactant N ₂ to 2.

2 N_{2} + 5 O_{2} ⟶ 2 N_{2} O_{5}

Element	Reactants	Products	Balanced?
N	2 $\times$ 2 = 4	2 $\times$ 2 = 4	4 = 4, yes
O	5 $\times$ 2 = 10	2 $\times$ 5 = 10	10 = 10, yes

The numbers of N and O atoms on either side of the equation are now equal, and so the equation is balanced.

Check your learning

Write a balanced equation for the decomposition of ammonium nitrate to form molecular nitrogen, molecular oxygen, and water. (Hint: Balance oxygen last, since it is present in more than one molecule on the right side of the equation.)

Answer:

2 {NH}_{4} {NO}_{3} ⟶ 2 N_{2} + O_{2} + 4 H_{2} O

Got questions? Get instant answers now!

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?

Aislinn Reply

tijani

what is titration

John Reply

what is physics

Siyaka Reply

A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance

Jude Reply

Can you compute that for me. Ty

Jude

what is the dimension formula of energy?

David Reply

what is viscosity?

David

what is inorganic

emma Reply

what is chemistry

Youesf Reply

what is inorganic

emma

Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes

Adjei

please, I'm a physics student and I need help in physics

Adjanou

chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.

Pedro

A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity

Krampah Reply

2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.

Sahid Reply

you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s

Samuel Reply

can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?

Joseph Reply

Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time

Joseph

Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing

Joseph

"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true

Ryan

what's motion

Maurice Reply

what are the types of wave

Maurice

answer

Magreth

progressive wave

Magreth

hello friend how are you

Muhammad Reply

fine, how about you?

Mohammed

Mujahid

A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?

yasuo Reply

Who can show me the full solution in this problem?

Reofrir Reply

Got questions? Join the online conversation and get instant answers!

Jobilize.com Reply

<< Chapter < Page Page > Chapter >>

Practice Key Terms 9

Read also:

Get Jobilize Job Search Mobile App in your pocket Now!

100% Free Mobile Applications
Receive real-time job alerts and never miss the right job again

Source: OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9

Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry' conversation and receive update notifications?

Ask

	2 Additional information in chemical equations By OpenStax Read Online Course
	2011 Dynamics CRM By Danielrosenberger Start Quiz
©flickr: Alexander	Mechanics I MCQ By Stephanie Redfern Start Quiz
©flickr: Iqbal	Liver Cancer By Darlene Paliswat Start Test
	Professional makeup certification Test By Angela January Start Quiz
	3 BOD GI quiz By Brooke Delaney Start Exam
	Chemistry Final By Briana Hamilton Start Flashcards
	Concepts of biology By OpenStax Read Online Course
	3 Neuroanatomy 03 Ventricular System By Stephen Voron Start Quiz
	1 Lec:1 Descriptive Epidemiology By Janet Forrester Start Quiz
	Biology Exam Final By Savannah Parrish Start Exam