# chemical equations

## steps in balancing a chemical equation

Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count the number of each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2. 2. Add coefficients to increase the number of atoms or molecules of reactants or products. Use the smallest coefficients possible. 3. Repeat steps 1 and 2 until the equation is balanced. Helpful Hint When you balance chemical equations, never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. Work through the Problem Solving examples below. Then do the You Try It! problems to check your understand- ing. If you need more help, go to this URL: (14:28). MEDIA Click image to the left or use the URL below. URL: Problem Solving Problem: Balance this chemical equation: N2 + H2 ! NH3 Hints for balancing 1. Two N are needed in the products to match the two N (N2 ) in the reactants. Add the coefficient 2 in front of NH3 . Now N is balanced. 2. Six H are now needed in the reactants to match the six H in the products. Add the coefficient 3 in front of H2 . Now H is balanced. Solution: N2 + 3H2 ! 2NH3 Problem: Balance this chemical equation: CH4 + O2 ! CO2 + H2 O Solution: CH4 + 2O2 ! CO2 + 2H2 O You Try It! Problem: Balance these chemical equations: Zn + HCl ! ZnCl2 + H2 Cu + O2 ! CuO

## using coefficients

Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2H2 . A coefficient of 1 usually isnt written. Coefficients can be used to balance equation 1 (above) as follows: Equation 2: 2H2 + O2 ! 2H2 O Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms. There are now four hydrogen atoms in both reactants and products. Is equation 2 balanced? Count the oxygen atoms to find out.

## conserving mass

Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions. How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. For this and other contributions, Lavoisier has been called the father of modern chemistry. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in Figure 8.5. As a result, any gases involved in the reactions were captured and could be measured. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was his discovery of oxygen. You can learn more about Lavoisier and his important research at:

## balancing chemical equations

Some chemical equations are more challenging to write. Consider the reaction in which hydrogen (H2 ) and oxygen (O2 ) combine to form water (H2 O). Hydrogen and oxygen are the reactants, and water is the product. To write a chemical equation for this reaction, you would start by writing symbols for the reactants and products: Equation 1: H2 + O2 ! H2 O Like equations in math, equations in chemistry must balance. There must be the same number of each type of atom in the products as there is in the reactants. In equation 1, count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced.

## writing chemical equations

A chemical equation is a symbolic representation of a chemical reaction. It is a shorthand way of showing how atoms are rearranged in the reaction. The general form of a chemical equation was introduced in this chapters lesson "Introduction to Chemical Reactions." It is: Reactants ! Products Consider the simple example in Figure 8.4. When carbon (C) reacts with oxygen (O2 ), it produces carbon dioxide (CO2 ). The chemical equation for this reaction is: C + O2 ! CO2 The reactants are one atom of carbon and one molecule of oxygen. When there is more than one reactant, they are separated by plus signs (+). The product is one molecule of carbon dioxide. If more than one product were produced, plus signs would be used between them as well.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

In chemical equations, reactants and products are represented by

``````a. plus signs and arrows.

b. elements and compounds.

c. coefficients and subscripts.

-->  d. chemical symbols and chemical formulas.
``````

A shorthand way of showing how atoms are rearranged in a chemical reaction is a chemical

``````a. symbol.

b. formula.

-->  c. equation.

d. letter.
``````

When there is more than one reactant in a chemical equation, they are separated by

``````a. arrows.

b. subscripts.

-->  c. plus signs.

d. coefficients.
``````

Which chemical equation is not balanced?

``````a. 2Na + Cl2  2NaCl

b. C + O2  CO2

-->  c. NO + O2  2NO2

d. N2 + 3H2  2NH3
``````

In the reaction represented by the chemical equation 2Cu + O2 2CuO, new bonds are formed in

``````a. 2Cu.

b. O2 .

-->  c. CuO.

d. none of the above
``````

If there is more than one product in a chemical equation, the products are separated by

``````-->  a. plus signs.

b. minus signs.

c. equals signs.

d. two-way arrows.
``````

What is the missing coefficient in the following chemical equation? CH4 + ?O2 CO2 + 2H2 O

``````a. 3

-->  b. 2

c. 1

d. 0
``````

Chemical equations must be balanced because matter cannot be

``````a. created.

b. destroyed.

c. changed.

-->  d. two of the above
``````

Which chemical equation is balanced?

``````-->  a. 2Na + Cl2  2NaCl

b. Na + 2Cl2  2NaCl

c. 2Na + 2Cl2  2NaCl

d. Na + Cl2  NaCl
``````

Which chemical equation correctly represents the reaction in which carbon combines with oxygen?

``````a. C2 + O2  2CO

b. C2 + 2O  C2 O2

-->  c. C + O2  CO2

d. 2C + O  C2 O
``````

What is the missing coefficient in the following chemical equation? ?NO + O2 2NO2 ?

``````a. 0

b. 1

-->  c. 2

d. 4
``````

The symbol 2O2 represents two molecules of oxygen.

``````-->  a. true

b. false
``````

A coefficient of 1 usually is not written.

``````-->  a. true

b. false
``````

Subscripts are used to balance chemical equations.

``````a. true

-->  b. false
``````

The general form of a chemical equation is Reactants = Products.

``````a. true

-->  b. false
``````

The first step in balancing a chemical equation is counting atoms.

``````-->  a. true

b. false
``````

A chemical equation must balance only when the reaction reversible.

``````a. true

-->  b. false
``````

The symbol CO2 represents two molecules of carbon monoxide.

``````a. true

-->  b. false
``````

The symbol 2H2 represents two atoms of hydrogen.

``````a. true

-->  b. false
``````

Coefficients are used to balance chemical equations.

``````-->  a. true

b. false
``````

In balancing chemical equations, you should use the smallest subscripts possible.

``````a. true

-->  b. false
``````

The number of each type of molecule must be the same on both sides of a chemical equation.

``````a. true

-->  b. false
``````

Changing coefficients changes the substances involved in a chemical reaction.

``````a. true

-->  b. false
``````

Chemists use a standard method to represent chemical reactions.

``````-->  a. true

b. false
``````

The chemical equation H2 CO3 H2 O + CO2 is balanced.

``````-->  a. true

b. false
``````

Water is the reactant in the chemical equation H2 O H2 + O2 .

``````-->  a. true

b. false
``````

symbolic representation of a chemical reaction

``````a. chemical symbol

b. coefficient

c. H2

-->  d. chemical equation

e. 2H

f. subscript

g. chemical formula
``````

example of a coefficient

``````a. chemical symbol

b. coefficient

c. H2

d. chemical equation

-->  e. 2H

f. subscript

g. chemical formula
``````

symbol of a chemical compound

``````a. chemical symbol

b. coefficient

c. H2

d. chemical equation

e. 2H

f. subscript

-->  g. chemical formula
``````

example of a subscript

``````a. chemical symbol

b. coefficient

-->  c. H2

d. chemical equation

e. 2H

f. subscript

g. chemical formula
``````

number showing how many atoms or molecules of a given element or compound are involved in a chemical

``````a. chemical symbol

-->  b. coefficient

c. H2

d. chemical equation

e. 2H

f. subscript

g. chemical formula
``````

symbol of an chemical element

``````-->  a. chemical symbol

b. coefficient

c. H2

d. chemical equation

e. 2H

f. subscript

g. chemical formula
``````

number showing how many atoms of a given element are in a molecule

``````a. chemical symbol

b. coefficient

c. H2

d. chemical equation

e. 2H

-->  f. subscript

g. chemical formula
``````

## diagram questions

No diagram questions associated with this lesson