Chemical Reactions

Writing and balancing chemical equations

Reactions

• Writing an equation to represent a chemical reaction is like writing a sentence.

• Sentences– Subject and a verb with punctuation.

• Equations– Reactants and products with an arrow in between.

Reactants and Products

• In a chemical reaction, the ways in which atoms are joined together are changed.– Reactants are converted to products when bonds

that hold atoms together are broken and new bonds form. (conservation of mass)

– Chemical reactions can be described in different ways.• Word equations

– Reactants are written on the left, products written on the right.

Symbols of Chemical Reactions

+ Used to separate two reactants or two products

→ "yields," separates reactants from products ↔ Used in place of an arrow for reversible

reactions (s) solid (l) liquid (aq) substance dissolved in water (g) gas

Writing Chemical Equations

• Important to remember:– Arrows separate the reactants (on the left) and

products (on the right)• Example:

– Fe + O2 Fe2O3

– Skeleton equations do not tell the relative amounts of the reactants and products.• A catalyst speeds up chemical reactions with being

used up itself. These are written above the arrow.– Example: H2O2(aq) MnO2 H2O(l) + O2(g)

Example

• Write a skeleton equation for this chemical reaction: When calcium carbonate is heated, calcium oxide and carbon dioxide are produced.

– Answer: (go through in class)

Writing Word Equations

• NaHCO3(s) + HCl(aq) NaCl(aq) + H2O + CO2(g)

– Answer: (go through in class)

Balancing Everyday Equations

• Analogy: Football Offensive Team– 1 center, 2 guards, 2 tackles, 1 tight end, 2 wide

receivers, 2 running backs and 1 quarterback– C + 2G + 2T + Te + 2Wr + 2Rb + Qb

CG2T2TeWr2Rb2Qb

Balancing Chemical Equations

• Important: – Each side of the chemical equation must have the

same number of atoms of each element.

Rules• Rules for Balancing Chemical Equations

1. Determine the correct formulas for all reactants and products.2. Write the formulas for the reactants on the left and the products on

the right with a “yields” sign in between. If two or more reactants are involved use a “plus” sign to separate their formulas.

3. Count the number of atoms of each element in the reactants and products.

4. Balance the elements one at a time by using coefficients (= a small whole number that appears in front of a chemical formula in an equation.)

When no coefficient is written, it is assumed to be 1.

5. Check each atom or polyatomic ion to be sure that the equation is balanced.

6. Make sure that all the coefficients are in the lowest possible ratio.

Example

• H2 (g) + O2(g) H2O (l)

• H2 (g) + O2(g) 2H2O (l)

• 2H2 (g) + O2(g) 2H2O (l)

Clarification

• What is the difference between 2O and O2?

• 2O = 2 single oxygen atoms• O2 = 1 oxygen molecule (O – O)

– Analogy: disposable razors• A single razor = R• A package of two razors = R2

• Two single razors = 2R• Two packages of razors = 2R2

Boxing

• AgNO3 (aq) + Cu(s) Cu(NO3)2 (aq) + Ag(s)

Types of Reactions

1. Combination– Two or more substances react to form a single substance

2. Decomposition– A single compound is broken down into two or more products

3. Single-Replacement– Atoms of an element replace the atoms of a second element in a

compound

4. Double-Replacement– Exchange of the positive ions between two compounds

5. Combustion– An element or a compound reacts with oxygen and produces heat

and light

Combination Reactions

• In a combination reaction, two or more substances react to form a single substance.– Example:

S + O2 SO2

Decomposition Reactions

• A single compound is broken down into two or more products.– Example:

CaCO3 CaO + CO2

Single-Replacement Reactions

• Atoms of an element replace the atoms of a second element.– Example:

Mg + Zn(NO3)2 Mg(NO3)2 + Zn

Mg + LiNO3 no reaction***

***Refer to Table 7.2 “Activity Series of Metals”A reactive metal will replace any metal below it in the activity

series.

Double-Replacement Reactions

• Involve an exchange of positive ions between two compounds.– Example:

Na2S + Cd(NO3)2 CdS + 2NaNO3

Combustion Reactions

• An element or a compound reacts with oxygen.– Example:

CH4 + 2O2 CO2 + 2H2O + heat and light

A combustion reaction between an element and oxygen is also an example of a combination reaction.

Predicting Products of a Chemical Reaction

• Table 7.3 “Keys to Indentifying Types of Chemical Reactions”