chapter 11 what is a chemical reaction?

Chapter 11 What is a chemical reaction?

• A chemical reaction describes a change in composition.

• In a chemical reaction, the original substances are the reactants, and the substances that are created are called the products.

• Observations that suggests that a chemical reaction is taking place are the production of :

• Energy (heat, light, or sound)

• Gas

• Precipitate

• Change in color

• Not all reactions are chemical.

• A physical change can occur rather than a chemical one.

• For example, water turning into ice.

Reactions involve rearrangements of atoms.

• Reactants are converted into products.

• The law of conservation of mass states that mass cannot be created nor destroyed.

• This means that the number of atoms in the product must be equal to the original reactants.

• Atoms do not become other kinds of atoms, nor do they appear or disappear.

Concept check

• List four observations that signify a chemical change.

• What happens to atoms during a chemical reaction?

Chemical reactions release or absorb energy

• Endothermic reaction- reaction in which energy is absorbed.

- Bond formation always requires energy.

• Exothermic reaction- reaction in which energy is released. – Bond breaking always releases energy.

• Spontaneous reaction- when a reaction is said to occur naturally or unaided.

For example a forest fire.

Spontaneous reactions can be endothermic or exothermic.

• Particles must collide for a chemical reaction to occur.

• Example, when a safety match is lit, the reaction begins when the two substances are brought together by striking the match head across the striking surface.

• If this collision happens with enough energy, the bonds in the reactants are broken, allowing new bonds to form between the atoms.

Types of Reactions

Types of Chemical Reactions

1) Synthesis (composition reaction)

2) Decomposition

3) Single replacement reactions

4) Double replacement reactions

Synthesis Reactions

• Two or more substances combine to form a new compound

A + X AX

Where A and X can be elements or compounds. AX is a compound.

Ex. A thin strip of magnesium metal is placed in an open flame, it burns with bright white light. When the metal strip is completely burned, only a fine white powder of magnesium oxide is left.

2 Mg (s) + O2 (g) 2 MgO (s)

• Some metals, such as iron, combine with oxygen to produce two different oxides

2 Fe (s) + O2 (g) 2 FeO (s)

4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)

The first reaction is Fe +2The second reaction is Fe +3

Decomposition Reactions

• Decomposition is a single compound undergoes a reaction that produces two or more simpler substances

AX A + X

AX is a compound. A and X can be elements or compounds

Most take place only when heat/electricity is added

Decomposition of binary compounds

• Simplest type of decomposition

• Binary compound is decomposed into its elements

2 H2O (l) 2 H2 (g) + O2 (g)

Decomposition of a substance by an electric current is called electrolysis

electricity

Decomposition of Metal Carbonates

• EX. When a metal carbonate is heated, it breaks down to produce a metal oxide and carbon dioxide gas

CaCO3 (s) CaO (s) + CO2

Δ means heated

Δ

Single Replacement Reactions

• Single replacement reactions – one element replaces a similar element in a compound

A + BX AX + BOr

Y + BX BY + X

A, B, X, and Y are elements. AX, BX, and BY are compounds

Replacement of a Metal in a Compound by Another metal

• A more active metal will replace a least active metal.

2 Al (s) + 3 Pb(NO3) (aq)

Looking at the activity series

Al is more reactive than Pb so Al WILL replace Pb to form:

3 Pb (s) + 2 Al(NO3)3 (aq)

2 Al (s)+ 3 Pb(NO3) (aq) 3 Pb (s) + 2 Al(NO3)3 (aq)

Double Replacement Reactions

• The ions of two compounds exchange places in an aqueous solution to form two new compounds.

• One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of solution or a molecular compounds, usually water.

• The other compound is often soluble and remains dissolved in solution

Double Replacement Reaction

AX + BY AY + BX

A, X, B, and Y in the reactants represent ions. AY and BX represent ionic or molecular compounds

Formation of a Precipitate

• The formation of a precipitate occurs when the cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound.

2 KI (aq) + Pb(NO3)2 (aq) PbI2 (s) + 2 KNO3 (aq)

Verify the products with the solubility sheet

Formation of a gas

• In some double replacement reactions, one of the products is an insoluble gas that bubbles out of the mixture

FeS (s) + 2 HCl H2S (g) + FeCl2 (aq)

Formation of Water

• In some double replacement reactions, a very stable molecular compound, such as water, is one of the products

HCl (aq) + NaOH (aq) NaCl (aq) + H2O(l)

Balancing Equations

Balancing requires patience

• To satisfy the law of conservation of mass, you should insert coefficients into the chemical equation.

• This ensures that there are equal numbers of atoms for each element on each side of the equation when an equation is balanced.

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation CH3CH2OH +O2 CO2 + H2O

Carbon atoms 2 1 No

Hydrogen atoms 6 2 No

Oxygen atoms 3 3 Yes

• To balance this equation, there are two carbon atoms in the reactant column and only one in the product column.

• To balance the number of carbon atoms you need to double the number of carbon dioxide molecules in the products.

CH3CH2OH + ? O2 2CO2 + ? H2O

The 2 is called a coefficient and indicates that there are two carbon dioxide molecules.

Coefficients are normal whole numbers, when a coefficient is equal to 1, the 1 is not written for simplicity.

Coefficients are written in front of the formula and multiply the entire formula.

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation CH3CH2OH +O2 2CO2 + H2O

Carbon atoms 2 2 Yes

Hydrogen atoms 6 2 No

Oxygen atoms 3 5 No

• The carbon atoms are now balanced by the hydrogen atoms are not. In addition the oxygen atoms have been thrown out of balance.

• Let’s start with hydrogen, there are six in the reactant column, so we must add three to the product column.

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation CH3CH2OH +O2 2CO2 + 3H2O

Carbon atoms 2 2 Yes

Hydrogen atoms 6 6 Yes

Oxygen atoms 3 7 No

• The oxygen atoms are still unbalance.

• Multiplying the number of oxygen molecules by three results in an equal number of oxygen atoms.

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation CH3CH2OH + 3O2 2CO2 + 3H2O

Carbon atoms 2 2 Yes

Hydrogen atoms 6 6 Yes

Oxygen atoms 7 7 Yes

• Now you have written a balanced chemical equation.

• Replacing the question marks in the formula equation with the right coefficients now give a correct chemical equation.

Tips for balancing equations

• 1. Save the elements that appear in several

reactants or products. (usually oxygen and

hydrogen )• 2. If the same polyatomic ion appears on

both sides, treat them as single units. • 3. After considering the first two steps,

balance the equation from left to right. • 4. For ionic equations, be sure that charges are

balanced.

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation NH3 + O2 NO + H2O

Nitrogen atoms 1 1 Yes

Oxygen atoms 2 2 Yes

Hydrogen atoms 3 2 No

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation NH3 + O2 NO + H2O

Nitrogen atoms

Oxygen atoms

Hydrogen atoms

Testing an equation for balance Reactants Products

Balance?

Unbalanced

Formula equation 4NH3 + 5O2 4NO + 6H2O

Nitrogen atoms 4 4 Yes

Oxygen atoms 10 10 Yes

Hydrogen atoms 12 12 Yes

9-2 cont..How are chemical equations for

reactions written?

• A chemical equation-describes the type and number of atoms that are rearranged during a reaction.

• A word equation describes a reaction. ethanol + oxygen carbon dioxide + water

The arrow means “react to form” and the plus sign means “and”

• A word equation can then be replaced with formulas to make a formula equation.

• The word ethanol can be replaced with its formula CH3CH2OH (l)

• The other three words in the equation are also replaced with the corresponding formula and symbol.

Word equation:

ethanol + oxygen carbon dioxide + water

Formula equation:

?CH3CH2OH (l) + ? O2 (g) ?CO2 (g) +? H2O(l)

Question marks are used to signify that the number of molecules for each is not yet known.

Numbers in Formulas

Coefficients Subscripts Superscripts

show show show

Amounts of reactants Number of Amount

and products in an each type of and type

Equation atom in a formula of charge on an ion.

Ex: 3NaOH Ex: H2O Ex: K+

Sample problem p. 315

Sometimes it is useful to write the equation in the form of a sentence.

The first step in the commercial production of nitric acid is the combustion of ammonia in the presence of a catalyst. The exothermic reaction produces nitrogen monoxide and water vapor. Write and balance the equation for this reaction.

• Step one: Write the word equation

ammonia + oxygen nitrogen monoxide + water

• Step two : Write the unbalanced formula equation.

? NH3 + ?O2 ?NO + ?H2O

• Step 3: Count the number of atoms for each element and Balance