Writing Chemical Reactions

Chemical reactions, or chemical changes, happen when the atoms in one or more chemicals split up and join together in new ways.

For example, hydrogen can react with oxygen :

hydrogen + oxygen water

Before the reaction the oxygen atoms go round in pairs, and the hydrogen atoms go round in pairs. When they react we get two new molecules, of the compound water.

Each of these molecules has three atoms, two hydrogen atoms joined to one oxygen atom.We can write this as H2O.

Types of Reactions

Many chemical reactions have defining characteristics which allow them to be classified as to type.

The five types of chemical reactions in this unit are:◦Combination◦Decomposition◦Single Replacement◦Double Replacement◦Combustion

Types of Chemical Reactions

The reaction of sugar with concentrated sulphuric acid.

Two or more substances combine to form one substance.◦The general form is A + X AX

Example:◦Magnesium + oxygen magnesium oxide

◦2Mg + O2 2MgO

Combination Reactions

Combination reactions may also be called composition or synthesis reactions.

Some types of combination reactions:◦ Combination of elements K + Cl2 One product will be formed

Combination Reactions

K + Cl2

Write the ions: K+ Cl-

Balance the charges: KCl

Balance the equation: 2K + Cl2 2KCl

Combination Reactions

Some types of combination reactions:◦Oxide + water Nonmetal oxide + water acid SO2 + H2O H2SO3

Metal oxide + water base BaO + H2O Ba(OH)2

Combination Reactions

Some types of combination reactions:◦Metal oxides + nonmetal oxidesNa2O + CO2 Na2CO3CaO + SO2 CaSO3

Combination Reactions

One substance reacts to form two or more substances. ◦The general form is AX A + X

Example:◦Water can be decomposed by electrolysis.

◦2H2O 2H2 + O2

Decomposition Reactions

Types of Decomposition Reactions:◦Decomposition of carbonates When heated, some carbonates break down to form an oxide and carbon dioxide. CaCO3 CaO + CO2 H2CO3 H2O + CO2

Decomposition Reactions

Types of decomposition reactions:◦Some metal hydroxides decompose into oxides and water when heated. Ca(OH)2 CaO + H2O

Note that this is the reverse of a similar combination reaction.

Decomposition Reactions

Types of decomposition reactions:◦Metal chlorates decompose into chlorides and oxygen when heated. 2KClO3 2KCl + 3O2 Zn(ClO3)2 ZnCl2 + 3O2

◦Some of these reactions are used in explosives.

Decomposition Reactions

Some substances can easily decompose:◦Ammonium hydroxide is actually ammonia gas dissolved in water. NH4OH NH3 + H2O

◦Some acids decompose into water and an oxide. H2SO3 H2O + SO2

Decomposition Reactions

Some decomposition reactions are difficult to predict.

The decomposition of nitrogen triiodide, NI3, is an example of an interesting decomposition reaction.

Decomposition Reactions

Nitrogen triiodide

A metal will replace a metal ion in a compound.◦The general form is A + BX AX + B

A nonmetal will replace a nonmetal ion in a compound.◦The general form is Y + BX BY + X

Single Replacement Reactions

Examples:◦ Ni + AgNO3 Nickel replaces the metallic ion Ag+. The silver becomes free silver and the

nickel becomes the nickel(II) ion. Ni + AgNO3 Ag + Ni(NO3)2

Balance the equation: Ni + 2AgNO3 2Ag + Ni(NO3)

Single Replacement Reactions

Not all single replacement reactions that can be written actually happen.

The metal must be more active than the metal ion.

Aluminum is more active than iron in Al + Fe2O3 in the following reaction:

Single Replacement Reactions

Al + Fe2O3 Aluminum will replace iron(III) as was seen in the video.

Iron(III) becomes Fe and aluminum metal becomes Al3+.

2Al + Fe2O3 2Fe + Al2O3

Thermite Reaction

An active nonmetal can replace a less active nonmetal.◦The halogen (F2, Cl2, Br2, I2) reactions are good examples.

◦F2 is the most active and I2 is the least.Cl2 +2 NaI 2 NaCl + I2

Single Replacement Reactions

Ions of two compounds exchange places with each other.◦The general form is AX + BY AY + BX

Metathesis is an alternate name for double replacement reactions.

Double Replacement Reactions

NaOH + CuSO4 The Na+ and Cu2+ switch places. Na+ combines with SO4

2- to form Na2SO4. Cu2+ combines with OH- to form Cu(OH)2 NaOH + CuSO4 Na2SO4 + Cu(OH)2 2NaOH + CuSO4 Na2SO4 + Cu(OH)2

Double Replacement

CuSO4 + Na2CO3 Cu2+ combines with CO3

2- to form CuCO3.

Na+ combines with SO42- to form

Na2SO4.CuSO4 + Na2CO3 CuCO3 + Na2SO4

Double Replacement

Na2CO3 + HCl Notice that gas bubbles were produced rather than a precipitate.

What was the gas?Write the double replacement reaction first.

Double Replacement

Na2CO3 + HCl Na+ combines with Cl- to form NaCl.H+ combines with CO3

2- to form H2CO3.

Na2CO3 + 2HCl 2NaCl + H2CO3H2CO3 breaks up into H2O and CO2.

Double Replacement

The gas formed was carbon dioxide.

The final balanced reaction is: Na2CO3 + HCl NaCl + H2O + CO2.

Balance the equation.Na2CO3 + 2HCl 2NaCl + H2O + CO2

Double Replacement

When a substance combines with oxygen, a combustion reaction results.

The combustion reaction may also be an example of an earlier type such as 2Mg + O2 2MgO.

The combustion reaction may be burning of a fuel.

Combustion Reaction

Methane, CH4, is natural gas.When hydrocarbon compounds are burned in oxygen, the products are water and carbon dioxide.

CH4 + O2 CO2 + H2OCH4 + 2O2 CO2 + 2H2O

Combustion Reaction

Combustion reactions involve light and heat energy released.

Natural gas, propane, gasoline, etc. are burned to produce heat energy.

Most of these organic reactions produce water and carbon dioxide.

Combustion Reactions

Classify each of the following as to type:

H2 + Cl2 2HCl◦Combination

Ca + 2H2O Ca(OH)2 + H2◦Single replacement

Practice

2CO + O2 2CO2◦Combination and combustion

2KClO3 2KCl + 3O2◦Decomposition

Practice

FeS + 2HCl FeCl2 + H2S◦Double replacement

Zn + HCl ?◦Single replacement◦Zn + 2HCl ZnCl2 + H2

Practice