Writing Chemical Reactions Types of Chemical Reactions 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.

Types of Chemical Reactions The five types of chemical reactions in this unit are: Combination Decomposition Single Replacement Double Replacement Combustion The reaction of sugar with concentrated sulphuric acid.

Combination Reactions 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 oxides Na2O + CO2  Na2CO3 CaO + SO2  CaSO3

Decomposition 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.

Nitrogen triiodide

Single Replacement Reactions 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:

Thermite Reaction 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

Single Replacement Reactions 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

Double 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 NaOH + CuSO4  The Na+ and Cu2+ switch places. Na+ combines with SO42- 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 CO32- 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 CO32- to form H2CO3. Na2CO3 + 2HCl  2NaCl + H2CO3 H2CO3 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

Combustion Reaction 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 + H2O CH4 + 2O2  CO2 + 2H2O

Combustion Reactions 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.

Practice Classify each of the following as to type: H2 + Cl2  2HCl Combination Ca + 2H2O  Ca(OH)2 + H2 Single replacement

Practice 2CO + O2  2CO2 2KClO3  2KCl + 3O2 Combination and combustion 2KClO3  2KCl + 3O2 Decomposition

Practice FeS + 2HCl  FeCl2 + H2S Zn + HCl  ? Double replacement Single replacement Zn + 2HCl  ZnCl2 + H2