Writing Chemical Reactions Chemical reactions or chemical changes

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 H 2 O.

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 or more substances combine to form one substance. � Two ◦ The general form is A + X AX � Example: ◦ Magnesium + oxygen magnesium oxide ◦ 2 Mg + O 2 2 Mg. O

Combination Reactions � Combination reactions may also be called composition or synthesis reactions. � Some types of combination reactions: ◦ Combination of elements �K + Cl 2 �One product will be formed

Combination Reactions �K + Cl 2 � Write the ions: K+ Cl- � Balance the charges: KCl � Balance the equation: 2 K + Cl 2 2 KCl

Combination Reactions �Some types of combination reactions: ◦ Oxide + water �Nonmetal oxide + water acid �SO 2 + H 2 O H 2 SO 3 �Metal oxide + water base �Ba. O + H 2 O Ba(OH)2

Combination Reactions �Some types of combination reactions: ◦ Metal oxides + nonmetal oxides � Na 2 O + CO 2 Na 2 CO 3 � Ca. O + SO 2 Ca. SO 3

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. ◦ 2 H 2 O 2 H 2 + O 2

Decomposition Reactions �Types of Decomposition Reactions: ◦ Decomposition of carbonates �When heated, some carbonates break down to form an oxide and carbon dioxide. �Ca. CO 3 Ca. O + CO 2 �H 2 CO 3 H 2 O + CO 2

Decomposition Reactions �Types of decomposition reactions: ◦ Some metal hydroxides decompose into oxides and water when heated. �Ca(OH)2 Ca. O + H 2 O 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. � 2 KCl. O 3 2 KCl + 3 O 2 �Zn(Cl. O 3)2 Zn. Cl 2 + 3 O 2 ◦ Some of these reactions are used in explosives.

Decomposition Reactions �Some substances can easily decompose: ◦ Ammonium hydroxide is actually ammonia gas dissolved in water. �NH 4 OH NH 3 + H 2 O ◦ Some acids decompose into water and an oxide. �H 2 SO 3 H 2 O + SO 2

Decomposition Reactions �Some decomposition reactions are difficult to predict. �The decomposition of nitrogen triiodide, NI 3, 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 + Ag. NO 3 �Nickel replaces the metallic ion Ag+. �The silver becomes free silver and the nickel becomes the nickel(II) ion. �Ni + Ag. NO 3 Ag + Ni(NO 3)2 �Balance the equation: �Ni + 2 Ag. NO 3 2 Ag + Ni(NO 3)

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 + Fe 2 O 3 in the following reaction:

Thermite Reaction �Al + Fe 2 O 3 �Aluminum will replace iron(III) as was seen in the video. �Iron(III) becomes Fe and aluminum metal becomes Al 3+. � 2 Al + Fe 2 O 3 2 Fe + Al 2 O 3

Single Replacement Reactions �An active nonmetal can replace a less active nonmetal. ◦ The halogen (F 2, Cl 2, Br 2, I 2) reactions are good examples. ◦ F 2 is the most active and I 2 is the least. � Cl 2 +2 Na. I 2 Na. Cl + I 2

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 � Na. OH + Cu. SO 4 � The Na+ and Cu 2+ switch places. � Na+ combines with SO 42 - to form Na 2 SO 4. � Cu 2+ combines with OH- to form Cu(OH)2 � Na. OH + Cu. SO 4 Na 2 SO 4 + Cu(OH)2 � 2 Na. OH + Cu. SO 4 Na 2 SO 4 + Cu(OH)2

Double Replacement �Cu. SO 4 + Na 2 CO 3 �Cu 2+ combines with CO 32 - to form Cu. CO 3. �Na+ combines with SO 42 - to form Na 2 SO 4. �Cu. SO 4 + Na 2 CO 3 Cu. CO 3 + Na 2 SO 4

Double Replacement �Na 2 CO 3 + HCl �Notice that gas bubbles were produced rather than a precipitate. �What was the gas? �Write the double replacement reaction first.

Double Replacement �Na 2 CO 3 + HCl �Na+ combines with Cl- to form Na. Cl. �H+ combines with CO 32 - to form H 2 CO 3. �Na 2 CO 3 + 2 HCl 2 Na. Cl + H 2 CO 3 �H 2 CO 3 breaks up into H 2 O and CO 2.

Double Replacement �The gas formed was carbon dioxide. �The final balanced reaction is: Na 2 CO 3 + HCl Na. Cl + H 2 O + CO 2. �Balance the equation. �Na 2 CO 3 + 2 HCl 2 Na. Cl + H 2 O + CO 2

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 2 Mg + O 2 2 Mg. O. �The combustion reaction may be burning of a fuel.

Combustion Reaction �Methane, CH 4, is natural gas. �When hydrocarbon compounds are burned in oxygen, the products are water and carbon dioxide. �CH 4 + O 2 CO 2 + H 2 O �CH 4 + 2 O 2 CO 2 + 2 H 2 O

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 to type: each of the following as �H 2 + Cl 2 2 HCl �Ca + 2 H 2 O Ca(OH)2 + H 2 ◦ Combination ◦ Single replacement

Practice � 2 CO + O 2 2 CO 2 ◦ Combination and combustion � 2 KCl. O 3 2 KCl + 3 O 2 ◦ Decomposition

Practice �Fe. S + 2 HCl Fe. Cl 2 + H 2 S ◦ Double replacement �Zn + HCl ? ◦ Single replacement ◦ Zn + 2 HCl Zn. Cl 2 + H 2