Writing Chemical Reactions Chemistry Lesson # 5 Writing Chemical Reactions

Chemical Equations A chemical reaction is a process by which chemical change happens. All reactions are also accompanied by changes in energy. Some reactions absorb energy as they proceed, and are called endothermic reactions, other release energy in the form of heat or light, and are called exothermic reactions. Some reactions happen quickly, and others slowly, and can be manipulated if you understand what is happening. A chemical equation uses either chemical names or formulas to describe the changes that occur during a chemical reaction.

General Formula for a Reaction A + B → C + D A and B are called the reactants C and D are called the products The addition sign separates each of the reactants or products if there are more than one of either in a reaction. There are reactions where there is only one reactant or product, or many. Usually words like “reacts with”, “and”, “mixed with” indicate an addition sign. The arrow signifies that a reaction is occurring. In words, you may see the words “produces”, “forms”, “creates”, or “yields” to indicate the separation between reactants and products. Writing word equations and formula (skeleton) equations is one way to represent a chemical reaction because it tells you what reacts and what is produced.

Predicting States of Matter Chemicals can be solid, liquid, gas, or aqueous (dissolved in water in a solution). Metals are solids, except for mercury, which is liquid. Non-metals are usually gas, except for bromine, which is liquid. Ionic and polyatomic compounds are usually aqueous, unless they are precipitates or coatings – in this case they are solid. Acids are aqueous. Molecular compounds are usually gas.

Hints for Writing out Chemical Equations First decide whether the chemical name is an element or a compound. A metal element will be a single word written as is from the periodic table, and is written with just the element’s symbol (no charge when alone). Example: Lithium – Copper – A non-metal element will be a single word too, but can be written as is, or with a two beside it if it is one of the diatomic molecules (hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine) Example Sulfur – Chlorine –

Hints (continued) A compound will be two words, with the second one ending in “ide”, “ate”, “ite”, or “ic acid”. You must decide whether they are ionic, polyatomic, or acid (in this case you will criss cross), or molecular (you will use the written prefixes to write the formula). Example: Sodium nitride – Magnesium phosphate – Sulfuric acid – Diphosphorus pentoxide –

Hints (continued) Next decide whether these elements and compounds are either reactants or products. Reactants tend to be written first, followed by the products, but read carefully to ensure this is truly the case. There can be one reactant with multiple products, multiple reactants with only one product, or many of each. If you can locate where the arrow is, you will know where reactants end and the products begin.

Examples Example – hydrochloric acid reacts with sodium hydroxide to produce water and a solution of sodium chloride. Example – carbon dioxide and water are produced when tricarbon octahydride reacts with oxygen.

Examples 1. Iron reacts with oxygen in air to form a protective coating of iron (III) oxide.

Examples 2. Magnesium oxide decomposes into solid magnesium metal and oxygen gas.

Examples 3. When aluminum foil is placed in a solution of copper (II) chloride, copper metal and aluminum chloride are formed.

Examples 4. When sodium sulfate and calcium nitrate solutions are mixed, a precipitate of calcium sulfate and sodium nitrate solution are formed.

Examples 5. Carbon tetrahydride reacts with oxygen to produce carbon dioxide and water vapour, along with heat and light.