Unit 3: Writing Chemical Reactions SCH3U/4C Ms. Richardson

Chemical Reactions A chemical reaction is a process that occurs in which substances combine and interact and result in the formation of a new product. Recall the evidence we use to indicate if a chemical change has occurred…

Evidence of a Chemical Change Light is produced Precipitate is formed Gas is produced Change in colour Change in odour

Describing Chemical Reactions Starting substances involved in a chemical reaction are known as reactants The new substances formed from the interaction of the reactants are known as products

Chemical Equations To make describing chemical reactions simpler, we use a condensed statement that represents the change occurring. These ‘statements’ are known as a chemical equations.

Types of Chemical Equations Chemical equations can be represented three ways: Word equations Skeletal equations Balanced Chemical Equations Symbols used in chemical reactions: + indicates more than 1 reactant/product reaction arrow indicates a reversible reaction

Word Equations Reactants and products are represented using only words A reaction arrow separates reactants from products. We read the reaction arrow as “reacts to produce” iron + chlorine  iron (III) chloride “iron combined with chlorine, reacts to produce iron (III) chloride”

Skeletal Equations Chemical formulas are used to represent reactants and products. Usually, the physical state of the substances involved are indicated. Symbol State (s) Solid (l) Liquid (g) Gas (aq) aqueous

Problems with Skeletal Equations Skeletal equations are preferred over word equations as they provide more information. However often times, the skeletal equations do not satisfy the Law of Conservation of Mass

Problems with Skeletal Equations Example: hydrogen + oxygen  water H2(g) + O2(g)  H2O(l) If we count the atoms involve: Reactants: 2 H atoms + 2 O atoms Products: 2 H atoms + 1 O atom

Satisfying the Law of Conservation of Mass Recall that the subscript refers to how many atoms of that element. All chemical reactions should have equal atoms going in and equal atoms being produced. But, we cannot change subscripts in a compound (that changes the chemical composition!)

Balancing Chemical Equations We can balance each side of the equation by adding coefficients. This increases the number of each molecule we have, so that the number of atoms of each element will be equal.

Rules for Balancing Equations Write the skeletal equation Count the atoms of each element in the reactants and products Add coefficients so that you have equal atoms on both sides H2 + I2  HI Reactants Products H = 2 H = 1 I = 2 I = 1 H2 + I2  2HI Reactants Products H = 2 I = 2

Balancing Chemical Equations If you have common multiples as your coefficients, you must reduce to the lowest ratio. See pages 118 – 120 in your text (MGH 11) for extra tips on balancing. Balancing takes practice, don’t let it frustrate you!!!!