Lesson 5 - Chemical Reactions

Compare the following videos Consider: 1)What is a chemical reaction? 2)How do we know a chemical reaction occurred? 3)What are the properties of a chemical change? What about a physical change?

Chemical vs. Physical Change Chemical: Transformation into a new substance Physical: Change in state ●formation of bubbles ●change in colour ●formation of a precipitate ●change in temperature ●solid ●liquid ●gas ●aqueous

Chemical reaction A chemical reaction is a process by which a chemical change occurs. These are always associated with a change in energy

Chemical equations To describe chemical reactions, we use equations. There are two types of equations we will use: word equations and chemical equations Ex. hydrogen + chlorine → hydrochloric acid H 2(g) + Cl 2(g) → 2HCl (aq)

Chemical equations Reactants → Products ● The arrow indicates the direction of the reaction (usually written left to write) ● Reactant are on the left side (“butt” of the arrow”) ● Products are on the right side (“head” of the arrow”) ● A ‘+’ sign is used when there are more than one reactant or product

Types of equations ● A word equation uses the written name of the reactants and products (e.g. IUPAC name or common name) instead of the chemical symbols. copper sulfate + sodium hydroxide → copper hydroxide + sodium sulfate ● A chemical equation (or symbol equation) uses compounds’ chemical symbols rather than their written names. CuSO 4(aq) + NaOH (aq) → CuOH (aq) + NaSO 4(aq)

Chemical equations Mg (s) + HCl (aq) → MgCl (aq) + H 2(g) ● Notice that each substance in the equation has a subscript (‘s’, ‘aq’, ‘l’ or ‘g’) ●T his indicates the state that the substance is in. (s) indicates a solid (g) indicates a gas (l) indicates a liquid (aq) indicates “aqueous solution” (the reactant or product is dissolved in water)

Recall the law of conservation of mass Matter can not be created or destroyed. Therefore… in a chemical reaction the mass of the products equals the mass of the reactants. The atoms of the reactants are rearranged to form the products.

Balancing chemical equations An unbalanced equation where the reactants and products are not equal is called a skeleton equation i.e. Mg (s) + HCl (aq) → MgCl 2(aq) + H 2(g) A balanced equation has an equal number of atoms in the reactants and the products. This is achieved by placing coefficients in front of the reactants or products. Mg (s) + 2 HCl (aq) → MgCl 2(aq) + H 2(g)

Steps for writing balanced equations 1)Pick an atom OTHER than H or O, and balance them by putting a coefficient (number) in front of either a reactant or product, as appropriate. (Don’t forget to count atoms in ALL compounds on BOTH sides!) 2)Pick another atom, and balance as in step 1. If you have to go back and re-do a previously balanced atom, do so! Keep going until you’ve done all but H and O. 3)When you’ve only got H or O left, balance whichever one looks easier! When you’re down to the last atom, it will be automatically done!

Ex. HCl + Al → AlCl 3 + H 2 1)Let’s start with Cl. The LHS has 3 Cl so add 3 to the RHS 3HCl + Al → AlCl 3 + H 2 1) Aluminum is already balanced so now we can move on to H. We have 3 H on the RHS and 2 on the LHS so we need to multiply both sides to get to 6. 6HCl + Al → AlCl 3 + 3H 2

Ex. HCl + Al → AlCl 3 + H 2 3)Now we need to go back and balance Cl again. We have 6 Cl on the RHS so add 3 on the LHS 6HCl + Al → 2AlCl 3 + 3H 2 4)Now only Al is unbalanced, There are 2 on the LHS so add 2 to the RHS 6HCl + 2Al → 2AlCl 3 + 3H 2

Another technique for balancing equations With practice you will discover what works best for you! URL:

Practice! 1)Go to: a) Practice with the introduction first b) Try some more practice with the game 2)Play balancing equations battleships at: