4.1 Introduction to Chemical Reactions

4.1 Introduction to Chemical Reactions pp. 152 - 155 Learning Goals By the end of this lesson, you should: ■ Be able to differentiate among a word equation, skeleton equation and balanced chemical equation ■ Understand how to show states of matter in skeleton & balanced chemical equations ■ Understand how to apply the Law of Conservation of Mass to balance chemical equations Chemical Reaction ■ ■ A processes in which one or more substances changes into one or more new substances Evidence of Chemical Reactions: ■ ■ ■ ■ Unexpected change in colour Energy is released or absorbed A gas is produced A precipitate (solid) forms when mixing two solutions Describing Chemical Reactions ■ ■ A word equation is a way of representing a chemical reaction. It tells you what reacts and what is produced. Specific conditions of the reaction may be included (e.g. catalyst – a substance that makes a reaction occur faster) Writing word equations ■ They are written in a particular order. ■ ■ ■ Reactants (what you start with) are always on the left side of the arrow Products (what you make) are always on the right side of the arrow Multiple reactants or products are separated by a + sign Skeleton Equations ■ A representation of a chemical reaction where the formulae of the reactants are connected to the formulae of the products by an arrow. ■ ■ e.g. CH4 (g) + O2 (g) 🡪 H2O (l) + CO2 (g) This provides more detail than a word equation (e.g. state of reactants / products) ■ (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (i.e. dissolved in water) The Conservation of Mass ■ In a chemical reaction, the total mass of the reactants is always equal to the total mass of the products ■ Atoms do not change in a reaction. The molecules that they form can be changed but the atoms themselves are not changed The Conservation of Mass Balanced Chemical Equations ■ Provide even more information than the skeleton equations (i.e. coefficients are used to show the ratio of chemicals which follows the law of conservation of mass) ■ This equation is balanced because the total number of atoms of each type are the same on both the reactant and product sides Steps to balancing an equation Step 1 ■ Write the word equation of the reaction ■ e.g. aluminum + bromine 🡪 aluminum bromide Step 2 ■ Write the skeleton equation by replacing each name with a correct formula. ■ e.g. Al +Br2 🡪 AlBr3 Steps to balancing an equation Step 3 ■ Count the number of atoms on each side of the equation Type of atom Reactants Products Al 1 1 Br 2 3 Steps to balancing an equation Step 4 ■ Multiply each of the formulae by the appropriate coefficients to balance the number of atoms. ■ DO NOT change any of the subscripts from your skeleton equation ■ Start out by picking the element with the most number of atoms and try to balance it first ■ Start with Bromine. ■ ■ ■ The 2 and 3 will be balanced if we multiply the reactant side by 3 which would give it 6 Br, and multiply the product side by 2 to give us 6 Br. Now we have 2 Al products which need to be balanced so we add a 2 to the Al on the reactant side. 2Al + 3Br2 🡪 2AlBr3 Tips for Balancing Chemical Equations ■ ■ ■ ■ Remember that H, O, F, Br, I, N, and Cl exist as diatomic elements (i.e. H2, O2, etc.) Balance compounds first and elements last Balance hydrogen and oxygen last If a polyatomic ion appears in both a reactant and a product, think of it as a single unit Tips for Balancing Chemical Equations ■ ■ Once you think the chemical equation is balanced, do a final check by counting the atoms of each element one more time If you go back and forth between two substances, using higher and higher coefficients, double-check each chemical formulae More examples on the board ■ sodium + chlorine 🡪 sodium chloride ■ nitrogen + hydrogen 🡪 ammonia ■ copper + silver nitrate 🡪 copper nitrate + silver Learning Goals By the end of this lesson, you should: ■ Be able to differentiate among a word equation, skeleton equation and balanced chemical equation ■ Understand how to show states of matter in skeleton & balanced chemical equations ■ Understand how to apply the Law of Conservation of Mass to balance chemical equations Homework ■ ■ Read pp. 152 – 155 Answer # 1, 3 – 5 on p. 155