Chemical Reactions and Balancing Equations

Evidence of Chemical Reactions Reactions and Balancing Equations Evidence of Chemical Reactions A chemical change occurs when new substances are made. Visual clues: Color change precipitate formation gas bubbles flame heat release cooling light Other clues: New odor permanent new state 3

Evidence of Chemical Reactions (cont.) Reactions and Balancing Equations Evidence of Chemical Reactions (cont.) Heat produced or absorbed Change in colour Change of state Change of state Name these compounds Write these equations Methane reacts with oxygen to produce heat HCl + Co(NO3)2 Na2Cr2O7 + Pb(NO3)2 forming a solid Calcium reacts with water to create hydrogen gas

Reactions and Balancing Equations Conservation of Mass Matter cannot be created or destroyed. In a chemical reaction, all the atoms present at the beginning are still present at the end. The number of atoms doesn’t change, just the arrangement Therefore the total mass cannot change. 5

Reactions and Balancing Equations Chemical Reactions Reactions involve chemical changes in matter that result in new substances. Reactions involve rearrangement and exchange of atoms to produce new compounds. Reactions may involve various states of matter: solid, liquid, gas 2

Reactions and Balancing Equations Chemical Reactions Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq) On the reactants side we have solid zinc and (aqueous) hydrochloric acid all acids are aqueous aqueous solutions are water based solutions On the products side we have gaseous hydrogen and aqueous zinc chloride 1 mole : 2 = 1 : 1

Chemical Reactions (con.) The number of atoms on the reactant side of the equation must to be the same as the number of atoms on the product side of the equation the equation must be balanced This is a reiteration of the Conservation of Mass

Reactions and Balancing Equations Chemical Reactions For example: methane + oxygen yields carbon dioxide + water CH4(g) + O2(g) CO2(g) + H2O(g) This equation is unbalanced Compare the number of hydrogens and the number of oxygens on both side Discuss – match up atoms on both sides

Reactions and Balancing Equations Chemical Reactions As seen on the previous slide: We need the same number of hydrogens on the both sides The same goes for carbon and oxygen Discuss – match up atoms on both sides

Balancing Reactions We will cover this on the board in class

Reactions and Balancing Equations Chemical Equations Shorthand way of describing a reaction. Provides information about the reaction: Formulas of reactants and products States of reactants and products Relative moles of reactant and product compounds (atoms) that are required Can be used to determine masses of reactants used and of products made Write an eqn on the board 4

The Four Types of Reactions Type of Reaction Characteristic Element Arrangement Combination Reactants form one product Decomposition One reactant forms two or more products Single Displacement One element displaces another in a compound Double Displacement Compounds “trade” elements

Examples of Combination Reactions

Examples of Decomposition Reactions

Examples of Single Displacement Reactions

The Activity Series Used to determine the outcome of a single displacement reaction See the next slide

Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg In a reaction an element in the activity series will displace an element below it from a compound. Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg increasing activity

Examples of Double Displacement Reactions

The following may result from a double displacement reaction: formation of a precipitate release of gas bubbles release or absorption of heat formation of water

Calculations from Chemical Equations Known as Stoichiometry

The Importance of a Balanced Chemical Equation Indicates the mole ratios of compounds in the reaction both on the reactant side and the product side

Mole Ratios Consider the following reaction: What are the mole ratios of: Al to H2? Al to HCl? AlCl3 to H2?

Stoichiometry The area of chemistry that deals with the quantitative relationships between reactants and products.

Mole Ratios in a Chemical Equation The molar coefficients in a balanced chemical equation form the basis for the solution of stoichiometry problems We use the ratios between them, known as the “molar ratios”, as the basis for solving the problem

Mole Calculations Stoichiometry problems may involve moles only. See the next two examples below.

For the synthesis of ammonia given below: How many moles of hydrogen gas are needed to react with 15.5 moles of nitrogen gas? How many moles of ammonia are produced?

In the following double displacement reaction, how many moles of PbCl2 are formed if 5.00 moles of NaCl react?

Mass Calculations In stoichiometry problems the quantities of the compounds involved could be expressed in terms of their masses. These problems require converting between mass and moles. See the two examples below.

Example 1, Mass Calculation Calculate the number of grams of H2 required to form 12.0 moles of NH3.

Example 2, Mass Calculations Consider the following reaction: Say we have 16.8 g of Fe. How many grams of hydrogen gas are produced?