SECTION 3.2 Writing Chemical Equations

Objectives At the end of this lesson, you will be able to: Translate chemical word equations into formula equations Balance simple chemical equations

Chemical Equations Chemical equations represent the process of a chemical reaction To write one, must know the reactants and products Need to know if a chemical change has occurred  Use our evidence of chemical changes- what are they? Takes everything we have learned so far and puts it into a condensed equation  i.e. compound names and formulas, states, ionic vs. molecular etc

Writing Word Equations A plus sign (+) groups the reactants together  It does not matter which order the reactants are written in An arrow (  ) separates the reactants from the products and is read “produces” A plus sign (+) also joins the products

Word Equations Taken from descriptions of a chemical reaction  Ex. Solid magnesium metal reacts with aqueous hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas  Evidence of a chemical change: gas, heat is given off How do we write this as a chemical equation? Using words?  magnesium + hydrochloric acid  magnesium chloride + hydrogen gas

Word Equations Another example: aluminium foil reacts with blue copper II sulphate solution and produced solid copper and colourless (grey?) aluminium sulphate solution As a word equation:

To Do: Writing Formula Equations from Word Equations

Objectives At the end of this lesson, you will be able to: Translate chemical word equations into formula equations Balance simple chemical equations

Formula equations Uses chemical formulas of reactants and products in a chemical equations to represent a reaction  Ex. hydrogen + oxygen  water  How do we write this as a formula equation?  H (2)g + O (2)g  H2O (l) This is called a skeleton equation (just shows what is involved with the reaction)  Doesn’t show correct proportions of reactants and products

Formula Equations - example Aluminium and copper II sulphate example: Aluminium (s) + copper II sulphate (aq)  copper (s) + aluminium sulphate (aq) As a formula equation:

Balancing Equations We know what the correct proportions are through:  Law of Conservation of Mass Developed by Antoine Lavoisier Total mass of the reactants equals the total mass of the products Using this, we can deduce that:  ** total # of atoms present before a reaction equals the total # of atoms after a reaction

Balancing Equations Let’s look at this example: Water decomposes (with electrical energy) to produce oxygen gas and hydrogen gas As a formula equation: Does this equation follow the conservation of mass? Why or why not?

Balancing Equations To get the number of atoms to stay the same, before and after the equation, more than one of a molecule may be involved in the reaction coefficient The number of molecules is represented by a number in front of the formula called the coefficient

Balancing Equations Look at the following reaction:  4 AlCl PbO 2  2 Al 2 O PbCl 4 An equation is balanced if the # of each type of atom on reactant side = # of each type of atom on product side – is it? The numbers in front are coefficients Cannot change formulas of any substances; you can only add coefficients to balance How is this related to the Law of Conservation of Mass?

Practice Keep a tally of the # atoms as you choose coefficients – you may change your mind, so change tally as you go  ___Zn + ___HCl  ___ZnCl 2 + ___H 2  ___Li 2 O + ___Mg 3 P 2  ___Li 3 P + ___MgO  ___K + ___O 2  ___K 2 O  ___CH 4 +___ O 2  ___CO 2 +___ H 2 O

Polyatomic Ions When dealing with polyatomic ions (i.e. SO 4 2- ), treat them as single units to be balanced Treat any number outside of the brackets as another coefficient Fe(NO 3 ) 2(aq) + Na 3 PO 4(aq)  NaNO 3(aq) + Fe 3 (PO 4 ) 2(s) How do we approach this problem? Which atoms should we start with first?

Practice Fe(NO 3 ) 2 + Na 3 PO 4  NaNO 3 + Fe 3 (PO 4 ) 2 H 2 SO 4 + NaOH  Na 2 SO 4 + H 2 O

Practice N 2(g) + H 2(g)  NH 3(g) CaC 2(s) + H 2 O (l)  Ca(OH) 2(s) + C 2 H 2(g) SiCl 4(s) + H 2 O (l)  SiO 2(s) + HCl (aq) H 3 PO 4(aq) + CaSO 4(s)  Ca 3 (PO 4 ) 2(s) + H 2 SO 4(aq) Methane + oxygen  carbon dioxide + water vapor Sodium chloride  sodium + chlorine Calcium nitrate + sodium sulfate  sodium nitrate + calcium sulfate Sulfur + oxygen  sulfur dioxide

Practice Try the following for more practice: P. 90 # 7-9 Chem Workbook:  “Balancing Formula Equations 1”  “Balancing Formula Equations 2”