Balancing Chemical Equations Chemistry Unit Science 10

What is a chemical equation? When a chemical reaction occurs, it can be described by an equation. This shows the chemicals that react (called the reactants) on the left-hand side, and the chemicals that they produce (called the products) on the right-hand side. The chemicals can be represented by their names or by their chemical symbols.

What is a chemical equation? For example: Aluminium + Oxygen Aluminium Oxide Numbers found in front of the elements are called coefficients. ex: 2Aluminium Reactants Product

To balance a chemical equation Step 1: Write the skeleton equation (aka, no numbers) Be sure you have the correct formulas for all the compounds. Watch especially for polyatomics Ex: Al + NO 3 Al 2 (NO 3 ) 3

To balance a chemical equation Step 2: Balance the elements or polyatomic ions that appear just once on each side first. Ex: Al + NO 3 Al 2 (NO 3 ) 3 2Al + 3(NO 3 ) Al 2 (NO 3 ) 3

To balance a chemical equation Step 3: Check for any terms you haven’t looked at and balance them so the total on the reactants side = the total on the products side. Ex: Al + NO 3 Al 2 (NO 3 ) 3 2Al + 3NO 3 Al 2 (NO 3 ) 3 6Al + __NO 3 __Al 2 (NO 3 ) 3

To balance a chemical equation Step 4: Double check the equation A table can be very helpful Ex: 2Al + 3NO 3 Al 2 (NO 3 ) 3 Al = 2 Al = 2 NO 3 = 3 NO 3 = 3

Example - Photosynthesis Word Equation: water + carbon dioxide + sun energy produces carbohydrate + oxygen Skeleton Equation: H 2 O + CO 2 + sun energy  C 6 H 12 O 6 + O 2

Example - Photosynthesis H 2 O + CO 2 + sun energy  C 6 H 12 O 6 + O 2 H appears only once on each side. Choose a coefficient 6 for H 2 O (to put 12 hydrogen on each side) (6x2 = 12) 6H 2 O + __CO 2 + energy  __C 6 H 12 O 6 + __O 2 C appears only once on each side. Choose a coefficient 6 for CO 2 (6x1 = 6) 6H 2 O + 6CO 2 + energy  __C 6 H 12 O 6 + __O 2

Example - Photosynthesis 6H 2 O + 6CO 2 + sun energy  C 6 H 12 O 6 + O 2 Oxygen appears more than once on both sides. Count the total on each side and make the adjustment as needed. 6O + 12O = 18O 6O + 6x2O = 18O 6H 2 O + 6CO 2 + energy  C 6 H 12 O 6 + 6O 2 H = 12 H = 12 C = 6 C = 6 O = 18 O = 18

Diatomic Elements Diatomic means that they exist as X 2 instead of just X in the natural environment. There are 7 diatomic elements: Hydrogen (H 2 ) Nitrogen (N 2 ) Oxygen (O 2 ) Fluorine (F 2 ) Chlorine (Cl 2 ) Iodine (I 2 ) Bromine (Br 2 )

Types of Reactions Reactions can be either Exothermic or Endothermic. Exothermic reactions: Reactions where energy is… Endothermic reactions: Reactions where energy is… released. absorbed

Types of Reactions In Photosynthesis, energy is required. It is on the reactant side of the equation. Therefore it is… In Cellular Respiration, energy is given off. It is on the product side of the equation. Therefore it is… * Even if a small amount of energy is needed to start the reaction (like a match), if there is energy generated by the reaction itself (like an explosion), it is exothermic Exothermic Endothermic

Types of Reactions: Synthesis When a reaction has two or more reactants that synthesize to form a new product. X + Y  XY ex: water is synthesised from hydrogen and oxygen gas: 2H 2(g) + O 2(g)  2H 2 O (l) + thermal energy

Types of Reactions: Decomposition A compound decays into two or more simpler compounds or elements. XY  X + Y ex: water + energy decays into hydrogen and oxygen gas (boiling water): 2H 2 O (l) + thermal energy  2H 2(g) + O 2(g)

Types of Reactions: Single Displacement One element takes the place of (displaces) another element in a compound A + BX  B + AX or AX+ Y  AY + X ex: 2Mg (s) + 2 HCl (aq)  2MgCl (aq) + H 2(g) 2AgNO 3(aq) + Cu (s)  Cu(NO 3 ) 2(aq) + 2Ag (s) One element takes the place of another

Types of Reactions: Single Displacement This is like someone cutting in on a couple dancing. Before the ‘cut-in’ there are two people dancing and one single. After the ‘cut-in’ there are still two people dancing but one has changed and there is a different single.

Types of Reactions: Single Displacement Predict the products of the following reaction: cobalt + aluminum chloride  The metal element (cobalt) replaces the positive ion (aluminium) in the compound. cobalt + aluminum chloride  aluminum + cobalt(II) chloride

Types of Reactions: Single Displacement How did we know the charge on cobalt was +2 since it is multi-valent (has more than 1 charge)? When predicting single displacement reaction products, we choose the most common charge for the metal ion. The most common charge is listed first on your periodic table of ions.

Types of Reactions: Double Displacement The positive ions of two different compounds change (displace) places, forming 2 new compounds. WX + YZ  WZ + YX ex: NaOH (aq) + HCl (aq)  NaCl (aq) + HOH (l)

Types of Reactions: Double Displacement This is like a dance where two couples are dancing and they exchange partners. Before the exchange everybody is dancing. After the exchange, everybody is still dancing but with a different partner.

Types of Reactions: Double Displacement Predict the products of the following reaction: –cobalt(II) nitrate + aluminum chloride  The metal ions (cobalt(II) and aluminium) switch places. cobalt(II) nitrate + aluminum chloride  cobalt(II) chloride + aluminum nitrate Notice that the charge on each metal ion remains the same on both sides of the reaction arrow. Remember to balance the reaction after you have predicted the products –3Co(NO 3 ) 2 + 2AlCl 3  3CoCl 2 + 2Al(NO 3 ) 3

Types of Reactions: Combustion The reaction of a substance with a diatomic oxygen molecule is said to be a combustion reaction. AB + O 2  AO + BO Combustion reactions are almost always exothermic (they give off heat). For example when wood burns, it must do so in the presence of O2 and a lot of heat is produced:

Types of Reactions: Combustion Wood as well as many common items that combust are organic (i.e., they are made up of carbon, hydrogen and oxygen). Organic chemistry is chemistry involving these 3 elements and has a lot to do with biological make up. When organic molecules combust the reaction products are carbon dioxide and water (as well as heat).

Which is the Which of Which one? ___ Pb(NO 3 ) 2  ___ PbO + ___ NO 2 + ___ O 2 ___ Ca + ___ SO 3  ___ Ca 3 (SO 3 ) 2 ___ Ca + ___ H 2 O  ___ Ca(OH) 2 + ___ H 2 ___ CrCl 3 + ___ H 2 SO 4  ___ Cr 2 (SO 4 ) 3 + ___ HCl ___ Fe(NO 3 ) 3 + ___ NH 4 OH  ___ Fe(OH) 3 + ___ NH 4 NO 3 ___ Al + ___ K 3 PO 4  ___ AlPO 4 + ___ K ___ Fe + ___ O 2  ___ Fe 2 O 3 Decomposition 2 Pb(NO 3 ) 2  2 PbO + 4 NO O 2 Synthesis 3 Ca + 2 SO 3  1 Ca 3 (SO 3 ) 2 1 Ca + 2 H 2 O  1 Ca(OH) H 2 Single Displacement Double Displacement 2 CrCl H 2 SO 4  1 Cr 2 (SO 4 ) HCl 1 Fe(NO 3 ) NH 4 OH  1 Fe(OH) NH 4 NO 3 Double Displacement 1 Al + 1 K 3 PO 4  1 AlPO K Single Displacement 4 Fe + 3 O 2  2 Fe 2 O 3 Combustion