Chemical Reactions Balancing Reactions

Rxn vs Equation Chemical reaction is a process where the atoms of 2 or more elements or compounds rearrange to form new substances. Chemical Equation summarizes the details of a particular chemical reaction.

Signs of a chemical reaction Change in color Formation of a gas (odor) Light/ Heat produced Change in temperature –(absorbs/releases energy) Formation of precipitate

Law of Conservation of Mass mass is neither created nor destroyed in a chemical reaction total mass stays the same atoms can only rearrange

Example 1: What is the total mass of reactants in the reaction below? 2 H2 + O2  2 H2OAnswer: __________g ? 72 g Example 2: What is the total mass of products in the reaction below? AgNO3 + NaCl  AgCl + NaNO3Answer: __________g 170 g 56 g ? Example 3: What is the mass of zinc produced in the reaction below? Ca + ZnCO3  CaCO3 + ZnAnswer: __________g 40 g 125 g 100 g ?

Chemical Equations Reactants Products 2H 2 (g) + O 2 (g)  2H 2 O(g) –Coefficient –Chemical formula –Physical state (s, solid; g, gas; aq, aqueous; l, liquid) Diatomic elements. H 2 N 2 O 2 F 2 Cl 2 Br 2 I 2

Some other stuff you may see: Δ heat added to reactants (triangle  appears above arrow) Catalysts are substances that increase the rate of the reaction but are not used up themselves. The formula for the catalyst is written above the arrow: (example MnO 2 )

Balancing Equations Copy down equation (leave space for #’s) Make an atom inventory –List elements on both sides (reactants/products) –Count # of atoms on each side Coefficient  subscript = # of atoms –Reduce if necessary! –Double √

Chemical Reactions Types of Reactions

Disclaimer The events depicted in this story are fictitious. Any similarity to any person living or dead is merely coincidental.

The Story Begins…

2 H 2 + O 2 → 2 H 2 O 3 Mg + N 2 → Mg 3 N 2 Synthesis Reaction A reaction in which two or more elements form a compound.

The Story Continues…

Decomposition Reaction 2 H 2 O → 2 H 2 + O 2 2HgO → 2 Hg + O 2 A reaction in which a compound is broken down into two or more elements.

The Story Continues…

Single Replacement Reaction 2 KCl + F 2 → 2 KF + Cl 2 Mg + 2 HCl → H 2 + MgCl 2 A reaction in which one element reacts with one compound to form another element and another compound.

The Story Continues…

Double Replacement Reaction Pb(NO 3 ) 2 + K 2 CrO 4  PbCrO KNO 3 NaCl + AgNO 3  AgCl + NaNO 3 A reaction in which the metals present in two compounds change places to form two new compounds.

The End! But…

Combustion When a carbon compound reacts with oxygen to make carbon dioxide and water. –Hint: It will always have O 2 on reactant side It will always have CO 2 and H 2 O on product side Example: CH 4 + O 2 → CO 2 + H 2 O

Examples 2NaCl  2Na + Cl 2 C 8 H 18 + O 2  H 2 O + CO 2 2HCl + F 2  2HF + Cl 2 HCl + NaOH  HOH + NaCl 2Na + Cl 2  2NaCl Decomposition Combustion Single Replacement Double Replacement Synthesis