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 H2O Answer: __________g ? 72 g

Example 2: What is the total mass of products in the reaction below?

AgNO3 + NaCl AgCl + NaNO3 Answer: __________g 170 g 56 g ---------?----------

Example 3: What is the mass of zinc produced in the reaction below?

Ca + ZnCO3 CaCO3 + Zn Answer: __________g

40 g 125 g 100 g ?

72

226

65

Chemical Equations

• Reactants

• Products

2H2(g) + O2(g) 2H2O(g)

– Coefficient

– Chemical formula

– Physical state (s, solid; g, gas; aq, aqueous; l, liquid)

• Diatomic elements. H2 N2 O2 F2 Cl2 Br2 I2

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 MnO2)

MnO22 KClO3(s)2 KClO3(s) 2KCl(s) + 3 O2(g)

Catalysts are not used up in the reaction so they appear above the arrow

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

DisclaimerDisclaimer

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

The Story

Begins…

2 H2 + O2 → 2 H2O3 Mg + N2 → Mg3N2

Synthesis Reaction

A reaction in which two or more elements form a compound.

The Story Continues…

Decomposition Reaction

2 H2O → 2 H2 + O2

2HgO → 2 Hg + O2

A reaction in which a compound is broken down into two or more elements.

The Story Continues…

Single Replacement Reaction

2 KCl + F2 → 2 KF + Cl2

Mg + 2 HCl → H2 + MgCl2

A reaction in which one element reacts with one compound to form another element and another compound.

The Story Continues…

Double Replacement Reaction

Pb(NO3)2 + K2CrO4 PbCrO4+ 2 KNO3

NaCl + AgNO3 AgCl + NaNO3

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 O2 on reactant side

• It will always have CO2 and H2O on product side

• Example:CH4 + O2 → CO2 + H2O

Examples• 2NaCl 2Na + Cl2

• C8H18 + O2 H2O + CO2

• 2HCl + F2 2HF + Cl2

• HCl + NaOH HOH + NaCl

• 2Na + Cl2 2NaCl

Decomposition

Combustion

Single Replacement

Double Replacement

Synthesis