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Chemical Reactions & Equations. +. Evidence of Chemical Reactions. Release of a gas Example: bubbles formed when magnesium and hydrochloric acid were mixed Color change Example: color went from clear to yellowish orange when potassium iodide was added to hydrogen peroxide

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evidence of chemical reactions
Evidence of Chemical Reactions
  • Release of a gas
    • Example: bubbles formed when magnesium and hydrochloric acid were mixed
  • Color change
    • Example: color went from clear to yellowish orange when potassium iodide was added to hydrogen peroxide
  • Formation of a precipitate
    • Example: cloudiness occurred when CO2 gas passed through limewater
  • Change in temperature, light, sound, smell
    • Example: temperature increased when hydrochloric acid and sodium carbonate were mixed
endothermic or exothermic

CaO(s) + H2O(l) Ca(OH)2(aq) + 82 kJ

NH4NO3(s) + H2O(l) + energy NH4NO3(aq)

Endothermic or Exothermic?

Since energy is a product, this reaction is exothermic.

This reaction will generate enough heat to fry an egg!

Since energy is a reactant, this reaction is endothermic.

This is the cooling reaction of a cold pack.

chemical equations


substances after the change


substances before the change









Chemical Equations

Propane and oxygenreact to produce carbon dioxide, water, and energy. This reaction is not yet balanced.

symbols in equations


“plus” or “and” indicates multiple reactants or products

“yields” or “produces” separates reactants & products, arrow points in the direction of the reaction




indicate phase of a reactant or product





“aqueous” dissolved in water (not the same as liquid (l))

indicates reaction is reversible


over the arrow, indicates heating is necessary


“no reaction”

Symbols in Equations
balancing equations

Balance the numbers of atoms of each element on both sides of the equation by determining the coefficients of the compounds.

Balancing Equations
  • When balanced, an equation accounts for the conservation of:
    • mass
    • charge
    • energy


an example forming water

Instead we double the coefficients of H2 and H2O

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

There are now 4 H’s and 2 O’s on each side.

An Example: Forming Water

H2 (g) + O2 (g) H2O (l)

On left: 2 H and 2 O On right: 2 H and 1 OTherefore, not balanced

We could use 1/2 O2 on the left but that is not done, we always want whole number coefficients.

rules for balancing equations




Rules for Balancing Equations
  • Write the formulas for all reactants and products
  • Don’t change formulas (subscripts), only coefficients.
  • Balance elements that only occur once on each side of the equation first
  • To determine the number of atoms of an element, multiply the coefficient by the subscript for the element.

Tips: for reactions involving oxygen, balance it last

polyatomic ions can be balanced as a unit

balancing example

C2H4 + O2 CO2 + H2O

C2H4 + O22CO2 + 2H2O

C2H4 + 3O2 2CO2 + 2H2O

Balancing Example

Write the equation for burning ethylene (C2H4) to produce carbon dioxide and water.

C and H both appear once on each side of the equation whereas O appears in both compounds on the right hand side.

Multiply CO2 by 2 to balance C

Multiply H2O by 2 to balance H

There are now 6 O’s on the right so multiply O2 by 3 to finish.

balancing practice 1

Balance F:

K + F22KF

Balance K:

2K + F22KF

Balancing Practice #1

Unbalanced equation:

K + F2 KF

balancing practice 2

Notice Ca is fine:

Ca + H2O Ca(OH)2 + H2

Balance O:

Ca + 2H2O Ca(OH)2 + H2

Balancing Practice #2


Ca + H2O Ca(OH)2 + H2

Hydrogen is already taken care of so we’re done.

balancing practice 3


2NH4Cl + Ca(OH)2 NH3 + H2O + CaCl2


2NH4Cl + Ca(OH)22NH3 + H2O + CaCl2


2NH4Cl + Ca(OH)2 2NH3 + 2H2O + CaCl2


2NH4Cl + Ca(OH)22NH3 + 2H2O + CaCl2

10 H on each side, so we’re done!

Balancing Practice #3

NH4Cl + Ca(OH)2 NH3 + H2O + CaCl2

5 types of chemical reactions
5 Types of Chemical Reactions
  • Combustion
  • Synthesis
  • Decomposition
  • Single Replacement
  • Double Replacement

Some reactions fall into multiple categories.

combustion reactions

Hydrocarbons (C, H, O) - heat homes, cook food, and power transportation. Always get CO2(g) and H2O(g).

hydrocarbon+ O2 (g)CO2 (g) + H2O(g)

Metals produce metal oxides (like rust and tarnishes).

2Mg(s)+ O2(g) 2MgO(s)

Combustion Reactions

A substance reacts with oxygen to create products containing oxygen. Heat may be required to initiate. Exothermic.

practice combustion

C7H16(g) + O2(g)

CO2(g) + H2O(g) + heat

C7H16(g) + 11 O2(g)

7 CO2(g) + 8 H2O(g)

Practice Combustion

Write the balanced equation for the combustion of heptane gas (C7H16)

Write out the reactants and products:

Balance the equation (ignore heat):

synthesis reactions

A + B AB

general form

Note: A & B can be elements or compounds


sodium chloride

2Na+ Cl2 2NaCl

magnesium oxide

2Mg+ O2 2MgO

Synthesis Reactions
  • The combining of two or more substances to form a more complex substance.
decomposition reactions

AB A + B

general form



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

NH4NO3(s)N2O(g) + 2H2O(g)

Decomposition Reactions
  • Compound breaks down into two or more simpler substances (elements or compounds).
single replacement reactions


Cl2(aq) + 2KBr(aq) 2KCl(aq) + Br2(aq)

Single Replacement Reactions

One element takes the place of another element. Metals replace metals. Non-metals replace non-metals. Not reversible.

A + BC BA + C

general forms

A + BC AC + B

double replacement reactions


general form


Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

Double Replacement Reactions

When solutions of two soluble ionic compounds are mixed, an insoluble product (precipitate) forms.

Remember “inny outy” rule

more on double replacement
More on Double Replacement
  • When 2 ionic solutions are mixed there are several events that could happen:
    • Nothing, everything is soluble
    • A precipitate forms (possibly two)
    • A gas is formed

(like FeS(s) + H2SO4(aq) --> FeSO4(aq) + H2S(g)

    • A covalent molecule forms (like H+ + OH- --> H2O)
  • Ions that remain in solution are called spectator ions because they don’t participate in the reaction.
practice identifying reaction types

Ca(s) + CuSO4(aq) CaSO4(aq) + Cu(s)

2AlCl3(l) 2Al(l) + 3Cl2(g)

ZnSO4(aq) + SrCl2(aq) ZnCl2(aq) + SrSO4(s)

4Na(s) + O2(g) 2Na2O(s)

Practice - Identifying Reaction Types

single replacement


double replacement

synthesis, combustion