Science 1206

Conservation of mass&

Balancing equations

Chemical Changes• A Chemical change

occurs when matter changes into a new and different form.   

• For instance when an electric current is passed though liquid water, the water changes into two gasses which are highly flammable; hydrogen and oxygen.

Hindenburg Disaster

• During a chemical reaction, the atoms are neither created nor destroyed. All that really happens to the atoms is that they are rearranged.

Water Hydrogen Oxygen

4 H’s and 2 O’s 4 H’s 2 O’s

• The old substances whose bonds are broken are called reactants

• The new substances that are formed as a result of the reaction are called products. (products are produced)

Reactants Products

Energy Changes in Chemical Reactions

1.1. ENERGYENERGY change• the absorption or the

release of heat and/or light, sound, electrical energy.

• exoexothermic reactions release energy (exo = out)

• endendoothermic reactions absorb heat (endo = in)

Reactions: Heat = energyHeat = energy

The brilliant white light associated with some fireworks is due to the release of energy release of energy when magnesium reacts with oxygen.

Equations

Chemical Equations

Equations are a way of representing chemical changes/reactions

Chemical changes can be communicated in sentence form or as chemical equations.

– Word equations – use of words to DESCRIBE what happens

– Chemical (skeleton) equations – use to symbols to SHOW what happens

Chemical changes can be communicated in sentence form or as chemical equations.

• In a chemical equation, the reactants are listed on the left side of an arrow symbol, and the products are listed on the right side.

A chemical Equation

Chemical equations have four parts:

1. Chemical formulas

2. Subscripts for the States of matter • (s) solid • (l) liquid • (g) gas • (aq) aqueous - dissolved in water

3. Numerical coefficients • indicates how many atoms/molecules are involved

4. Reaction symbols • the "+" sign on the reactants (left) side is read as "reacts

with" • the arrow (   ) is read as "to produce" • the "+" sign on the products (right) side is read as "along

with".

Parts of a Chemical Equation/Reaction

Numerical CoefficientsSubscripts

Equation PracticeGiven the equation:

Pick outa. A subscript ______________b. The symbol meaning “to produce” ______c. A chemical formula ____________d. The symbol meaning “combines with”

_______e. A coefficient ______________f. A reactant _______________g. The symbol meaning “along with” _______

2( ) 2 ( ) 6 12 6( ) 2( )6 6 1 6g l s gCO H O C H O O

Complete the following:

• Chemistry Worksheet 2-1

Chemical Equations can be translated into a sentence

• Two molecules of diesel fuel reacts with 49 molecules of oxygen to produce 32 molecules of carbon dioxide along with 34 molecules of water.

Words to use for formulas when writing chemical equations as sentences

Formula Word

Ionic Formula Unit

Molecular Molecule

Metal Element Atom

Nonmetal Element Molecule

Let’s Try an Example

Writing equations as sentences - Practice

The Law of Conservation of Mass

• In a chemical reaction the mass of the reactants before a chemical reaction equals the mass of the products after the reaction is complete.

mass of reactants = mass products

• In other words, the mass you end up with equals the mass you started with.

Ex: Burning Paper is a reaction that obeys the Law of Conservation of Mass

+ Oxygen +

20 grams 20 grams

Balancing EquationsAny representation of a chemical equation

must reflect the Law of Conservation of Mass

• In order to write a chemical equation that obeys the law of conservation of mass, the number of atoms of each element must be the same on both

sides of the equation.

the left hand Side = the right hand side

• Ex:

Balancing Equations - Practice

Let’s have a look at this equation and try to balance it.

Note: To balance equations we write Coefficients in front of each formula.

___ HgO (s) ___ Hg (l) + ___ O2 (g)

Rules for Balancing Equations

1. When you insert a coefficient, each subscript in the formula is Multiplied by the coefficient.

Ex:

2. The number of atoms of each element MUST be

the same on the left and right sides.

3. Practice, Practice, Practice

2

2 2( ) 4 ' =

2 1( )2

2 '

x H H sH O

x O O s

Let’s Try another

___ Fe (s) + ___ O2 (g) ___ Fe2O3 (s)

• Write a balanced chemical equation for the reaction of cobalt with oxygen to produce cobalt (III) oxide

1.  ___Mg(s)  + ___O2(g)  →  ___MgO(s) 

2.  ___Cu(s)  + ___AgNO3(aq)  →  ___Ag(s) +  ___Cu(NO3)2 (aq)   

3.  ___KI(aq)  + ___Pb(OH)2 (aq)  →  ___PbI2(s) +  ___KOH (aq)

4.  ___P4 (s)  + ___F2 (g)  →  ___PF3 (l)

5.  ___Cr2O3 (s)  →  ___Cr(s) +  ___O2 (g)

Practice

• Complete the following activities– Chemistry Worksheet 2-2