Chemical Reactions and

Balancing Equations

Lecture 14

Chem 101

2

Chemical & Physical Changes

Physical change, the

chemical composition of

the substance remains

constant.

Chemical change, the

chemical composition of

the substance changes; a

chemical reaction occurs.

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Evidence for Chemical Reactions

There are four observations which indicate a

chemical reaction is taking place.

1. Release of gas:

Release of gas can be observed in different

ways; from light fizzing to heavy bubbling.

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Release of hydrogen

gas from the reaction

of magnesium metal

with acid.

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Evidence for Chemical Reactions

2. Production of an insoluble solid.

A precipitate, or insoluble solid, forms by adding

two aqueous solutions together.

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Production of

an insoluble

solid.

7

Evidence for Chemical Reactions

3. Observation of a permanent color change.

Many chemical reactions involve a permanent

color change.

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Permanent color change as an indication of a chemical reaction

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Evidence for Chemical Reactions

4) A heat energy change is observed.

Exothermic reaction: Heat is released by the

reaction.

Endothermic reaction: Heat is absorbed by the

reaction.

Examples: Observed heat and light given off.

10

Heat and light

given off as an

indicator

of chemical

reaction..

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Writing Chemical Equations

Chemical equation describes a chemical reaction using

formulas and symbols. General formula of a chemical

equation is:

A + B → C + D

A and B are reactants and C and D are products.

Adding a catalyst, reaction speeds up without the catalyst

being consumed. A catalyst is written above the arrow.

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States of Matter

When writing chemical equations, specify the physical

state of the reactants and products.

A(g) + B(l) → C(s) + D(aq)

g: gaseous state,

l: liquid state,

s: solid state,

aq: aqueous state

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Chemical Equation Symbols

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Chemical Reactions

HC2H3O2(aq) + NaHCO3(s) → NaC2H3O2(aq) + H2O(l) + CO2(g)

This reaction shows

– Aqueous acetic acid is added to solid sodium

carbonate

to produce

– Aqueous sodium acetate, liquid water, and carbon

dioxide gas.

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Diatomic Molecules

Seven nonmetals occur naturally as diatomic

molecules and these elements are written as diatomic

molecules in the chemical reactions.

Diatomic Molecules:

hydrogen (H2),

nitrogen (N2),

oxygen (O2),

and the halogens, F2, Cl2, Br2, and I2.

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Cont’d…Diatomic Molecules

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Rules of Balancing Chemical Equations

When balancing a chemical equation:

The number of atoms of each element must be the

same on both sides of the arrow.

To balance place a whole number coefficient in

front of each substance.

A coefficient multiplies all subscripts in a chemical

formula:

– 3 H2O has 6 hydrogen atoms and 3 oxygen atoms

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Balancing Equations

Before placing coefficients in an equation, check that

the formulas are correct.

Never change the subscripts in a chemical formula to

balance a chemical equation.

Balance each element in the equation starting with

the most complex formula.

Balance polyatomic ions as a single unit if it appears

on both sides of the equation.

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Balancing Equations

The coefficients must be whole numbers. If

coefficient is a fraction, multiply the whole equation by

the denominator to get whole numbers:

[H2(g) + ½ O2(g) → H2O(l)]

2

2 H2(g) + O2(g) → 2 H2O(l)

After balancing the equation, check, there must be the

same number of atoms of each element (or polyatomic

ion) on both sides of the equation:

4 H, 2 O → 4 H, 2 O

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Balancing Equations

Finally, check that you have the smallest whole

number ratio of coefficients. If all the coefficients can be

divided by a common factor, do so to complete balancing

of the reaction.

[2 H2(g) + 2 Br2(g) → 4 HBr(g)]

2

H2(g) + Br2(g) → 2 HBr(g)

2 H; 2 Br → 2 H; 2 Br

Balancing a Chemical Equation

Balance the following chemical equations:

__H2O(l) + __Mg(s) → __H2(g) + __MgO(aq)

__Al2(SO4)3(aq) + __Ba(NO3)2(aq) → __Al(NO3)3(aq) + __BaSO4(aq)

__Al(s) + __CuSO4(aq) → __Cu(s) + __AlSO4(aq) (Check Formulas)

Balancing a Chemical Equation

Balance the following chemical equations:

N2 + H2 → NH3

NaCl + F2 → NaF + Cl2

C8H18 +O2 → CO2 + H2O

AgNO3 + MgCl2 → AgCl + Mg(NO3)2

aluminum + hydrochloric acid → aluminum chloride + hydrogen

calcium hydroxide + phosphoric acid → calcium phosphate + water

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Review

4 ways to understand if a chemical reaction occurred:

1. A gas is detected.

2. A precipitate is formed.

3. A permanent color change is seen.

4. Heat or light is given off.

An exothermic reaction gives off heat and an

endothermic reaction absorbs heat.

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There are 7 elements that exist as diatomic molecules:

– H2, N2, O2, F2, Cl2, Br2, and I2

When we balance a chemical equation, the number of

each type of atom must be the same on both the product

and reactant sides of the equation.

We use coefficients in front of compounds to balance

chemical reactions.

Review