hchemchapter6balancing&writingchemicalequations

Chemical Reactions:An Introduction

Chapter 6

Copyright © Houghton Mifflin Company.All rights reserved. 6–2

Chemical Reactions

• Reactions involve chemical changes in matter resulting in new substances

• Reactions involve rearrangement and exchange of atoms to produce new molecules– Elements are not transmuted during a reaction

Reactants Products

Nylon being drawn from the boundary between two solutions containing different reactants.


Evidence of Chemical Reactions

• a chemical change occurs when new substances are made

• visual clues (permanent)– color change, precipitate formation, gas bubbles,

flames, heat release, cooling, light

• other clues– new odor, permanent new state

Figure 6.1: Bubbles of hydrogen and oxygen gas form when an electric current is used to decompose water.


Figure 6.3a: When colorless hydrochloric acid is added to a red solution of cobalt(II) nitrate, the solution turns blue, a sign that a chemical

reaction has taken place.


A hot pack used to warm hands and feet in winter. When the package is opened, oxygen from the air penetrates a bag containing solid chemicals. The resulting

reaction produces heat for several hours.

Figure 6.3b: A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate.

Figure 6.3c: Bubbles of hydrogen gas form when calcium metal reacts with water.


Chemical Equations

• Shorthand way of describing a reaction

• Provides information about the reaction– Formulas of reactants and products– States of reactants and products– Relative numbers of reactant and product

molecules that are required– Can be used to determine weights of reactants used

and of products that can be made


Conservation of Mass

• Matter cannot be created or destroyed

• In a chemical reaction, all the atoms present at the beginning are still present at the end

• Therefore the total mass cannot change

• Therefore the total mass of the reactants will be the same as the total mass of the products


Combustion of Methane

• methane gas burns to produce carbon dioxide gas and liquid water– whenever something burns it combines with O2(g)

CH4(g) + O2(g) CO2(g) + H2O(l)

H

HC

H

HOO+

O

O

C + OH H

1 C + 4 H + 2 O 1 C + 2 O + 2 H + O1 C + 2 H + 3 O

Figure 6.3d: Methane gas reacts with oxygen to produce a flame in a bunsen burner.


Combustion of MethaneBalanced

• to show the reaction obeys the Law of Conservation of Mass it must be balanced

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

H

HC

H

H

OO

+

O

O

C +

OH H

OO

+O

H H

+

1 C + 4 H + 4 O 1 C + 4 H + 4 O


Writing Equations

• Use proper formulas for each reactant and product• proper equation should be balanced

– obey Law of Conservation of Mass

– all elements on reactants side also on product side

– equal numbers of atoms of each element on reactant side as on product side

• balanced equation shows the relationship between the relative numbers of molecules of reactants and products – can be used to determine mass relationships


Symbols Used in Equations

• symbols used after chemical formula to indicate state– (g) = gas; (l) = liquid; (s) = solid– (aq) = aqueous, dissolved in water


Sample – Recognizing Reactants and Products

• when magnesium metal burns in air it produces a white, powdery compound magnesium oxide– burning in air means reacting with O2

– Metals are solids, except for Hg which is liquid write the equation in words

– identify the state of each chemicalmagnesium(s) + oxygen(g) magnesium oxide(s)

write the equation in formulas– identify diatomic elements– identify polyatomic ions– determine formulas

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


Balancing by Inspection

Count atoms of each elementa polyatomic ions may be counted as one “element” if it

does not change in the reaction

Al + FeSO4 Al2(SO4)3 + Fe

1 SO4 3

b if an element appears in more than one compound on the same side, count each separately and add

CO + O2 CO2

1 + 2 O 2


Balancing by Inspection

Pick an element to balancea avoid elements from 1b

Find Least Common Multiple and factors needed to make both sides equal

Use factors as coefficients in equationa if already a coefficient then multiply by new

factor

° Recount and Repeat until balanced


Examples


write the equation in words

– identify the state of each chemical

magnesium(s) + oxygen(g) magnesium oxide(s) write the equation in formulas

– identify diatomic elements

– identify polyatomic ions

– determine formulas



Examples


count the number of atoms of on each side

– count polyatomic groups as one “element” if on both sides

– split count of element if in more than one compound on one side


1 Mg 1

2 O 1


Examples


pick an element to balance

– avoid element in multiple compounds

° find least common multiple of both sides & multiply each side by factor so it equals LCM


1 Mg 1

1 x 2 O 1 x 2


Examples


± use factors as coefficients in front of compound containing the element if coefficient already there, multiply them together

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

1 Mg 1

1 x 2 O 1 x 2


Examples


Recount

Mg(s) + O2(g) 2 MgO(s) 1 Mg 2

2 O 2 Repeat

2 Mg(s) + O2(g) 2 MgO(s) 2 x 1 Mg 2

2 O 2


Examples

• Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water

• write the equation in words

– identify the state of each chemical

ammonia(g) + oxygen(g) nitrogen monoxide(g) + water(g) write the equation in formulas

– identify diatomic elements

– identify polyatomic ions

– determine formulas

NH3(g) + O2(g) NO(g) + H2O(g)


Examples


count the number of atoms of on each side

– count polyatomic groups as one “element” if on both sides

– split count of element if in more than one compound on one side

NH3(g) + O2(g) NO(g) + H2O(g)

1 N 1

3 H 2

2 O 1 + 1


Examples


pick an element to balance– avoid element in multiple compounds

° find least common multiple of both sides & multiply each side by factor so it equals LCM

NH3(g) + O2(g) NO(g) + H2O(g)1 N 1

2 x 3 H 2 x 3 2 O 1 + 1


Examples


± use factors as coefficients in front of compound containing the element

2 NH3(g) + O2(g) NO(g) + 3 H2O(g)

1 N 1

2 x 3 H 2 x 3

2 O 1 + 1


Examples


Recount

2 NH3(g) + O2(g) NO(g) + 3 H2O(g)2 N 16 H 6

2 O 1 + 3 Repeat

2 NH3(g) + O2(g) 2 NO(g) + 3 H2O(g) 2 N 1 x 2

6 H 6 2 O 1 + 3


Examples


´ Recount

2 NH3(g) + O2(g) 2 NO(g) + 3 H2O(g)

2 N 2

6 H 6

2 O 2 + 3


Examples


Repeat– A trick of the trade, when you are forced to attack an element that is in

3 or more compounds – find where it is uncombined. You can find a factor to make it any amount you want, even if that factor is a fraction!

– We want to make the O on the left equal 5, therefore we will multiply it by 2.5

2 NH3(g) + 2.5 O2(g) 2 NO(g) + 3 H2O(g)

2 N 2

6 H 6

2.5 x 2 O 2 + 3


Examples


Multiply all the coefficients by a number to eliminate fractions– x.5 2, x.33 3, x.25 4, x.67 3

2 x [2 NH3(g) + 2.5 O2(g) 2 NO(g) + 3 H2O(g)]

4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)

4 N 4

12 H 12

10 O 10

Figure 6.4: The reaction between methane and oxygen to give water and carbon dioxide.


Figure 6.5: The reactants (a) potassium metal and (b) water. (c) The reaction of potassium with water.

Zinc metal reacts with hydrochloric acid to produce bubbles of hydrogen gas.


A bombardier beetle defending itself.


Decorations on glass are produced by etching with hydrofluoric acid.

hchemchapter6balancing&writingchemicalequations

Science

chemical reactions reactions

reaction reactants products

reaction formulas of

evidence of chemical

gas bubbles

chemical changes

resulting reaction

total mass