copyright©2000 by houghton mifflin company. all rights reserved. 1 chemical reactions: an...

Copyright©2000 by Houghton Mifflin Company. All rights reserved.

1

Chemical Reactions:An Introduction

Chapter 6


2

Chemical Reactions

• Reactions involve chemical changes in matter resulting in new substances

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

Reactants Products


3

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 cluesnew odor, permanent new state


4

Figure 6.2: Hot and cold pack reactions


5

Figure 6.5(b)&(c): The reaction of potassium with water


6

Figure 6.3 (a): Chemical reactions


7

Figure 6.3 (b): Chemical reactions


8

Figure 6.3 (c): Chemical reactions


9

Chemical Equations

• Shorthand way of describing a reaction

• Provides information about the reactionFormulas of reactants and productsStates of reactants and productsRelative numbers of reactant and product

molecules that are requiredCan be used to determine weights of reactants

used and of products that can be made


10

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


11

Combustion of Methane

• methane gas burns to produce carbon dioxide gas and liquid waterwhenever 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


12

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


13

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


14

Symbols Used in Equations

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


15

Sample – Recognizing Reactants and Products• when magnesium metal burns in air it produces a

white, powdery compound magnesium oxideburning 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 formulasidentify diatomic elementsidentify polyatomic ionsdetermine formulas

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


16

Balancing by Inspection• Count atoms of each element

– 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

– 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


17

Figure 7.6: The thermite reaction gives off so much heat that the iron formed is molten


18

Balancing by Inspection

• Pick an element to balance– avoid elements from 1b

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

• Use factors as coefficients in equation– if already a coefficient then multiply by new

factor

• Recount and Repeat until balanced


19

Exampleswhen magnesium metal burns in air it produces a white, powdery compound magnesium oxideburning in air means reacting with O2

• write the equation in wordsidentify the state of each chemical

magnesium(s) + oxygen(g) magnesium oxide(s)




20

Examples• when magnesium metal burns in air it produces a


• count the number of atoms of on each sidecount polyatomic groups as one “element” if on both sidessplit count of element if in more than one compound on

one side


1 Mg 1

2 O 1


21

Examples• when magnesium metal burns in air it produces a


• pick an element to balanceavoid 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


22

Examples• when magnesium metal burns in air it

produces a white, powdery compound magnesium oxideburning in air means reacting with O2

• 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


23

Exampleswhen magnesium metal burns in air it produces a white, powdery compound magnesium oxideburning in air means reacting with O2

• 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


24

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 wordsidentify the state of each chemical

ammonia(g) + oxygen(g) nitrogen monoxide(g) + water(g)


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


25



• count the number of atoms of on each sidecount polyatomic groups as one “element” if on both sidessplit 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


26

Examples• Under appropriate conditions at 1000°C ammonia

gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water

• pick an element to balanceavoid 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


27

Examples• Under appropriate conditions at 1000°C ammonia

gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water

• 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


28



• 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


29

Examples• Under appropriate conditions at 1000°C ammonia gas

reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water

• Recount

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

2 N 2

6 H 6

2 O 2 + 3


30



• 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


31

Examples• Under appropriate conditions at 1000°C ammonia gas

reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water

• Multiply all the coefficients by a number to eliminate fractions

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

copyright©2000 by houghton mifflin company. all rights reserved. 1 chemical reactions: an...

Documents

o slide

houghton mifflin company

balanced slide

water slide

chemical reactions reactions

ol h h c h h oo

molten slide

g mgos slide