Chapter 7 – Chemical Reactions

Physical vs Chemical Changes

Physical changes only alter the state of matter; the composition of matter is not changed.
Ex. water (H2O) I 20

s
j

y
->

it it
(e)

The state of matter depends on temperature and pressure

To go from one state to another add or release energy

A chemical reaction, or chemical change, alters the composition of matter and physical and chemical
properties are different from those of the elements of which they are composed.
H! (#) + O! (#) → 2 H! O(*)
rammables
Example:

I neither
combustion

Parts of a Chemical Equation

Chemical reactions are written in the form of a chemical equation

Chemical equations show the result of a chemical reaction using symbols and formulas.
etc
5 yields, goes to, produces,
how many miles
-> 2 +! (#) + ,! (#) → 2 +! ,(*)
or molecules/ I
formula units
realtants products

(left) crignt)

Matter cannot be created or destroyed and therefore mass is always conserved in a chemical reaction.
Consider the following chemical equation and determine if matter is conserved: start end
-

When matter is conserved, the chemical equation is said to be balanced

H2
=
+ 02 -
20 1
The chemical equation above did not need to be balanced. However, most chemical equations need to be balanced.

Rules for Balancing Equations: Steps for Balancing Equations:

1. The number of atoms in the reactants must equal the 1. List the atoms and their number under the reactants.
number of atoms in the products. 2. List the atoms and their number under the products.
2. Subscripts can NOT change! (H2) 3. Adjust the coefficients until each atom is balanced.
3. If there is no coefficient in front of a molecule, there
is an understood, invisible “1”.

2
___./0 + ___1
1 ! → ___.1
22 + ___/0
o !

k - l K -

Br -

2
BV - I

F I
f- 2

___.2*,
2
" → ___.2* + ___,!
2 3

K -
l K -
I

I
C. 1- 11-1

f- 3
0-2

" )! → ___343," + ___675,

I
___34 ! 5,# + ___67(3,
1 2 1 #
Na - Z Na - I
s -
l
I
f-
0 -
7
o - 10
tb - I
I
Pb -

1773 I
N -

µ -

___2
2 + , + ___,
$ % ! 14
15 ! → ___2,! + ___+
6 !,

7=14 l l M
6- -
=

H 6=12
H 2 12
-
-
=

d- 4=6
0-3=34
Balancing advice: try to balance coefficients of “1” first. When possible, save oxygens for last. It’s okay
to use fractions but the final answer should be whole numbers. Make sure coefficients are reduced,
and most importantly just keep going – unless you’ve gone too far – then stop. J

2
 fp
___+" 6,# + 3 3 !O
___.,+ → ___." 6,# + ___+

1+-4=6 H -

2=6
p .
, .
1 _
, =
,
K
K =3 3=3
-

-
I

0-5=7 0-5=7
Write a balanced chemical equation for the reaction between solid cobalt(III) oxide and solid carbon that
produces solid cobalt and carbon dioxide gas.

2102031-36 → 4 lot 3102

Stoichiometric Calculations (Stoichiometry) -26.98

At
What relationship do you use to convert between grams and moles?
O
- 15.999×3
molar mass
1. How many moles are in 98.3 grams of aluminum hydroxide? H - 1.007×3

"h÷i¥+ –4
' -26m
""

Al / OH ) }

2.3xio-9mol-p-m.gr/-c9slP0n)z
2. How many grams are in 2.3 x 10 moles of calcium phosphate?

120<237
(a = 40.078
•

071g 12=30.97×2
6 C. 94
The coefficient used for balancing a chemical equation is called the
0=15.999×8
gtoilhiometvil loefflilnts
127.992
Look at the following chemical reaction: 2 8# + ,! → 2 8#,
The coefficients tell us that for every 2 moles of Mg and every 1 mole of O2 consumed there is 2 moles of
MgO produced. These stoichiometric coefficients are based on moles of a substance not the mass of a
substance, because every element has a different mass.
Consider the following decomposition reaction: 2 9*! ," (:) → 4 9*(:) + 3 ,! (#)
If two moles of Al2O3 are consumed, how many moles of Al are produced? O2?
2 Moles All 03 2 molls 1-1203

y moles At 3 moles 02
Chemists use these molar ratios (coefficients) as a predictive tool.

3
 For example: Hydrogen and oxygen gas undergo a synthesis reaction to produce water. How many moles
of H2O are produced when 5.00 moles of oxygen are used?
1. Write the balanced chemical equation. 2- Hz 1- 02 → 2-11-20
2. Use the molar ratios to convert between moles of oxygen to moles of water.

gmot.es#Z-E Imo I 02
= 10.0 moles H2O

If 3.00 moles of water are produced, how many moles of oxygen must be consumed?
g molts H2O 1 molls 02
= 1 . 5 moles 02
2 miles Hzo

Any comparison of one element or compound to another MUST be made in MOLES

In general, stoichiometry follows the scheme:

A
B

A B

Given the following reaction, how many moles of water are produced from 19.2 g of B2H6?
I
/! +% + -3 ,! → -2 +/,! + -2 +! ,

_1mtf
2 MOI 1+20
= t.gg moles H2O

27.369 1 not BZH 6

When sodium azide (NaN3) is activated in a car airbag, nitrogen gas and solid sodium are produced
according to the chemical equation below. If 0.500 moles of NaN3 decompose, what mass of nitrogen
would be produced?
2 343" (:) → 2 34(:) + 3 3! (#)

# N2
500 moles NaN 3 moles
.
}
=
21g Nz
moles Naw} twirl Nz

4
 "
3×10 kg 02

The combustion of fossil fuels adds 8.2 x 1012 kg of carbon into the atmosphere every year in the form of
CO2. What is the mass (in kg) of this much CO2?
2(:) + ,! (#) → 2,! (#)

""""i÷÷÷t÷÷4÷÷i÷:
How many grams of Na2S2O3 are needed to react completely with 42.7 g of AgBr? :::
2 Na S O + AgBr NaBr + Na3[Ag(S2O3)2]
2 2 3
"

→i÷÷÷t÷÷;:÷%i:
"

71.9g Nash 03
Limiting Reactants and Theoretical Yield
When all the reactants of a reaction are completely consumed, the reaction is in perfect stoichiometric
proportions. Often, a reaction is not in perfect stoichiometric proportions, leading to a situation in which
the entirety of one reactant is consumed, but there is some of another reactant remaining.
reactant
The reactant that is entirely consumed is called the limiting
Any reactants remaining are called Wiess real tant

Determine the limiting and excess ingredient(s).

limiting -
meat

there and bread

excess -

Now determine the limiting and excess reactant on the molecular level.
2+# (#) + 2 ,! (#) → 2,! (#) + 2 +! ,(*)

malt ants products is city

limiting
Cf " less is oz

21 Hy
yµ
5 02 5
 Carbon dioxide reacts with hydrogen gas according to the equation below. If 5.0 moles of CO are exposed
to 7.50 mol of H2, what is the limiting reactant?
To determine the limiting reactant, you must calculate how much product each reactant will make if it
were completely consumed.
5.0 mol 7.5 mol
2,(#) + 2 +! (#) → 2+" ,+(#)

realtant

10oCH3= 5 mol CH
limiting
is #2

amount

/morOH
least
3.75 mol CHO
=
produces
of product

Silicon dioxide (quartz) is usually quite unreactive but reacts readily with hydrogen fluoride according to
the equation below. If 6.0 mol of HF are exposed to 4.5 mol of SiO2, which is the limiting reactant?
7.5MM 6mol
5=,! (:) + 4 +1(#) → 5=1# (#) + 2 +! ,(*)

In this case we have two products, which product do you pick?

doesn't matter, but must pill same product for
each calculation

mission/mirsie= 1mirsiin Alsify 1.5

= mil sit in

site is limiting realtant

Determine the limiting reactant when there are 0.055 g H2 and 0.48 g O2.
·0039. 489
2 +! (#) + ,! (#) → 2+! ,(*)

greatwill
i s
reauthnt H2
=

=0029 mol H20

limiting

more ent

6
Identifying the limiting reactant can be challenging. It is tempting to assume that the reactant with the
smaller starting mass is the limiting reactant, but that is usually not the case. (Grams don’t react with
grams; moles react with moles).

Suppose a sealed reaction vessel contains 16 g CH4 (methane) and 48 g O2. Determine which is the
limiting reactant and how much (in grams) of the excess reactant will be left over.
16 18
2+# (#) + 2 ,! (#) → 2,! (#) + 2 +! ,(*)

left over g CHy

Start (9) CHy-consumed gCHy=
Step 1: Determine the limiting reactant. 02
limiting:

/molemore= 99mo e

02/mosieurete =75 mil 182

Step 2: Determine how much methane will be consumed.

902- g(Hy

/molmilao,y= 12.88 g e

Step 3: Find the excess by subtracting the consumed amount from the original mass.

12.03 CHY
16
3.97g
=
-

The above calculation (step 1) not only identified the limiting reactants, but it also determined the
theoretical yield
The theoretical yield is the maximum amount of product possible in a chemical reaction for the given
quantities of reactants. This is also called the stoichiometric yield
limiting ->
theoretical amount
reALAYt
7
Percent Yield: Actual vs Theoretical
Go back to our sandwiches. Suppose we had 10 pieces of bread, 12 pieces of cheese, and 6 slices of
bologna. According to the following recipe how many sandwiches could you make?

We have enough bread to make?

S
We have enough cheese to make?
4
We have enough bologna to make?
I
What is our theoretical yield of sandwiches?
4
Unfortunately, as we were making the last sandwich, we accidentally dropped 3 pieces of cheese on the
ground and our dog ate them. How many sandwiches did we actually produce? 3
Often times, like making sandwiches, reactants do not react completely resulting in a smaller amount of
product formed than calculated from the theoretical yield. The amount of product(s) that is produced
during a reaction is called the

It is useful to distinguish between the theoretical and actual yields of a chemical reaction and determine
the
4?AE4* F=>*C
6>0?>@A B=>*C = I 100%
Aℎ>H0>A=?4* F=>*C

Calculate the percent yield for our sandwiches.

100% 79%
-5 x
=

8
 Chlorobenzene, C6H5Cl, is used in the production of many important chemicals such as aspirin, dyes, and
disinfectants. One industrial method of preparing chlorobenzene is to react benzene, C6H6, with chlorine
gas as shown in the chemical reaction below. When 36.8 g of C6H6 reacts with an excess of Cl2, the actual
yield of C6H5Cl is 38.8 g. What is the percent yield?
36.89 ? 9
2% +% (*) + 2*! (#) → 2% +& 2*(:) + +2*(#)

%%÷ 38.89
'
too = 73.2101 .

1. = ✗ =

53

"Hf÷¥÷fÉEÉ"ˢ
"

78.11 1m01 H, IMAI 1611-561

53g
=

theoretical
Y' 119
Aqueous hydrobromic acid reacts with solid sodium hydroxide to produce aqueous sodium bromide and
liquid water. If 57.5 g of sodium bromide is produced from the reaction of 52.6 g of hydrobromic acid and
42.9 g of sodium hydroxide, calculate the percent yield of sodium bromide.
HBV + NaOH → Na Br + H2O

52.6g 42-99 57 limiting

.gg HBr=

52.bg#tsr-f=EHNPr=.6smi1NaBr
80.912g HPV 1hr01 HBV

m-99NaÉBr gNaoH 3%997 [ moi NaOH

=
" 07 MOINA Br

57.5

t.mil#B&mi1NaDr
bz.bgttbu-11-MHB-n-XN.am
✗ " " = 102.894
-

66-89
HBV
80.912g
85.96° to 66
9