1
Enthalpy of Reaction 1
CSCOPE Unit 12 Lesson 02 Day 4
Vocabulary
Endothermic process a heat absorbing process
Enthalpy the heat content of a system; the symbol is “ H ”; the symbol for the change in enthalpy is “ 
H ”
Exothermic process a heat releasing process
Heat the energy (total kinetic energy) that is transferred from one body to another because of a temperature difference
Since enthalpy ( H ) cannot be measured directly, chemists measure the change in enthalpy
(

H ), defining the enthalpy of any element in its standard state to be exactly zero.
The change in enthalpy (

H ) equals the heat gained or lost by the system. The enthalpy of reaction (Δ H rxn
) equals the difference between the enthalpy of formation of the products and the enthalpy of formation of the reactants. enthalpy of reaction = (enthalpy of products) – (enthalpy of reactants)
Δ H rxn
= or
Δ H f o
(of products) – Δ H f o
(of reactants)
If an enthalpy change for a reaction, Δ H rxn
, is negative, the total energy of the final system is less than that of the initial system, so energy is released to the environment. Chemical reactions that release heat energy (Δ H rxn
values are negative) are termed exothermic.
If an enthalpy change for a reaction, Δ H rxn
, is positive, the total energy of the final system is more than that of the initial system, so energy is absorbed from the environment. Chemical reactions that absorb heat energy (Δ H rxn
values are positive) are termed endothermic.
Step 1: Write the balanced equation.
Step 2: Using a table of standard enthalpies of formation (page 7 of this CSCOPE), find the
sum of the Δ H f o
of all of the products. The number of moles in the balanced equation
must be used as a factor; in other words, the Δ H f o
of each product must be multiplied
by its coefficient in the balanced equation.
Step 3: Using a table of standard enthalpies of formation (page 7 of this CSCOPE), find the
sum of the Δ H f o
of all of the reactants. The number of moles in the balanced equation
must be used as a factor; in other words, the
Δ
H f o of each reactant must be multiplied
by its coefficient in the balanced equation.
Step 4: Find the difference between the sum of all of the enthalpies of products and the sum
of all of the enthalpies of reactants.
CSCOPE Unit 12 Lesson 02 Day 4

2
Example:
Calculate the enthalpy change for the following equation:
4 NH
3
(g) + 5 O
2
(g)

4 NO (g) + 6 H
2
O (g)
Step 1
The equation is already balanced.
Step 2
Using a table of standard enthalpies of formation (page 7), find the sum of the
Δ H f o
of all of the products:

Look up the Δ H f o of each product.

Determine the coefficient of each product from the balanced equation and multiply the Δ H f o by the coefficient.

Add all of the results of the products together:
Substance
Δ H o f from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (products)
Step 3
Using a table of standard enthalpies of formation (page 7), find the sum of the
Δ H f o of all of the reactants:

Look up the Δ H f o of each reactant.

Determine the coefficient of each reactant from the balanced equation and multiply the Δ H f o by the coefficient.

Add all of the results of the reactants together:
Substance
Δ H f o from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (reactants)
Step 4: Find the difference between the sum of all of the enthalpies of products and the sum
of all of the enthalpies of reactants.
CSCOPE Unit 12 Lesson 02 Day 4

Exercises
1) Calculate the enthalpy change for the following equation:
C (s) + O
2
(g)

CO
2
(g)
Step 1
This equation is already balanced.
Step 2
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the products:
Substance
CO
2
(g)
Δ H f o from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (products)
Step 3
Using a table of standard enthalpies of formation, find the sum of the
Δ H of all of the reactants:
Substance
C
O
2
(s)
(g)
Δ H o f from table
Hint: This is an element in its standard state.
Hint: This is an element in its standard state.
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o of (reactants)
Step 4: Find the difference between the sum of all of the enthalpies of products and
the sum of all of the enthalpies of reactants.
Δ H rxn
= Δ H f o
(of products) – Δ H f o
(of reactants)
3
CSCOPE Unit 12 Lesson 02 Day 4

2) Calculate the enthalpy change for the following equation:
CH
4
(g) + NH
3
(g)

HCN (g) + H
2
(g)
Step 1
This equation will need to be balanced.
Step 2
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the products:
Substance
HCN (g)
H
2
(g)
Δ H f o from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (products)
Step 3
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the reactants:
Substance
CH
NH
4
3
(g)
(g)
Δ H o f from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (reactants)
Step 4: Find the difference between the sum of all of the enthalpies of products and
the sum of all of the enthalpies of reactants.
Δ H rxn
= Δ H f o
(of products) – Δ H f o
(of reactants)
4
CSCOPE Unit 12 Lesson 02 Day 4

3) Calculate the enthalpy change for the following equation:
CaCO
3
(s)

CaO (s) + CO
2
(g)
Step 1
Step 2
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the products:
Substance
Δ H f o from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (products)
Step 3
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the reactants:
Substance
Δ H o f from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (reactants)
Step 4: Find the difference between the sum of all of the enthalpies of products and
the sum of all of the enthalpies of reactants.
5
CSCOPE Unit 12 Lesson 02 Day 4

4) Calculate the enthalpy change for the following equation:
C
2
H
4
(g ) + F
2
(g)

CF
4
(g) + HF (g)
Step 1
Step 2
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the products:
Substance
Δ H f o from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o
of (products)
Step 3
Using a table of standard enthalpies of formation, find the sum of the
Δ H f o
of all of the reactants:
Substance
Δ H o f from table
Coefficient from balanced equation
Coefficient times
Δ H f o
SUM:
Δ H f o of (reactants)
Step 4: Find the difference between the sum of all of the enthalpies of products and
the sum of all of the enthalpies of reactants.
6
CSCOPE Unit 12 Lesson 02 Day 4

7
The
Standard Heats of Formation
Δ H f o for any element in its standard state is defined to be exactly zero.
Name
CaCO
3
(s)
CaO (s)
Ca(OH)
2
(s)
CCl
4
(g)
CCl
4
(l)
CF
4
(g)
CH
4
CH
3
OH ( l )
C
2
H
2
( g )
C
2
H
4
( g )
C
2
H
5
OH ( l )
C
3
H
8
( g )
CO
2
(g)
FeCO
3
(s)
Fe
2
O
3
(s)
H (g)
H
2
O (g)
H
2
O (l)
H
2
O
2
(l)
H
2
S (g)
HCl (g)
HCN (g)
HF (g)
KCl (s)
KClO
3
(s)
N
2
O (g)
NaCl (s)
Na
2
CO
3
(s)
NaOH (s)
NH
3
(g)
NH
4
NO
3
(s)
NO (g)
NO
2
(g)
PCl
3
(g)
PCl
5
(g)
SiO
2
(s)
SO
2
(g)
SO
3
(g)

H
 f
(kJ/mol)
– 1207
.
1
– 635
.
5
– 986.6
– 100
.
4
– 132 .
6
– 933
– 74
.
9
– 238
.
6
– 226 .
7
– 52
.
3
– 277
.
0
–104
.
0
– 393
.
3
– 740
.
6
– 824
.
2
+ 218
– 241
.
8
– 285 .
8
– 187
.
4
– 20 .
1
– 92
.
5
+ 130 .
5
– 268
.
6
– 436
– 391 .
2
+ 81 .
6
– 410 .
9
– 1130
.
9
– 430 .
5
– 45
.
6
– 365
.
3
+ 90 .
4
+ 33 .
9
– 278
.
7
– 371
.
1
– 859 .
4
– 296
.
2
– 395 .
4
CSCOPE Unit 12 Lesson 02 Day 4