KEY
Questions and Problems for 9.5: Energy in Chemical Reactions
9.5 Energy in Chemical Reactions (Read pgs. 269 - 273 in the chemistry textbook)
1. Define each of the following in terms of a chemical reaction:
(a) system
the reactants and products in a chemical reaction
(b) surroundings
all the things that contain and/or interact with the system
(c) heat of reaction
generic for the amount of heat absorbed or released during a chemical
reaction
(d) enthalpy of reaction (ΔHrxn)
 another word for heat of reaction
 determined by the difference in energy of the products
and the reactants
 must occur AT A CONSTANT PRESSURE (like the
pressure of the Earth's atmosphere)
(e) endothermic and how it relates to ΔHrxn (change of enthalpy of reaction)
 ΔHrxn = (+)
reactants → products
ΔHrxn = (+)
(f) exothermic and how it relates to ΔHrxn (change of enthalpy of reaction)
 ΔHrxn = (−)
reactants → products
ΔHrxn = (−)
(g) thermochemical equation
an equation with ΔHrxn written directly after
Examples:
Endothermic: CO2(g) → CO(g) + O2(g)
ΔHrxn = + 570 kJ
Exothermic: CO(g) + O2(g) → CO2(g)
ΔHrxn = − 570 kJ*
*Note: if you "flip" the equation you "flip" the sign of ΔHrxn
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2. Determine if each of the following reactions is endothermic or exothermic:
3.
(a) 2HgO(s) → 2 Hg(l) + O2(g)
ΔHRXN = +185 kJ
Endothermic
(b) N2(g) + 3H2(g) → 2NH3(g)
ΔHRXN = −92.2 kJ
Exothermic
(c) CO2(g) + 2H2O(l) → CH4(g) + 2 O2(g)
ΔHRXN = + 890 kJ
Endothermic
(d) CH4(g) + 2 O2(g) → CO2(g) + 2H2O(l)
ΔHRXN = −890 kJ
Exothermic
(a) If ΔHrxn is (−) energy was released and can be considered a product of the chemical reaction.
N2(g) + 3H2(g) → 2NH3(g) + 92.2 kJ
(b) If ΔHrxn is (+) energy was absorbed and can be considered a product of the chemical reaction.
CO2(g) + 2H2O(l) + 890 kJ → CH4(g) + 2 O2(g)
(c) Using the position of the energy term, determine if each of the following reactions is endothermic
or exothermic
(i) C(s) + O2(g) → CO2(gl) + 393.5 kJ
Exothermic
(ii) O2(g) + 495.0 kJ → 2 O(g)
Endothermic
(iii) 1299.6 kJ + 2CO2(g) + H2O(l) → C2H2(g) + 5/2 O2(g)
Endothermic
(iv) CuO(s) + H2(g) → 129.7 kJ + Cu(s) + H2O(l)
Exothermic
(d) Insert the energy term as either a product or reactant based on the given ΔHrxn
(i) CH4 (g) + 2 O2(g)
(ii) 2 C(s) + H2(g)
(iii) NO(g) + O3 (g)
+ 226.8 kJ
→ CO2(gl) + 2 H2O(l)
+ 890 kJ*
→ C2H2(g)
→ NO2(g) + O2(g)
ΔHrxn = −890 kJ
ΔHrxn = +226 kJ
+ 199 kJ
ΔHrxn = −199 kJ
+ 91 kJ
→ CO(g) + 2 H2(g)
ΔHrxn = + 91 kJ
*NOTE: there is no (+) or (−) sign associated with the energy when it is
inserted into the chemical equation.
(iv) CH3OH(g)
2
4. When it comes to determining ΔHrxn - REMEMBER:
(a) REACTANT BOND BREAKING:
endothermic, ΔHbonds broken = (+)
(b) PRODUCT BOND MAKING:
exothermic, ΔHbonds made = (−)
5. What is the equation to determine ΔHrxn from ΔHreactants and ΔHproducts?
ΔHrxn = ΔHbonds broken − ΔHbonds made
6. The reaction of 1 mole of glucose C6H12O6(s) with oxygen gas to produce carbon dioxide gas and liquid
water is a combustion reaction and the energy of the products is LESS THAN the energy of the reactants by
≈ 2804 kJ
(a) Write a balanced chemical equation for this combustion reaction.
C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(l)
(b) It takes a total of 1273 kJ of energy to break the bonds of glucose and oxygen in this reaction. At the
same time, 4076 kJ of energy are released by the formation of carbon dioxide and water. Is this reaction
exothermic or endothermic? Explain
C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(l)
energy in = break bonds
1273 kJ
energy out = make bonds
4076 kJ
exothermic, more energy out than in
(c) Calculate ΔHrxn in kilojoules for the combustion of glucose.
1273 kJ − 4073 kJ = − 2803 kJ =ΔHRXN
3
7. Methanol (CH3OH), which is used as a cooking fuel, undergoes combustion reaction.
(a) Write a balanced chemical equation for this reaction.
2 CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(g)
(b) 402.4 kJ of energy are required to combust the methanol. The formation of carbon dioxide and
water releases 1932.3 kJ of energy. Calculate ΔHRXN for this reaction.
2 CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(g)
energy in = break bonds
402 kJ
energy out = make bonds
1930 kJ
402 kJ − 1930 kJ = −1528 kJ = ΔHRXN
exothermic, b/c ΔHrxn = (−)
(c) is this reaction exothermic or endothermic? Explain
8. In water solid sodium hydrogen carbonate will decompose into aqueous sodium hydroxide and carbon
dioxide gas.
(a) Write a balanced chemical equation for this decomposition reaction.
NaHCO3 (s) → NaOH(aq) + CO2(g)
(b) 948 kJ are required to break the bonds of the sodium hydrogen carbonate. The formation of the
aqueous sodium hydroxide and the carbon dioxide releases 863 kJ of energy. Calculate ΔHRXN
for this reaction.
NaHCO3 (s) → NaOH(aq) + CO2(g)
energy in = break bonds
948 kJ
energy out = make bonds
863 kJ
948 kJ − 863 kJ = + 85 kJ = ΔHRXN
(c) is this reaction exothermic or endothermic?
Explain
endothermic, b/c ΔHrxn = (+)
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9. Answer the following:
(a) In an exothermic reaction, is the energy of the products (Hproducts) less or greater than that of the
reactants(Hreactants)? Explain.
less than - energy absorbed < energy released - the products
are at a LOWER energy than were the reactants
(b) In an endothermic reaction, is the energy of the products less or greater than that of the reactants?
Explain.
greater than - energy absorbed > energy released - the
products are at a HIGHER energy than were
the reactants
10. When 2 moles of liquid water decomposes into its constituent elements, the energy of the products is
572 kJ more than the reactants.
(a) Is this reaction exothermic or endothermic? Explain.
Endothermic, b/c energy absorbed > energy released
(b) Write the balanced thermochemical equation for this decomposition reaction.
2 H2O(l) → O2(g) + 2 H2(g)
ΔHrxn = + 575 kJ
11. When nitrogen gas and oxygen gas form nitrogen monoxide gas the energy of the products is 90.2 kJ
less than the energy of the reactants.
(a) Is this reaction exothermic or endothermic? Explain.
Exothermic, b/c energy absorbed < energy released
(b) Write the balanced thermochemical equation for this synthesis reaction.
N2(g) + O2(g) → 2 NO(g)
ΔHrxn = − 90.2 kJ
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12. When solid silicon and chlorine gas react to form gaseous silicon tetrachloride and the energy of the
products is 657 kJ less than the energy of the reactants.
(a) Is this reaction exothermic or endothermic? Explain.
Exothermic, b/c energy absorbed < energy released
(a) Write the balanced thermochemical equation for this synthesis reaction.
Si(s) + 2 Cl2(g) → SiCl4(g)
ΔHrxn = −657 kJ
13. Classify the following as exothermic or endothermic reactions:
Exothermic
(a) 550 kJ of heat is released
Endothermic
(b) The energy level of the products is higher than that of the reactants.
Exothermic
(c) The metabolism of glucose provides energy for the body.
Exothermic
(d) The energy level of the products is lower than that of the reactants.
Endothermic
(e) In the body, the synthesis of proteins requires energy.
Endothermic
(f) 125 kJ of energy is absorbed.
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STOICHIOMETRY w/ ΔHrxn
steps:
1. determine the ratio moles: ΔHrxn for the given chemical by using the
coefficients of the balanced chemical equation
2. determine the molar mass of given chemical
3. calculate the moles of given chemical (if necessary)
4. calculate ΔHrxn for moles of given chemical
1. The thermochemical equation for the formation of ammonia from hydrogen and nitrogen is shown below.
Calculate the heat, in kilojoules, released when 50.0 g of ammonia forms.
N2(g) + 3 H2(g) → 2 NH3(g)
ΔHrxn = − 92.2 kJ
(i) determine the ratio of moles ammonia to ΔHrxn
2 moles NH3 : − 92.2 kJ
so:
− 𝟗𝟐.𝟐 𝐤𝐉
𝟐 𝐦𝐨𝐥𝐞𝐬 𝐍𝐇𝟑
or
𝟐 𝐦𝐨𝐥𝐞𝐬 𝐍𝐇𝟑
− 𝟗𝟐.𝟐 𝐤𝐉
𝒈
(ii) determine the molar mass of ammonia: 17.04 𝒎𝒐𝒍𝒆𝒔
(iii) calculate the moles of ammonia:
𝒈
50.0 g ÷ 17.04 𝒎𝒐𝒍𝒆𝒔 = 2.93 mole
(iv) calculate ΔHrxn for 2.93 moles NH3
2.93 moles NH3 ×
− 𝟗𝟐.𝟐 𝐤𝐉
𝟐 𝐦𝐨𝐥𝐞𝐬 𝐍𝐇𝟑
=
−135 kJ
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2. Mercury (II) oxide decomposed to mercury and oxygen as shown below:
2 HgO(s) → 2 Hg(l) + O2(g)
(a) Is this reaction exothermic or endothermic?
ΔHrxn = 182 kJ
endothermic
(b) How many kJ are needed if 25.0 g of mercury (II) oxide react?
(i) determine the ratio of moles HgO to ΔHrxn
2 moles HgO : 182 kJ
so:
𝟏𝟖𝟐 𝐤𝐉
𝟐 𝐦𝐨𝐥𝐞𝐬 𝐇𝐠𝐎
or
𝟐 𝐦𝐨𝐥𝐞𝐬 𝐇𝐠𝐎
𝟏𝟖𝟐 𝐤𝐉
𝒈
(ii) calculate the molar mass of HgO: 216.59 𝒎𝒐𝒍𝒆𝒔
(iii) calculate the moles of HgO:
𝒈
25.0 g ÷ 216.59 𝒎𝒐𝒍𝒆𝒔 = 0.115 mole
(iv) calculate ΔHrxn for 0.115 moles HgO
0.115 moles HgO ×
𝟏𝟖𝟐 𝐤𝐉
𝟐 𝐦𝐨𝐥𝐞𝐬 𝐇𝐠𝐎
=
10.5 kJ
8
3. Sodium and chloride react to form sodium chloride as shown below:
2 Na(s) + Cl2(g) → 2 NaCl(s)
(a) Is this reaction exothermic or endothermic?
ΔHrxn = − 819 kJ
exothermic
(b) How many kJ are needed if 15.0 g of chlorine gas react?
(i) determine the ratio of moles Cl2 to ΔHrxn
1 moles Cl2 : −819 kJ
so:
− 𝟖𝟏𝟗 𝐤𝐉
𝟏 𝐦𝐨𝐥𝐞𝐬 𝐂𝐥𝟐
or
𝟏 𝐦𝐨𝐥𝐞𝐬 𝐂𝐥𝟐
− 𝟖𝟏𝟗 𝐤𝐉
(ii) calculate the molar mass of Cl2:
70.90
𝒈
𝒎𝒐𝒍𝒆𝒔
(iii) calculate the moles of Cl2:
𝒈
15.0 g ÷ 70.90 𝒎𝒐𝒍𝒆𝒔 = 0.212 mole
(iv) calculate ΔHrxn for 0.212 moles Cl2
0.212moles Cl2 ×
− 𝟖𝟏𝟗 𝐤𝐉
𝟏 𝐦𝐨𝐥𝐞𝐬 𝐂𝐥𝟐
=
− 174 kJ
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4. Propane combusts as shown below:
2 C3H8 (g) + 5 O2(g) → 2 CO2(g) + 4 H2O(l)
(a) Is this reaction exothermic or endothermic?
ΔHrxn = − 2220 kJ
exothermic
(b) How many kJ are needed if 75.0 g of water are formed?
(i) determine the ratio of moles water to ΔHrxn
4 moles H2O : −2220 kJ
so:
− 𝟐𝟐𝟐𝟎 𝐤𝐉
𝟒 𝐦𝐨𝐥𝐞𝐬 𝐇𝟐 𝐎
or
𝟒 𝐦𝐨𝐥𝐞𝐬 𝐇𝟐 𝐎
− 𝟐𝟐𝟐𝟎 𝐤𝐉
(ii) calculate the molar mass of H2O: 18.02
𝒈
𝒎𝒐𝒍𝒆𝒔
(iii) calculate the moles of H2O:
𝒈
75.0 g ÷ 18.02 𝒎𝒐𝒍𝒆𝒔 = 4.16 mole
(iv) calculate ΔHrxn for 4.16 moles H2O
4.16 moles H2O ×
− 𝟐𝟐𝟐𝟎 𝐤𝐉
𝟒 𝐦𝐨𝐥𝐞𝐬 𝐇𝟐 𝐎
=
− 2308 kJ
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