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Enthalpy (Heat of Reaction)

 

EXOTHERMIC REACTIONS- release energy and therefore ΔH  is negative. Heat is released by the system due to the reactants of the reaction having a greater enthalpy than the products. The energy term will be included in the reaction on the product side.

CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l)                ΔH=-890.4kJ

can also be written as  

CH4(g) + 2O2(g) ==> CO2(g)+ 2H2O(l) + 890.4kJ

 

ENDOTHERMIC REACTIONS- absorb energy and therefore ΔH  is positive. Heat is absorbed by the system due to the products of the reaction having a greater enthalpy than the reactants. The energy term will be include in the reaction on the reactants side.

CO2(g) + 2H2O(l)--> CH4(g) + 2O2(g)            ΔH=+890.4kJ

can also be written as  

890.4kJ  + CO2(g) + 2H2O(l)--> CH4(g) + 2O2(g)  

 

To determine energy from a certain number of moles of a substance in a certain reaction, you must look at the coefficients and the energy either produced or absorbed.

CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l) + 890.4kJ

When 1 mole of CO2(g) is produced 890.4kJ is released.

When 2 mole of H2O(l) is produced 890.4kJ is released.

So if asked about 1 mole of H2O(l) half of 890.4kJ is released. So 445.2kJ is produced.

Stoichiometry and Enthalpy Calculations

 

Example 1 Given the decomposition of sodium hydrogen sulfate reaction:

2 NaHSO4(s) ----> Na2SO4(s) + H2O(g) + SO3(g)

DH = -231.3 kJ

If 3.60 grams of NaHSO4(s) reacts, how much heat is released?

Step 1--add the heat to the reaction 

(DH =  -  = exothermic; energy is a product)

2 NaHSO4(s) ----> Na2SO4(s) + H2O(g) + SO3(g) +231.3kJ

Step 2- find the moles

3.60g NaHSO4 X

1 mole

=0.0300 mol NaHSO4

120.1g

Step 3-Set up a proportion using the coefficients and heat to determine the energy

0.0300 mol =  x kJ
2 mol NaHSO4 231.3kJ

X=3.47 kJ

 

Example 2 Given the reaction of N2H4 and O2:

 N2H4(l) + 3 O2(g)----> 2 H2O(g) + 2NO2(g)

DH = -400. kJ

If 9.20 grams of NO2(g) is produced, how much heat is released?

Step 1--add the heat to the reaction 

(DH =  -  = exothermic; energy is a product)

N2H4(l) + 3 O2(g)----> 2 H2O(g) + 2NO2(g) +400kJ

Step 2- find the moles

9.20g NO2 X

1 mole

=0.200 mol NO2

46.0g

Step 3-Set up a proportion using the coefficients and heat to determine the energy

0.200 mol =  x kJ
2 mol NO2 400kJ

X=40 kJ

 

A few notes about this table:

If any of these reaction are reverse the sign of ΔH must also be reversed (positive becomes negative). Endothermic reaction reversed is exothermic. The Law of Conservation of energy must be followed.

Example

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

ΔH=-890.4kJ

so when reversed

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

ΔH=+890.4kJ

If the reaction is to be written with energy included it would look like this:

CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l) + 890.4kJ

or reversed

890.4kJ  + CO2(g) + 2H2O(l)--> CH4(g) + 2O2(g)

NOTE****ΔH can be + or -, but if the energy is included in the reaction the + or - tell us if energy is a product or reactant. Include it as if it were another substance, DO NOT indicate a + or -.

To determine energy from a certain number of moles of a substance in a certain reaction, you must look at the coefficients and the energy either produced or absorbed.

CH4(g) + 2O2(g) --> CO2(g) + 2H2O(l) + 890.4kJ

When 1 mole of CO2(g) is produced 890.4kJ is released.

When 2 mole of H2O(l) is produced 890.4kJ is released.

So if asked about 1 mole of H2O(l) half of 890.4kJ is released. So 445.2kJ is produced.

Regents Questions-highlight to reveal answers.

1. According to Reference Table I, which compounds form exothermically?

A.Hydrogen Fluoride

B. Hydrogen Iodide

C.Ethene

D. Ethyne

Answer-->

Choice A,Exothermic reactions have a negative H. Choices B, C, D have positive a ΔH

2. Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, the most energy would be released when oxygen reacts completely with 1.0 mole of

A. C6H12O6

B.CH4

C. CO

D. CH3OH

Answer-->-->

Choice A,ΔH =-2804

3. Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, the decompostion of which compound would be exothermic?

A. NH3(g)

B.CO2(g)

C. H2O(g)

D. NO2(g)

Answer-->

Choice D,Exothermic reactions have a negative H. Choices A, B, C have positive a ΔH

4.Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, which compound would form exothermic?

A. C2H4

B.C2H6

C. NO

D. NO2

Answer-->

Choice B,Exothermic reactions have a negative H. Choices A, C, D have positive a ΔH

5.Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, the dissolving of which salt is accompanied by the release of energy?

A. LiBr

B. NH4Cl

C. NaCl

D. KNO3

Answer-->

Choice A, releasing energy is exothermic and they have a negative ΔH . Choices B, C, D have positive a ΔH

6.Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, the greatest amount of energy would be given off by the complete oxidation of 1 mole of

A. C3H8

B.CH4

C. C6H12O6

D. C2H5OH

Answer-->

Choice C, ΔH =-2804

 

7. Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, how many kilojoules of heat are given off when 0.200 mole of CO2(g) is formed from its elements?

A. 113.2kJ

B. 2,830 kJ

C. 7,870kJ

D. 78.7kJ

Answer-->

Choice D, ΔH =-393.5kJ/mol *0.200mol=-78.7kJ. - indicated exothermic or given off.

7. Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, how many kilojoules of heat are absorbed when 0.500 mole of Al2O3(s) is decomposed its elements?

A. 838 kJ

B. 3,351 kJ

C. 1,676 kJ

D. 13,404 kJ

Answer-->

Choice A, ΔH =+3351kJ/2mol Al2O3*0.500molAl2O3=+838kJ.+ indicated endothermic or absorbed.

 

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