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Enthalpy (Heat of Reaction)

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The total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead. Enthalpy change is defined by the following equation:

 

          or products minus reactants

ΔH  is the enthalpy change

H_final is the final enthalpy of the system, measured in joules. In a chemical reaction, H_final is the enthalpy of the products.

H_initial is the initial enthalpy of the system, measured in joules. In a chemical reaction, H_initial is the enthalpy of the reactants.

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.

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.

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. NH_3(g)

B.CO_2(g)

C. H₂O_(g)

D. NO_2(g)

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

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

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

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 CO_2(g) is formed from its elements?

A. 113.2kJ

B. 2,830 kJ

C. 7,870kJ

D. 78.7kJ

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 Al₂O_3(s) is decomposed its elements?

A. 838 kJ

B. 3,351 kJ

C. 1,676 kJ

D. 13,404 kJ

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