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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
Step 3-Set up a proportion using the coefficients and heat to determine the energy
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
Step 3-Set up a proportion using the coefficients and heat to determine the energy
X=40 kJ
Regents Questions-highlight to reveal answers. 1. According to Reference Table I, which compounds form exothermically?
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
3. Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, the decompostion of which compound would be exothermic?
4.Based on Heat of Reactions at 101.3 kPa and 298 K chemistry reference table, which compound would form exothermic?
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?
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
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?
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?
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