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Homework answers / question archive / This experiment examines a law of chemistry known as Hess’s Law

This experiment examines a law of chemistry known as Hess’s Law. Hess’s Law states that a reaction may be done in parts, and the sum of the enthalpies of those parts is equal to the enthalpy of the reaction overall. Parts 2 and 3 of this experiment are Part 4 broken down into two steps. Therefore the enthalpies of parts 2 and 3 should sum to the enthalpy of part 4. Please fill in the data tables from the video data. Instructions for calculating the values may be found after each table. Part 2: Enthalpy of dissolution NaOH(s) trial (A) NH4NO3 (s) trial (B) a. Mass of substance dissolved (g) b. Mass of calorimeter (g) c. Mass of calorimeter + water (g) d. Mass of water (g) e. Total mass of reactants (g) f. Initial temperature of water (°C) g. Final temperature of solution (°C) h. Change in the temperature, ΔT (°C) i. Moles of solid dissolved in water (mols) j. Heat lost or gained by the water, Δqm(J) k. Heat absorbed by the calorimeter, Δqc(J) l. Heat absorbed or released by the dissolving solute Δqt =m.Δq + Δq (J)of solution, ΔHrxn (kJ/mol) Enthalpy n. Theoretical enthalpy, calculated ΔHrxn(kJ/mol) o. Percent error To calculate moles of solute (i): mass of substance/molar mass of substance To calculate total mass of reactants (e): mass of water + mass of substance dissolved To calculate heat lost or gained by water (j): total mass of reactants (e) x the specific heat of water (4.184 J/g°C) x ΔT (h) To calculate the heat gained or lost by the calorimeter (k): Ccal (6.410 J/°C) x ΔT (h) To calculate the total heat gained or lost by the reaction (l): Δqm(j)+ Δqc (k) To calculate the enthalpy of solution (m): - Δqt (l)/moles of salt dissolved (i) convert to kilojoules/mole! note: don’t forget to To find the theoretical enthalpy for NaOH dissolution follow these steps: Formula equation: NaOH(s) + H2O(l)→ Na+(aq) + OH-(aq) + H2O(l) Write the net ionic equation: Using the net ionic equation, find the values of heat of formation (ΔH°f) and plug them into this formula: Theoretical ΔHrxn = ΣΔH°f products - = ΣΔH°freactants Note: Σ means “sum” This reaction is ____________thermic. To find the theoretical enthalpy for NH4NO3 dissolution follow these steps: Formula equation: NH4NO3 (s) + H2O(l)→ NH4+(aq) + NO3-(aq) + H2O(l) Write the net ionic equation: Using the net ionic equation, find the values of heat of formation (ΔH°f) and plug them into this formula: Theoretical ΔHrxn = ΣΔH°f products - = ΣΔH°freactants This reaction is ____________thermic. Note: Σ means “sum” Part 3: Enthalpy of Neutralization a. Formula of the acid HCl(aq) b. Formula of the base NaOH(aq) c. Mass of empty calorimeter A (g) d. Mass of calorimeter + acid (g) e. Mass of acid solution (g) f. Mass of calorimeter B (g) g. Mass of calorimeter + base (g) h. Mass of base solution (g) i. Total mass of reactants (g) j. Initial temperature of the acid (°C) k. Initial temperature of the base (°C) l. Average initial temperature (°C) m. Final temperature of mixture (°C) n. Change in the temperature of the mixture, ΔT (°C) o. Total mass of solution (g) p. Change in heat of solution, Δqm(J) q. Change in heat of the calorimeter, Δqc(J) r. Heat produced by the reaction, Δq = Δq of + Δq (J) produced (mols) s. Moles water t. Enthalpy of neutralization, ΔHrxn (kJ/mol) u. Theoretical enthalpy, calculated ΔHrxn(kJ/mol) v. Percent error To calculate the average initial temperature (l): (Initial temp of acid (j) + initial temp of base (k))/2 To calculate the change in temperature (n): final temp (m) –average initial temp (l) To calculate the change in heat of the solution (p): total mass of solution (o) x specific heat of water (4.184 J/g°C) x ΔT (n) To calculate the change in heat of the calorimeter (q): Ccal (6.410J/°C) x ΔT (n) To calculate the total heat gained or lost by the reaction (r): Δqm(p)+ Δqc (q) To calculate the number of moles of water produced (s): volume of acid solution in liters x molarity of acid solution (watch the video for these values!) To calculate the enthalpy of neutralization (t): - Δqt(r)/number of moles of water produced (s) Note: don’t forget to convert to kJ/mol! To find the theoretical enthalpy for neutralization follow these steps: Formula equation: NaOH(aq) + HCl(aq)→ Na+(aq) + Cl-(aq) + H2O(l) Write the complete ionic equation: Write the net ionic equation: Using the net ionic equation, find the values of heat of formation (ΔH°f) and plug them into this formula: Theoretical ΔHrxn = ΣΔH°f products - = ΣΔH°freactants Note: Σ means “sum” Part 4: Hess’s Law a. Mass of NaOH dissolved (g) b. Mass of calorimeter (g) c. Mass of calorimeter + HCl solution (g) d. Mass of HCl solution (g) e. Total mass of reactants (g) f. Initial temperature of solution (°C) g. Final temperature of solution (°C) h. Change in the temperature, ΔT (°C) i. Moles of NaOH dissolved in water (mols) j. Heat lost or gained by the solution, Δqm(J) k. Heat absorbed by the calorimeter, Δqc(J) l. Heat absorbed or released by the reaction Δqt = Δqm+ Δqc (J) m. Enthalpy of neutralization, ΔHrxn (kJ/mol) n. Theoretical enthalpy, calculated ΔHrxn(kJ/mol) o. Percent error To calculate the number of moles of NaOH dissolved (i): mass of NaOH (a)/molar mass of NaOH To calculate total mass of reactants (e): mass of HCl solution (d) + mass of NaOH (a) To calculate the heat lost or gained by the solution (j): total mass of reactants (e) x specific heat of water (4. 184 J/g°C) x ΔT (h) To calculate the heat absorbed by the calorimeter (k): Ccal (6.410J/°C) x ΔT (h) To calculate the total heat absorbed or released by the reaction (l): Δqm(j)+ Δqc (k) To calculate the enthalpy of neutralization (m): - Δqt(l)/moles of NaOH (i) Note: don’t forget to convert to kJ/mol! To find the theoretical enthalpy for neutralization follow these steps: Formula equation: NaOH(s) + HCl(aq)→ Na+(aq) + Cl-(aq) + H2O(l) Write the complete ionic equation: Write the net ionic equation: Using the net ionic equation, find the values of heat of formation (ΔH°f) and plug them into this formula: Theoretical ΔHrxn = ΣΔH°f products - = ΣΔH°freactants Note: Σ means “sum” Processing your data: Confirmation of Hess’s Law Write the three net ionic equations (use the NaOH trial of part 2) from the three experiments: Add two of the reactions together to produce the third: Note: Things on both sides of the reaction cancel! Add your experimentally determined enthalpies in the same way as above: Explain how the sum of your experimental enthalpies is consistent with Hess’s Law:

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