Hess Law Lab DESIGN Aspect 1:Problem: What is the molar enthalpy of formation of magnesium oxide? Variables: Manipulated: None Responding: None Controlled: Isolation of calorimeter, concentrations of substances involved. Aspect 2:Background Information: Assumptions: Specific heat capacity of water, we assume that the acid has the same qualities as water including heat capacity, and we assume the enthalpy of formation for magnesium oxide from the data booklet for theoretical value.
Hess’s Law: Hess’ law states that the energy change for any chemical or physical process is independent of the pathway or number of steps required to complete the process provided that the final and initial reaction conditions are the same. In other words, an energy change is path independent, only the initial and final states being of importance. Waste disposal and safety: Corrosive substance which causes severe but delayed burns. DO NOT INGEST. Avoid Inhalation. Dispose as instructed by the lab technician or the teacher. Aspect 3:Apparatus Diagram: Figure from Inquiry into chemistry textbook pg 352
Materials: •1. 00 mol/L HCl(aq) •MgO(s) powder •Magnesium ribbon •Simple calorimeter •100 mL graduated cylinder •Scoopula •Electric balance •Thermometer •Sand paper Procedure: 1. Mg(s)+ O2(g) MgO(s) 2. MgO(s)+2HCl(aq) MgCl2(aq)+H2O(l) 3. Mg(s)+2HCl(aq) MgCl2(aq)+H2(g) 4. H2(g)+ O2(g) H2O(l) + 285. 8kJ Part 1: Determining ? H of Reaction (2) 1. Set up the simple calorimeter refer to figure 9. 16 above. Using a graduated cylinder, add 100 mL of 1. 00mol/L HCl(aq) to the calorimeter. 2. Record the initial temperature of the HCl(aq) to the nearest tenth of a degree. 3.
Find the mass of no more than 0. 80 grams of MgO(s) powder. Record exact mass. 4. Add the MgO(s) powder to the calorimeter containing the HCl(aq) swirl the solution gently, recording the highest temperature reached. 5. Dispose as directed by teacher. Part 2: Determining ? H of reaction (3) 1. Using a graduated cylinder, add 100mL of 1. 00mol/L HCl to the calorimeter. 2. Record initial temp. of hydrochloric acid to the nearest tenth of a degree. 3. If you are using magnesium ribbon, sand ribbon to desired mass. Determine mass no more than 0. 50 grams of magnesium.
Record the exact mass. 4. Add the Mg(s) to the calorimeter containing the HCl(aq). Swirl the solution gently, recording the highest temperature, tf , reached. 5. Dispose as directed. DATA COLLECTION & PROCESSING (DCP) Aspect 1:Recording Raw Data: Temperature of hydrochloric acid and mass of Magnesium oxide Mass of Magnesium oxide (g)Initial Temperature (°C)Final Temperature (°C) 0. 7523. 5029. 50 Temperature of hydrochloric acid and mass of magnesium Mass of magnesium (g)Initial Temperature (°C)Final Temperature (°C) 0. 4824. 0045. 00 Aspect 2:Processing Raw Data:
Enthalpy change of Magnesium oxide MgO(s)+2HCl(aq) MgCl2(aq)+H2O(l) Q=mc? t ? t= 29. 50°C-23. 50°C? t = 6 °C Q= (0. 00075kg) x x 6°C Q= 0. 018855kJ/mol MgO Enthalpy change of magnesium reaction Mg(s) +2HCl(aq) MgCl2(aq)+H2(g) Q=mc? t ? t=45. 00°C-24. 00°C? t= 21°C Q= (0. 00048kg) x x 21°C Q= 0. 0422352kJ/mol Mg ?HH2O = -285. 8 kJ/mol (given in data booklet) ?Hrxn = 0. 018855kJ/mol MgO + 0. 0422352kJ/mol Mg + 285. 8kJ/mol H2O ?Hrxn = – [285. 86109] kJ/mol MgO ? H system = -? H surroundings %error= x 100% %error= %error= -52. 48%