Hess\'s law lab report PDF

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Hess’s Law Lab Report Completed by -Partners: -On October 24th, 2019

Abstract: This lab was conducted to determine the enthalpy for the combustion of magnesium metal using Hess’s Law. The temperature of Hydrochloric acid was taken, and then taken once again after reacting with magnesium. The same was done for magnesium oxide, and this had given the data needed to calculate enthalpy change, It had been concluded that the enthalpy for the combustion of magnesium was -477.9kj/mol, which is an exothermic reaction. Introduction:

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The lab was conducted to determine the enthalpy for the combustion of magnesium using Hess’s Law. Enthalpy is measured by calorimetry, which is, as defined by Libretexts, “the process of measuring the amount of heat released or absorbed during a chemical reaction.” (Libretexts, "Calorimetry", 2019). The lab used Hess’s Law to determine the enthalpy for the combustion of magnesium metal. Libretexts states, “Hess’s law states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.” (Libretexts, "Hess's Law", 2019); Which determines the enthalpy change value. Hess’s Law is written as ΔH target= ΔH1 + ΔH2 + ΔH3… or ΔHtarget= Σ ΔH known. The enthalpy change determines whether a reaction releases heat, which is exothermic, or absorbs heat, which is endothermic. Calorimetry is the process of measuring such, and both reactions in the lab had an increase in temperature, making the reactions both exothermic. That was found using calorimetry, and then related to Hess’s law, where when adding the enthalpies for each step, the net enthalpy will predict whether a reaction will be endothermic or exothermic. Liquid water has a standard enthalpy of formation of 286KJ/mol, and so that is the energy needed to produce one mole of water in liquid form from hydrogen gas and oxygen gas. Experimental: Using a graduated cylinder, 100.0 mL of 1.0 mol/L hydrochloric acid was measured then put into a calorimeter. A thermometer was used to measure the initial temperature of the hydrochloric acid, and then 0.500g of magnesium metal ribbon was added to the calorimeter after being rubbed with steel wool. The maximum temperature was recorded after mixing, and then this process was repeated for the second reaction, but with 1.00g of solid magnesium oxide. Materials used throughout the lab were, steel wool, 1.0mol/L hydrochloric acid, solid magnesium metal ribbon, and solid magnesium oxide.Apparatus’ used throughout the lab were, a thermometer, a coffee cup calorimeter, and a graduated cylinder.

Results and Discussions: Table 1: Temperature of Magnesium And Magnesium Oxide reacting separately with Hydrochloric acid Mg(s) + HCl(aq)

MgO(s) + HCl(aq)

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Initial Temperature

20.2°C

Final Temperature 33.3°C

Initial Temperature

21.0°C

Final Temperature 25.1°C

ΔT

13.1°C

ΔT

4.10°C

Mass of Mg(s)

0.510g

Mass of MgO(s)

1.00g

a.) 100mL=100g (assuming same density and build as water) q=MCΔT Reaction 1 m=100g (HCl) ΔT=13.1 °C C=4.18j/g °C q=(100g)(4.18j/g °C) (13.1°C) q=5475.8j Reaction 2 q=(100g)(4.18j/g °C)(4.10°C) q=1713.8j b.) Reaction 1 ΔH=-q =-5475.8j ΔH=nΔHr or ΔHr=ΔH/n n=m/M n=0.51g/24.31g/mol n=0.020979021mol ΔHr=ΔH/n ΔHr=-5475.8j/0.020979021mol ΔHr=-261013j/mol or -261kj/mol Therefore, the enthalpy change per mole of magnesium in this reaction is -261kj/mol. Reaction 2 ΔH=-q ΔH=-1713.8j ΔH=nΔHr or ΔHr=ΔH/n n=m/M

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n=1.00g/40.31g/mol n=0.02480774mol ΔHr=ΔH/n ΔHr=-1713.8j/0.02480774mol ΔHr=-69083j/mol or -69.1j/mol Therefore, the enthalpy change per mole of magnesium oxide in this reaction is -69.1kj/mol. c.) Mg(s) + ½ O2(g) → MgO(s) Reaction 1: Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq) Reaction 2: H2O(l) + MgCl2(aq) →MgO(s) + 2HCl(aq) (reversed) Reaction 3: H2(g) + ½ O2(g) → H2O(l) _________________________________________________ Mg(s) + ½ O2(g) → MgO(s) ΔH1=-261kj/mol ΔH2=69.1kj/mol ΔH3=-286kj/mol ΔH1+ΔH2+ΔH3=-477.9kj/mol Therefore, the sum of the ΔH values for each reaction, and overall molar enthalpy of combustion of magnesium metal is -478kj/mol, which is exothermic. d.) Percentage Difference= |(theoretical - experimental)/theoretical|*100% = |(-602kj/mol - -478kj/mol)/-602kj/mol|*100% = 21% Therefore, the percent difference between the theoretical and experimental values is 21%. e.) One error that could account for the difference calculated could be the fact that the scale used to weigh the magnesium ribbon and magnesium oxide had been faulty. The scale had consistently went up and down about 0.10g, and had not stopped on a given value, which had possibly caused an inaccurate measurement of the mass of each material. This could affect calculations immensely. Along with that, another error that could have occurred could be that the magnesium ribbon and magnesium oxide had not been left long enough in the hydrochloric acid, and so giving an inaccurate final temperature, affecting the change in temperature and calculations. Lastly, another error that could have occurred is if some heat had transferred to the air or calorimeter itself. If this occurred energy released from the reaction would be transferred to the solution and the calculated energy change would be lower.

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Conclusion: The lab had concluded that the molar enthalpy of combustion of magnesium metal was -478kj/mol. Using Hess’s Law the enthalpies for each reaction were calculated and then added. The enthalpy for reaction one was -261kj/mol, and the enthalpy for reaction two was 69.1kj/mol. The third reactions enthalpy had been given, which was -286kj/mol. The accepted value for the enthalpy of magnesium combustion had been -602kj/mol, and so with the experimental value being -478kj/mol, there had been a percentage difference of 21%. The experimental value had been off a significant amount due to presumed error. Based on these results Hess’s Law has the potential for accurately predicting enthalpy for the combustion of magnesium, if done with more accurate equipment.

References: Libretexts. (2019, June 23). Calorimetry. Retrieved November 7, 2019, from https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Cqqqqhemistry _Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry) /Thermodynamics/Calorimetry.

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Libretexts. (2019, September 30). Hess's Law. Retrieved November 7, 2019, from https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Tex tbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Ther modynamics/Thermodynamic_Cycles/Hess's_Law.

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