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Hess’s Law: Additivity of Heat of
Reactions
Introduction
According to Hess’s law, if a reaction can be carried out in a series of steps, the sum of the enthalpies of the individual steps should equal the enthalpy change for the overall reaction. The reactions we will be using in this experiment are:
- Solid sodium hydroxide dissolved in water to form an aqueous solution of ions. NaOH(s) Na+(aq) + OH-(aq)
- Solid sodium hydroxide reacts with aqueous hydrochloric acid to form water and an aqueous solution of sodium chloride. NaOH(s) + H+(aq) + Cl-(aq) H 2 O (^) (l) + Na(aq) + Cl-(aq)
- Solutions of aqueous sodium hydroxide and hydrochloric acid react to form water ioned aqueous sodium chloride. Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) H 2 O (^) (l) + Na+(aq) + Cl-(aq)
Equipment
- A temperature sensor
- 250ml beaker
- Polystyrene coffee cup.
- Magnetic stirrer.
- 50ml of 1.0M NaOH
- 50ml of 1.0M HCl
- 100ml of 0.5M HCl
- 100ml of water
- 4.00g of solid NaOH
- A MultiLog
Equipment Setup Procedure
- Connect the MultiLog to the serial port of the computer.
- Connect the temperature sensor to the I/O 1 port of the MultiLog.
- Assemble the equipment as illustrated in figure 1.
Fig-
4.Turn the MultiLog on.
Set the MultiLog up according to the setup specified below. You can set up the MultiLog in two ways: -Use the keyboard of the MultiLog, or -Select the Control Panel from the Logger menu.
beaker. In step 8, instead of solid NaOH, add 50.0ml of 1.0M NaOH.
Data Analysis
1. Determine the temperature change, ∆t, for each reaction. Use the Markers or
the Grid to find the appropriate values:
Markers - double click with the left mouse button on the first required value. A marker will pop-up on the graph. Drag the marker with the left mouse button to the point on the graph you want to measure. Double click again on the second value. You get a second marker. Repeat the “drag” process with the second marker. You get both readings at the bottom of the display as well as the difference in the values - ∆ t and ∆ y. Grid - choose View\Display from the menu. Mark the option Grid in the Option Display Window.
2. Determine the mass of 100ml of solution for each reaction (assume the density of
each solution is 1.00g/mL)
- Calculate the heat released by each reaction. q, (cp=4.18J/g *C) q = Cp x m x∆t
- Find ∆H (∆H = -q)
- Calculate moles of NaOH used in each reaction.
- Determine ∆H/mol NaOH in each of the three reactions.
- Combine the heat of reaction (∆H/mol) 1 and 3.
An example of the graph obtained in this experiment is shown on the following page.
Questions
- According to your data, is the heat of reaction for the reaction equal to the sum of heats of formation of the individual components of the reaction?
- Find the percent error for the experiment.
Further Suggestions
- Dissolve different amounts of NaOH. Follow changes in temperature.
- Follow changes in pH in each case.
- Dissolve a hydrous CuSO 4 in water and CuSO 4 x 5H 2 O(s) in water and calculate Hess’ Law for: CuSO 4 (s) + 5H 2 O CuSO 4 x 5H 2 O
