Lab.9. Calorimetry. J. A. Beran; Laboratory Manual for Principles of General Chemistry, pp

Transcription

1 Key words: Heat, energy, exothermic & endothermic reaction, calorimeter, calorimetry, enthalpy of reaction, specific heat, chemical & physical change, enthalpy of neutralization, law of conservation of energy, final temperature, initial temperature, lattice energy, hydratation energy, enthalpy of solution Literature: J. A. Beran; Laboratory Manual for Principles of General Chemistry, pp J.E. Brady, F. Senese: Chemistry Matter and its Changes, 4th ed. Wiley 003, Chapters 7, 0,. M. Hein and S. Arena: Introduction to Chemistry, 13 th ed. Wiley 011; pp J. Crowe. T. Bradshaw, P. Monk, Chemistry for the Biosciences. The essential concepts., Oxford University Press, 006; pp Theoretical background Accompanying all chemical and physical changes is a transfer of heat (energy); heat may be either evolved (exothermic) or absorbed (endothermic). A calorimeter (Fig. 1) is the laboratory apparatus that is used to measure the quantity and direction of heat flow accompanying a chemical or physical change. The calorimeter is well-insulated so that, ideally, no heat enters or leaves the calorimeter from the surroundings. For this reason, any heat liberated by the reaction or process being studied must be picked up by the calorimeter and other substances in the calorimeter. The heat change in chemical reactions is quantitatively expressed as the enthalpy (or heat) of reaction, H, at constant pressure. H values are negative for exothermic reactions and positive for endothermic reactions.

2 Fig. 1. A set nested coffee cups is a good constant pressure calorimeter. Two quantitative measurements of heat are detailed in his experiment: measurements of the heat accompanying an acid-base reaction, and the heat associated with dissolution of a in water. Specific Heat The energy (heat, expressed in joules, J) required to change the temperature of one gram of a substance by 1⁰C is the specific heat of that substance: specific J heat g C energy mass J g T C (1) or, rearranging for energy, energy J () g C J specific heat mass g T C T is the temperature change of the substance. Although the specific heat of a substance changes slightly with temperature, for our purposes, we assume it is constant over the temperature changes of this experiment. Enthalpy (Heat) of neutralization of an Acid-Base Reaction The reaction of a strong acid with a strong base is an exothermic reaction that produces water and heat as products.

3 Definition: Enthalpy of neutralization: energy released per mole of water formed in an acidbase reaction-an exothermic quantity. aq OH aq H O l heat H3O The enthalpy (heat) of neutralization, H n, is determined by (1) assuming the density and specific heats of the acid and base solutions are equal to that of water and () measuring the temperature change, T, when the two are mixed. H n specific heat H O combined masses acid base T (3) H n is generally expressed in units of kj/mol of acid (or base) reacted. The mass (grams) of the solution equals the combined masses of the acid and base solutions. Enthalpy (Heat) of Solution for the Dissolution of a Salt Lattice energy: energy required to vaporize one mole of into its gaseous ions-an endothermic quantity. Hydratation energy: energy released when one mole of a gaseous ion is attracted to and surrounded by water molecules forming one mole of hydrated ion in aqueous solution-an exothermic quantity. When a dissolves in water, energy is either absorbed or evolved depending upon the magnitude of the s lattice energy and the hydration energy of its ions. For the dissolution of KI: KI H O K I H s =+13 kj/mol s aq aq The lattice energy (an endothermic quantity) of a, H LE, and the hydration energy (an exothermic quantity), H Hyd, of its composite ions account for the amount of heat evolved or absorbed when one mole of the dissolves in water. The enthalpy (heat) of solution, H s, is the sum of two terms (Fig. ): H H H (4) s LE Hyd Whereas H LE and H Hyd are difficult to measure in the laboratory, H s is easily measured.

4 Fig.. Energy changes in the dissolving of solid KI in water. The enthalpy of solution for the dissolution of a, H s, is determined experimentally by adding the heat changes of the and water when the two are mixed. H s is expressed in units of kilojoules per mole of. s heat change heat change H (5) H O H S specific heat H O massh O TH O specific heat mass T (6) mole mole A temperature rise for the dissolution of a, indicating an exothermic process, means that the H Hyd is greater than the H LE for the ; conversely, a temperature decrease in the dissolution of the indicates that H LE is greater than H Hyd and H s is positive.

5 Table 1. Specific Heat of Some Salts Salt Formula Specific Heat (J/g ⁰C) Ammonium chloride NH 4 Cl 1.57 Ammonium nitrate NH 4 NO Ammonium sulfate (NH 4 ) SO Calcium chloride CaCl Sodium carbonate Na CO Sodium hydroxide NaOH 1.49 Sodium sulfate Na SO Sodium thiosulfate pentahydrate NaS O 3 5H O 1.45 Potassium bromide KBr Potassium nitrate KNO Plot the Data. Plot the temperature ( y axis) versus time ( x axis) on the top half of a sheet of linear graph paper. The maximum temperature is the intersection point of two lines: (1) the best line drawn through the data points on the cooling portion of the curve and () a line drawn perpendicular to the time axis at the mixing time [when the acid is added to the base or when a is added to the water]. (Fig.3).

6 Fig. 3. Extrapolation of temperature vs. time data (not scale) for an exothermic process. EXPERIMENTAL PART PART 1 Enthalpy (Heat) of Neutralization for an Acid-Base Reaction Calculate the enthalpy of neutralization reaction: q = m c ΔT = m c ( T f - T i ) H = q /n, where c - specific heat of mixture, n moles of water created Step 1 Measure 50.0 ml of 1.1 M HCl in a clean, graduated cylinder. Measure temperature with temperature sensor and record it in Table. Step Using another clean, graduated cylinder transfer 50.0 ml of a standard 1.0 M NaOH solution to the dry calorimeter. Record the temperature and exact molar concentration of the NaOH solution in Table.

7 Step 3 Carefully, but quickly add the acid to the base, replace the calorimeter lid, and swirl gently. Read and record the temperature and time every 5 seconds for 1 minute and thereafter every 15 seconds for other minutes. Step 4 Plot the temperature (y axis) versus time (x axis) on the top of half of a sheet of linear graph paper. Determine the maximum temperature following instruction presented on Fig. 3. [THEORETICAL BACKGROUND]. Step 5 Repeat the acid-base experiment. Plot the data on the bottom half of the same sheet of graph paper. Step 6 Calculate the thermodynamic data and fill the proper places in Table 3. Step 7 Repeat steps 1-6, for 1.1 M HNO 3. Table. Data for acid base neutralization 1. Volume of acid (ml). Temperature of acid ⁰C 3. Volume of NaOH (ml) 4. Temperature of NaOH (⁰C) 5. Exact molar concentration of NaOH (mol/l) 6. Maximum temperature from graph( ⁰C) HCl + NaOH HNO 3 + NaOH Trial 1 Trial Trial 1 Trial Table 3. Calculation for Enthalpy (Heat) of Neutralization for an Acid-Base Reaction Average initial temperature of acid and base ( ⁰C ) Temperature change, T ( ⁰C ) HCl + NaOH Trial Trial 1 HNO 3 + NaOH Trial 1 Trial

8 Volume of final mixture (ml) Mass of final mixture (g) (Assume the density of the solution is 1.0 g/ml) Specific heat of mixture 4.18J/g C 4.18J/g C Heat evolved (J) Amount of OH - reacted, the limiting reactant (mol) Amount of H O formed (mol) Heat evolved per mole of H O, H n (kj/mol H O) Average H n (kj/mol H O) Answer the question: Is the enthalpy different for both neutralization reactions or is the same (in frame of experimental error)? Comment on your answer. Disposal: Rinse the calorimeter twice with deionized water before next step PART. Enthalpy (Heat) of Solution for the Dissolution of a Salt Calculation for Enthalpy: H S specific heat H O mass mole H O T H O specific heat mole mass T Step 8. Prepare the. On weighing paper, measure about 5.0 g of the assigned. Record the name of the in Table 4.

9 Step 9 Using your clean, graduated cylinder, add 0 g (0.0 ml) of deionized water to the calorimeter and record its temperature in Table 4. Secure the thermal sensor below the water surface. Step 10 Carefully add (do not spill) the to the calorimeter, replace the lid, and swirl gently. Read and record the temperature and time at 5-second intervals for 1 min and thereafter every 15 seconds for min. Record the data. Step 11 Plot the temperature (y axis) versus time (x axis) on the top half of the sheet of linear graph paper. Determine the maximum (for an exothermic process) or minimum (for an endothermic process) temperature as done in first part under point 4. Step 1 Calculate thermodynamic data for and fill the Table 4. Step 13 Repeat steps 7-1, for other assigned by TA. Disposal: Discard the solution into the Waste Salts container, followed by additional tap water. Table 3. Enthalpy (Heat) of solution for the Dissolution of a Salt Mass of (g) Moles of Mass of water (g) Initial temperature of water ( ⁰C ) Name of the Final temperature of mixture from the graph ( ⁰C )

10 Table 4.Calculation for Enthalpy (Heat of Solution for the Dissolution of a Salt Name of Change in temperature of solution Heat loss of water (J) Heat loss of (J) Total enthalpy change H s (J/mol ) Average H s (J/mol ) Additional information Table 1. Specific Heat of Some Salts Salt Formula Specific Heat (J/g ⁰C) Ammonium chloride NH 4 Cl 1.57 Ammonium nitrate NH 4 NO Ammonium sulfate (NH 4 ) SO Calcium chloride CaCl Sodium carbonate Na CO Sodium hydroxide NaOH 1.49 Sodium sulfate Na SO Sodium thiosulfate pentahydrate NaS O 3 5H O 1.45 Potassium bromide KBr Potassium nitrate KNO