Heats of Reaction lab. Enthalpy

Transcription

1 Heats of Reaction lab tonight s QuestiONs Is the amount of heat given off or absorbed, q sys, by a chemical reaction an intensive or extensive property? For an acid-base reaction, does the heat of reaction depend on which acid and base react? Does the physical state of a reactant (solid, liquid, gas, aqueous solution) affect the heat of reaction? What is Hess Law and can we verify it? Enthalpy Enthalpy (H) is defined as internal energy + pressure x volume (work): H = E + PV When the system changes at constant pressure, the change in enthalpy is H = E + P V Since E = q + w and w = -P V, we can substitute these into the enthalpy expression: H = E + P V = (q+w) w H = q (at constant pressure) Thus, enthalpy is a measure of the heat flow between a system and the surroundings 1

2 Enthalpy of Reaction, H The change in enthalpy, H, after a reaction is the enthalpy of the products minus the enthalpy of the reactants: H = H products H reactants H is called the enthalpy or heat of reaction H depends on the states of products and reactants. H is numerically the same, but opposite sign for reverse reaction hess Law Hess s law states that If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps. Intermediate step is formation of carbon monoxide (incomplete combustion), followed by further combustion to carbon dioxide. 2

3 MOre ON hess Law Hess' law allows ΔH rxn to be calculated even when it can t be measured directly. To do this, we perform arithmetic operations on chemical equations and known ΔH values. Chemical equations may be multiplied or divided by a whole number. When an equation is multiplied by a constant, so is ΔH If an equation is reversed, ΔH for reaction is reversed (-ΔH) Addition of chemical equations leads to a net equation If ΔH for each equation is added, the result will be the ΔH for the net equation. Let s try ONe Use the thermochemical equations shown below to determine the enthalpy for the reaction: 2NH 3 (g) N 2 (g) + 3H 2 (g) CH 2 O(g) + N 2 (g) + 3H 2 (g) N 2 H 4 (l) + CH 4 O(l) N 2 H 4 (l) + H 2 (g) 2NH 3 (g) CH 2 O(g) + H 2 (g) CH 4 O(l) N 2 H 4 (l) + CH 4 O (l) CH 2 O(g) + N 2 (g) + 3H 2 (g) 2NH 3 (g) N 2 H 4 (l) + H 2 (g) CH 2 O(g) + H 2 (g) CH 4 O(l) 2NH 3 (g) N 2 (g) + 3H 2 (g) ΔH=18.5 KJ Δ H=-9.0 KJ Δ H=32.5 KJ Δ H= KJ Δ H=+9.0 KJ Δ H= 32.5 KJ Δ H= 23.0 KJ For more fun like this: Scroll down the menu for the Hess Law DP (drill and practice) 3

4 Calorimetry: measurement of heat flow We can t know exact enthalpy of reactants and products, but we can measure H at constant pressure using calorimetry, the measurement of heat flow. If we perform a reaction in aqueous solution, the molecules/atoms involved in the reaction are the system and the water/container are the surroundings We can measure T of the surroundings easily T = T final T inital Using T and information on the mass and specific heat capacities of the water and container, we can calculate q surroundings and thus, H of the reaction Specific Heat capacity Look at the units for specific heat: J/g-K Specific heat = heat transferred (J) Mass (m) temp change ( T) C s = q m T or q = C s m T 4

5 Back to the coffee cup By carrying out a reaction in aqueous solution in a calorimeter, we can estimate the heat change for the system by measuring ΔT for the contents & calorimeter (the surroundings). Δq sys = - Δq surr = ΔH rxn Δq surr = Δq contents + Δq cup Δq contents = C s (water) m contents ΔT Δq cup = C (whole cup) ΔT ΔH rxn = q sys /moles of reaction (page 11) For today s lab; C s (water) = J/g- o C C (whole cup) = 50.0 J/ o C Another cup of coffee? When 50.0 ml of M AgNO 3 and 50.0 ml of M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from C to C. The temperature increase is caused by the following reaction:. AgNO 3 (aq) + HCl(aq) AgCl(s) + HNO 3 (aq) Calculate ΔH for this reaction in kj/mol AgNO 3, assuming the solution has a mass of g and a specific heat of 4.18 J/g C. ΔH rxn = q rxn = -C s x m x ΔT = J/g C x g x 0.81 C = -339 J # mol AgNO 3 = L x M AgNO 3 = mol AgNO 3 ΔH rxn in kj/mol = 339 J/ mol AgNO 3 x 1kJ/1000 J = -68 kj/mol 5

6 today s reactions 1. NaOH(aq) + HCl(aq) NaCl(aq) + H 2 O(l) H rxn = 55.8 kj 2. 2NaOH(aq) + H 2 SO 4 (aq) Na 2 SO 4 (aq) + 2H 2 O(l) H rxn = kj 3. NaOH(aq) + HNO 3 (aq) NaNO 3 (aq) + H 2 O(l) H rxn = 55.8 kj 4. NaOH(s) + HCl(aq) NaCl(aq) + H 2 O(l) H rxn = kj 5. We will also study the following physical change (dissolution of NaOH): NaOH(s) NaOH(aq) H rxn = 44.2 kj In class: Do calculations for all reactions For lab: Work with a partner with drawer near you Collect data on your reaction w/lab partner Do calculations for your reaction Check results with me, then enter your data into spreadsheet on the computer In class calculations Reaction 1: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) Record the molarity of the HCl(aq) For this reaction, assume 95.0 ml of 1.00 M NaOH(aq) will be used. Calculate the volume in ml of the HCl(aq) needed to neutralize the given amount of NaOH(aq). Reaction 2. 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) Record the molarity of the H2SO4(aq) For this reaction, assume 95.0 ml of 1.00 M NaOH(aq) will be used. Calculate the volume in ml of the H2SO4(aq) needed to neutralize the given amount of NaOH(aq). Be careful to use the correct stoichiometry for this reaction! 6

7 In Class calculations Reaction 3. NaOH(aq) + HNO3(aq) NaNO3(aq) + H2O(l) Record the molarity of the HNO3(aq): For this reaction, assume 95.0 ml of 1.00 M NaOH(aq) will be used. Calculate the volume in ml of the HNO3(aq) needed to neutralize the given amount of NaOH(aq) Reaction 4. NaOH(s) + HCl(aq) NaCl(aq) + H2O(l) Record the molarity of the HCl(aq): For this reaction, assume 95.0 ml of the HCl(aq) solution will be used. Calculate the mass in grams of solid NaOH needed to neutralize the given amount of HCl (aq). Equipment and Reagents Foothill Coffee Cup calorimeter (stockroom) Digital thermometer (check out from stockroom) Approximately 1 M solutions of each of the following: HCl(aq), HNO3(aq), H2SO4(aq) and NaOH(aq) in the hood. The acid solutions are standardized; the molarities written on the reagent bottles must be recorded. Sodium hydroxide pellets Note: Make mass measurements using the top loading balances in the lab room. DO NOT use the analytical balances! 7

8 Procedures Work with a partner. You and your partner will be assigned to collect data on one reaction system. You will do two trials for your reaction. Obtain the materials needed for your reaction For all reactions, the masses must be measured accurately using the top loading balances For reactions 1,2,3, the volume of acid must be carefully measured, since we will be using volume to figure out the number of moles of acid. The reactions go quickly, so begin collecting data as soon as possible after adding the reactants together. Page 6: for Reactions 1,2,3 1. Record the volume of acid that you decided on; use this volume for both trials. 2. Weigh and record the mass of your clean, dry calorimeter (with the lid). 3. Using a clean, dry graduated cylinder, measure 95.0 ml of the 1M NaOH(aq) 4. Pour the 95.0 ml of NaOH(aq) into the calorimeter. 5. Using a clean, dry graduated cylinder measure out the proper volume of acid. 6. First measure and record the initial temperature (Ti) of the base solution in the calorimeter. Remove the thermometer and have your calorimeter lid and thermometer ready before completing the next step. 7. Carefully pour the acid solution into the base solution in the calorimeter. Replace the lid. Insert the thermometer as quickly as possible. Continuously swirl the contents with the thermometer to ensure complete mixing. 8. While swirling, take the temperature readings every seconds until the temperature passes through a maximum and starts to decline. Record the maximum temperature obtained as T f. 9. IMPORTANT! After recording T f, remove the thermometer and weigh and record the final total mass of the calorimeter (with the lid) plus its contents. 8

9 Page 7: for Reaction 4 Record the #of NaOH(s) pellets you decided on; Use this # for both trials. Weigh and record the mass of your clean, dry calorimeter (with the lid). Using a clean, dry graduated cylinder, measure 95.0 ml of the HCl(aq) solution. Pour the acid into calorimeter. Weigh /record total mass of calorimeter (w/lid) Measure and record the initial temperature (T i ) of the acid in the calorimeter. Remove the thermometer. Bring the bottle of NaOH pellets directly to your bench. As quickly as possible., use your curved scoopula, to measure the number of solid NaOH pellets decided upon, placing them in a small beaker. Carefully add the measured NaOH pellets to the calorimeter contents, avoiding splashing the solution. Replace lid and insert thermometer into solution. Continuously swirl contents with the thermometer. While swirling take temperature readings every seconds until the temp passes hits a maximum and starts to decline. Record the max temperature obtained as T f. IMPORTANT! After recording T f, remove thermometer and weigh/ record the total mass of the calorimeter (with lid) plus its contents page 8: Reaction 5 You will use NaOH pellets (2 to 3 g). Each set of partners will use a different number of pellets (work together to figure out) Record the volume of water and # of pellets you and your partner decided upon; use the same amounts for both trials Weigh and record the mass of your clean, dry calorimeter (with the lid) Using a clean graduated cylinder, measure your volume of deionized water and water into calorimeter. Weigh/ record the mass of calorimeter (w/lid) Measure/ record initial temp (T i ) of the water. Remove thermometer. Bring the bottle of NaOH pellets directly to your bench. As quickly as possible, use your curved scoopula, to get the # of NaOH pellets decided upon, placing them in a small beaker. Carefully add the NaOH pellets to the calorimeter contents, avoiding splashing the solution. Replace lid and insert thermometer into solution. Continuously swirl contents with thermometer. While swirling take temp readings every secs until temp hits a maximum and starts to decline. Record max temp as T f. IMPORTANT! After recording T f, remove thermometer and weigh/ record the total mass of the calorimeter (with lid) plus its contents 9

10 Data calculations To find q sys, remember q sys + q surr = 0 (1 st Law TD) ΔT = T final - T initial q sys = - q surr q surr = q contents + q cup q contents = C s (water) m contents ΔT q cup = C (whole cup) ΔT ΔH rxn = q sys /moles of reaction (page 11) For today s lab; C s (water) = J/g- o C C (whole cup) = 50.0 J/ o C Note that the specific heats are given in J, but your answers should be in kj Waste disposal Use your large beaker as a waste beaker to collect all contents of the calorimeter after use and any rinses containing acid or base Dispose of waste beaker contents into the labeled hazardous waste container in the hood. Be careful not to spill drops of waste outside the container or to overfill the waste containers Rinse calorimeter, lid and thermometer after using. Dry and return calorimeter and digital thermometer to stockroom 10

11 Dire Warnings NaOH chemical burn Dispose of acid or basic waste into proper container in the hood Be very careful not to spill or leave any NaOH pellets on the lab benches, etc. It will attract water and look like a harmless drop, but is very corrosive! Use only the top loading balances today and keep the balance area clean Upcoming events In lab today: Standardization of NaOH Lab Report due Heats of reaction pre-lab due On Thursday: Second Lab Quiz Purity of a Hydrate Solubility Rules/Reactions/Net Ionic Equations Titrations: Standardization of NaOH / Molar Mass of Acid Finish Heats of Reaction Lab Molar Mass of Acid Lab Report due 11