Enthalpy Stoichiometry BINGO

Transcription

1 Enthalpy Stoichiometry BINGO Please write the following answers randomly on your BINGO card: kj kj 88.1 kj 34.8 kj -3.7 kj kj 538 g 1520 g

2 BINGO Problem #1 How much heat will be released when 76.0 g of O 2 reacts according to the following equation: 2H 2 + O 2 2H 2 O H = kj

3 BINGO Problem #2 How much heat will be released when 1.48 g of chlorine reacts with excess phosphorus according to the following equation: 2P + 5Cl 2 2PCl 5 H = 886 kj

4 BINGO Problem #3 What mass of propane, C3H8, must be burned in order to produce 76,000 kj of energy? C 3 H 8 + 5O 2 3CO 2 + 4H 2 O H = 2200 kj

5 BINGO Problem #4 How much heat will be absorbed when 38.2 g of bromine reacts with excess H2 according to the following equation: H 2 + Br 2 2HBr H = kj

6 BINGO Problem #5 How much heat will be released when 6.44 g of sulfur reacts with excess O2 according to the following equation: 2S + 3O 2 2SO 3 H = kj

7 BINGO Problem #6 What mass of iron must react to produce 3600 kj of energy according to the following equation: 3Fe + 2O 2 Fe 3 O 4 H = kj

8 BINGO Problem #7 How much heat will be absorbed when 13.7 g of nitrogen reacts with excess O2 according to the following equation: N 2 + O 2 2NO H = kj

9 BINGO Problem #8 How much heat will be released when 4.72 g of carbon reacts with excess O2 according to the following equation: C + O 2 CO 2 H = kj

10 Measuring H Through Calorimetry M S. G R O B S K Y

11 Determining ΔH using Calorimetry We measure the H (the transfer of heat at a constant pressure) of a chemical reaction by a technique called calorimetry What is calorimetry? The process of measuring heat based on observing the temperature change when a body absorbs or releases energy as heat energy Based on First Law of Thermodynamics The total heat of the system and the surroundings remains constant

12 Calorimetry We use an insulated device called a calorimeter to measure this heat transfer A typical device is a coffee cup calorimeter Reaction is open to the atmosphere Therefore, constant pressure

13 How Does Calorimetry Work? In calorimetry: The heat released by the system is equal to the heat absorbed by its surroundings The heat absorbed by the system is equal to the heat released by its surroundings q system + q surroundings = 0 In other words: The heat generated by the reactants is absorbed by the water We know the mass of the water, mwater We know the change in temperature of the water, Twater We also know that water has a specific heat of c water = 4.18 J/ C g. So, we can calculate the enthalpy of reaction by using the relationship: qsys = H = qsurr qrxn = H rxn = qwater qrxn = H rxn = -(mwater Cwater Twater)

14 Calorimetry Example heatcapacitymetals/heat_metal.htm

15 Enthalpy and Hess Law Enthalpy is a state function Remember, we cannot measure enthalpy directly The reactants start with some heat content The products end up with some heat content Therefore, DH going from some initial state to some final state is pathway independent Does not depend in any way on the system s past (or future) Examples of other state functions: Energy Elevation (non-scientific analogy) Non-examples of state functions: Work Heat (q) Distance travelled (non-scientific analogy)

16 H and Hess Law Start Finish Both lines accomplished the same result They went from start to finish So, the net result is the same Similarly, H for a process involving the transformation of reactants into products is not dependent on pathway Therefore, we can pick any pathway to calculate H for a reaction

17 Campsite to Illustrate Altitude as a State Function

18 Hess s Law States that for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation is the sum of the enthalpy changes for the individual steps

19 A B + C DH = x B + C D DH = y A D DH =? DH = x + y

20 Enthalpy Diagram Illustrating Hess s Law

21 Hess Law and Manipulating H If you reverse a reaction, the sign of H changes H 2 O (l) H 2 O (s) H = kj If you multiply both sides of the equation by a factor n, then H must change by the same factor n 2H 2 O (s) 2H 2 O (l) H = 2 x 6.01 = 12.0 kj