Solid potassium fluoride is dissolved in water in a perfect calorimeter. When 10.00 grams of potassium fluoride is dissolved in 120.0 grams of water initially at a temperature of 20.00 °C, the temperature of the solution increases to 25.61 °C. Calculate AH' for the reaction below in kj/mol. KF (s)-K* (aq) + F¯¯ (aq) AH'=7 [You may assume that no heat escapes the calorimeter and that all solutions have the same specific heat capacity as pure water (4.184 J/gºC)] • Was energy released by the salt into the water or was energy absorbed by the salt from the water? Think about how the temperature of the water changed. What does this tell you about AH for the reaction taking place? Is the process of dissolving the salt endothermic or exothermic? What is the sign of AH? AH positive (Remember this!) Energy does not escape the calorimeter. So, if energy is released by the reaction it must be take into (absorbed) by the water. If energy is absorbed by the reaction, it must have been taken out of the water. We must account for all of that energy. Remember, with energy (q), the sign tells us direction of the transfer. The magnitude of the energy change is the same, but its sign is different depending on whether we are looking at the water (solution) or the salt (reaction). Greaction=-solution- How do we relate energy (g) to temperature change (AT)? They are related though a property called the Specific Heat Capacity of the substance. Specific Heat Capacity is given the symbol "Cp" or sometimes just "S.H.". q= mass Cp AT or q=mass-S.H.AT So, we can relate the temperature change for the solution to "q" for the chemical reaction taking place in the solution. When we measure the temperature in our calorimeter, we are measuring the temperature change for the solution, AT solution solution = mass-Cp.AT solution and from above: reaction-solution The two equations above can be combined in order to directly relate the heat change for the reaction to the measured temperature change for the solution greaction-mass-Cp AT solution With a "perfect" calorimeter, we assume that the calorimeter itself absorbs no heat and does not allow any energy to escape. Thus, all of the energy is transferred to or from the contents of the calorimeter. What is the "total mass of the contents of the calorimeter? • We need to know the specific heat capacity of the solution in the calorimeter. If the solution is sufficiently dilute (not much salt compared to water) then the specific heat capacity of the water with the salt dissolved in it (the solution) will be very close to that of pure water. For water Cp = 4.184 //gºC AT solution was measured. What is the value of AT solution in this experiment? AT solution - Now can calculate from the mass, Cp. and AT. Answer in Joules. Greaction • This value is NOT AH for the reaction. AH is the quantity of heat transeferred per mol of salt used. This experiment did not use 1 mole of salt. It only used 10.00 grams of KF. How many moles of KF were actually used? Moles of KF moles of KF • AH is expressed as Greaction/(moles of salt used). It is usually expressed in units of kJ/mol. Calculate AH for the reaction. AH- kj