Figure 6 12 A bomb calorimeter ConstantVolume Calorimetry

Figure 6. 12: A bomb calorimeter.

Constant-Volume Calorimetry The calorimeter constant, C, includes both the surrounding water and metal bomb. qrxn = - qbomb = Cbomb t Reaction at Constant V H = qrxn /n H ~ qrxn /n No heat enters or leaves! E = qrxn /n 6. 4

Heat Capacity and Specific Heat l For water, C = 4. 18 J/(g o. C), and also C = 1. 00 cal/(g o. C) l Thus, for water: l it takes a long time to heat up, and l it takes a long time to cool off! l Water l Note is used as a coolant! Figure 11. 7, page 297 8

A Problem to Consider l Suppose a piece of iron requires 6. 70 J of heat to raise its temperature by one degree Celsius. The quantity of heat required to raise the temperature of the piece of iron from 25. 0 o. C to 35. 0 o. C is:

Measuring Heats of Reaction l Heat capacities are also compared for one gram amounts of substances. The specific heat capacity (or “specific heat”) is the heat required to raise the temperature of one gram of a substance by one degree Celsius. To find the heat required you must multiply the specific heat, s, of the substance times its mass in grams, m, and the temperature change, T.

A Problem to Consider l Calculate the heat absorbed when the temperature of 15. 0 grams of water is raised from 20. 0 o. C to 50. 0 o. C. (The specific heat of water is 4. 184 J/g. o. C. )

Heats of Reaction: Calorimetry l. A calorimeter is a device used to measure the heat absorbed or evolved during a physical or chemical change. (see Figure 6. 11) The heat absorbed by the calorimeter and its contents is the negative of the heat of reaction.

A Problem to Consider l When 23. 6 grams of calcium chloride, Ca. Cl 2, was dissolved in water in a calorimeter, the temperature rose from 25. 0 o. C to 38. 7 o. C. If the heat capacity of the solution and the calorimeter is 1258 J/o. C, what is the enthalpy change per mole of calcium chloride?

Heats of Reaction: Calorimetry l First, let us calculate the heat absorbed by the calorimeter. Now we must calculate the heat per mole of calcium chloride.

Heats of Reaction: Calorimetry l Calcium chloride has a molecular mass of 111. 1 g, so Now we can calculate the heat per mole of calcium chloride.

q = quantity of heat transferred m = mass of substance in grams = temperature change (ºC or K) How much heat is needed to raise 10. 0 g of aluminum from 22ºC to 42ºC?

What is the molar heat capacity of aluminum? 24 J/mol·K

Thermochemistry l The enthalpy of sublimation of magnesium at 25 ℃ is 148 k. J/mol. How much heat (at constant temperature and pressure) must be supplied to 1 g of solid magnesium metal to produce a gas composed of Mg+2 ions and electrons? Mg(s) Mg+2(g)+2 e-(g) Sublimation Mg(s) Mg(g) +148 k. J /mol First ionization Mg(g) Mg+(g)+e-(g) +738 k. J /mol Second ionization Mg+(g) Mg+2(g)+ e-(g) +1451 k. J/mol