Calorimetry AP Chemistry

Calorimetry Calorimetry is the measurement of heat flow. It allows us to calculate the amount of energy required to heat up a substance or to make a substance change states.

Calorimetry Terms Molar Heat of Fusion — The heat absorbed by one mole of a substance when changing from a solid to a liquid. For water, it = 6.0 kiloJoules/mole Heat of solidification is opposite of heat of fusion (heat is released).

For water, it = 40.7 kiloJoules/mole. Molar Heat of Vaporization — The heat absorbed by one mole of a substance when changing from a liquid to a gas. For water, it = 40.7 kiloJoules/mole. Heat of condensation is the opposite of heat of vaporization (heat is released) Heat Required For a Phase Change – Or Latent Heat Process Heat Absorbed or Released = q q = (moles) x (Molar Heat Fusion/Vaporization)

Calculating Heat Required To Change State Example #1: How much heat is needed to melt 56.0 grams of ice into liquid (the molar heat of fusion for ice is 6.0 kJ/mol)? 56.0 g 1 mole H2O 6.0 kJ = 18.0 g 1 mole = 18.7 kJ will be absorbed q = + 18.7kJ

Example #2 How much heat energy will be released when 200grams steam condenses back to a liquid water? Molar Heat condensation = 40.7kJ/mol q = (moles) x (Molar Heat condensation) 200gram 1 mole 40.7 kJ 18gram 1 mole = 452 kJ released So, q = - 452kJ

Heating a Substance with No Phase Change Also known as a sensible heat process. Depends on Specific Heat Capacity Specific Heat Capacity--The amount of energy required to raise one gram of a substance one degree Celcius. Every substance and every phase of that substance has its own unique specific heat capacity. Water’s Specific Heat (as a liquid) = 4.184 Joules/gram oC or unit cans be J/gK

Specific heat (J/goC) = heat capacity (J/oC) You will see both terms used. You can also use Molar Heat Capacity This is the heat energy required to raise 1 mole of a substance 1oC or 1K. Molar heat capacity can be determined from the specific heat by multiplying it by the molar mass of the substance.

Energy to Change Temperature q = (mass) ( C ) ( T ) Change in Temperature Tfinal – Tinitial In OCelcius or kelvin Heat Measured in Joules Specific Heat Capacity Mass In grams

Exothermic vs. Endothermic Exothermic Change – Heat Energy is released to the surroundings. q and ∆H are (-) Molecules slow down, extra energy is transferred to surrounding. Cooling phase changes are exothermic Endothermic Change – Heat energy is absorbed by the system. q and ∆H are (+) -Molecules move faster as they absorb energy -Phase changes that require energy (or heat) are endothermic

Changes In State Solid Liquid Gas Freezing Sublimation Deposition Melting Vaporization Liquid Gas Condensation

Example #3 How much energy is needed to heat 80grams of water from 10oC to 55oC at constant pressure? q = m C T = m C (Tfinal – Tinitial ) q = (80grams) ( 4.184 J/goC) (55oC – 10oC) q = + 15062 joules divide by 1000 to get kilojoules 15062 J 1 kJ = 1000J 15.06 kJ absorbed

Example #4 -How much energy is needed to change 150grams of ice from 0oC to 50oC? This problem requires two steps. Since water is solid ice at 0oC, we need to melt the ice and then heat it up to 50oC. Step 1 – Calculate heat required to melt 150grams ice 150g 1 mole 6.0 kJ = 50 kJ 18grams 1 mole Step 2 - Calculate heat required to heat liquid water from 0oC to 50oC q = mC T = (150g)(4.184 J/goC)(50oC) = 31380 J  convert to kJ = 31.38kJ *Add both heat values together for your final answer 50 kJ + 31.38kJ = 81.38 kJ heat absorbed.

Multiple Step Calorimetry Problems Add each individual energies together for total Use q = Moles x Molar Heat vap/fus e Gas Heats Vaporization d c Liquid Heats b Solid Heats melting a Use q = mC T qtotal = a + b + c + d + e

Calorimetry Coffee Cup Calorimetry Experimental way of measuring heat generation/consumption by essentially catching all the heat energy in a water bath or water bath + metal apparatus. Coffee Cup Calorimetry Styrofoam cup insulates the contents so any heat generated or consumed in the water can be measured by the temperature change q = -(Hrxn) = mCH2O∆T Reactions must take place in water, then you measure the change in temperature.

Bomb-Calorimeter Used to measure heats of reaction. Usually do combustion reactions in it. Metal innards absorb heat. You have to keep track of heat absorbed by metal and water. q = mCH2O∆T + Cp ∆T qwater + qapparatus Cp is the heat capacity of the apparatus – determined experimentally.

Conservation of Energy Under conditions of constant pressure Heat Lost = Heat gained qp-lost = qp-gained For a Reaction heat gained by water = heat released by reaction.

Example- When 100mls of 0. 1M HCl is mixed with 100mls of 0 Example- When 100mls of 0.1M HCl is mixed with 100mls of 0.1M NaOH in a coffee cup calorimeter, the temperature increases from 25oC to 29.8oC. Assume that the coffee cup is a perfect insulator, pressure is constant and the density of the solution is 1g/ml and specific heat capacity for the mixture is that of water, 4.18 J/gK. What is the enthalpy change for the reaction? ∆H = q = mC∆T

Since density is 1g/mL , all volumes are gram values. Total volume must be used = 200mLs = 200grams. ∆T = Tfinal – Tinitial = 29.8oC – 25oC = 4.8oC q = mC ∆T = (200g)(4.18J/gK)(4.8K) q = 4013 J = 4.013 kJ ∆H = - 4.013 kilojoules Since temperature of water increased, it is exothermic, which means a - ∆H