Experiment 5 (thermochemistry)

Question 1

Enthalpy

Answer 1

-a thermodynamic quantity equivalent to the total heat content of a system
-sum of a thermodynamic system’s internal energy and the product of its pressure and volume
-H = U + PV
-ΔH = qp
-ΔU = qv
-intrinsic value (per mole, does not change with change with change in moles)

Question 2

Endothermic

Answer 2

-reaction accompanied by or requiring the absorption of heat
-ΔH > 0
-Usually not spontaneous (but can be if entropy, S, is large enough)

Question 3

Exothermic

Answer 3

-reaction accompanied by the release of heat
-ΔH < 0
-Usually spontaneous (unless S is large negative number)
-When reaction is exothermic, the heat given off from the reaction will be absorbed by the solvent, raising its temperature
-Solvent is part of surroundings while reaction is system

Question 4

Can reaction be both endothermic and exothermic?

Answer 4

A reaction cannot be both exothermic and endothermic, however an exothermic process can combine with an endothermic process

Question 5

Hess’s Law

Answer 5

-the enthalpies of reactions can be added in the same manner as the reaction equations
-Thermodynamic functions are state functions (q and Δw are not state functions)

Question 6

Enthalpy (heat) of formation (ΔHf0)

Answer 6

-the heat absorbed/given off when one mole of this compound is produced from the reaction of the elements in their standard physical states
-Example: in H2(g) + 1/2O2(g) → H2O (l), ΔH(rxn) = ΔHf0 (reactants are in standard state and one mole of a compound is produced)

Question 7

Calorimetry

Answer 7

-the measurement of heat involved in physical or chemical processes, typically determined by observing temperature changes in a calorimeter
-How one measures the amount of heat lost/absorbed
-When there is no phase transition
–Q = CΔT

Question 8

Heat capacity

Answer 8

-the amount of heat required to raise the temperature of an object 1 degree C/K
-Cv = heat capacity measured at constant volume
-Cp= heat capacity measured at constant pressure
-Cs = specific heat
-C = mCs and Q = mCsΔT
-m= mass of solution (g) = volume of solution (mL)
–Total mass of solution (solvent’s mass + solute’s mass)
—Eg. 100 g of water + 10 g of salt = total mass of 110g

Question 9

ΔT

Answer 9

-change in temperature
-ΔT = T(f) - T(i)

Question 10

Tf

Answer 10

-T(f) = final temperature
-Not the highest temperature (Tmax)
-Line of best fit must be applied to cooling portion of graph
-Tf is y-intercept of cooling (point of graph when graph begins to slope downwards

Question 11

Ti

Answer 11

T(i) = initial temperature

Question 12

Q and ΔH relationship/formula

Answer 12

-Q is not equal to ΔH
-Heat of surroundings is being measured, not system
-H = -Q / moles of limiting reagent (moles of water produced)

Question 13

Example: determining heat of formation (ΔHf0) of MgO

Answer 13

-Mg(s) + ½O2(g) → MgO(s)
-ΔHf= ΔH1+ ΔH2 + ΔH3
-Q = mCsΔT
-ΔHrxn= -Q / moles of limiting reagent (Mg/MgO)

Question 14

Example: Determining the molarity of a NaOH unknown

Answer 14

-HCl(aq) + NaOH(aq) → H2O (l) + NaCl (aq)
-H+(aq) + OH-(aq) → H2O
-ΔHrxn= -Q / moles of limiting reagent (NaOH)
–moles of limiting reagent (NaOH) = -Q / ΔHrxn
–Q = mCsΔT
–Allows us to calculate molarity of NaOH (M = mol/V)