Chapter 10 Causes of Change Section 1 Energy Transfer

Energy as Heat Heat is energy that is transferred between objects that are at different temperatures Measure is joules (J) Law of Conservation of Energy

What is Thermodynamics? The branch of science concerned with the energy changes that accompany chemical reactions and physical changes

Temperature A measure of how hot or how cold something is A measure of the average kinetic energy of the particles in an object Measured in Celsius degrees (˚C)

Heat and Temperature are Different Heat is an extensive property, which means the energy transferred as heat by a sample of a substance depends on the amount of the substance Temperature is an intensive property, which means that the temperature of a substance does not depend on the amount of the substance

Enthalpy It is impossible to measure the total amount of energy present in a sample of matter. We can measure changes in energy content. Enthalpy is the total energy content of a sample

Enthalpy (con’t) If pressure remains constant, the heat absorbed or released during a chemical reaction is equal to the enthalpy change for a reaction. symbol: ΔH = final enthalpy - initial enthalpy note: enthalpy changes depend only on initial and final states, not on the route between them! enthalpy changes are slightly different from thermal energy changes constant pressure processes must use a little energy to push back the atmosphere enthalpy change is thermal energy change, minus work against atmosphere, for a constant pressure process http://antoine.frostburg.edu/chem/senese/101/thermo/#notes

Molar Heat Capacity The energy as heat needed to increase the temperature of 1 mol of the substance by 1 K. Has the symbol C The unit is J/K·mol

Molar Heat Capacity (con’t) We use the equation q = nCΔT Where q = heat needed to raise the temperature of n moles of a substance by ΔT , or Tfinal - Tinitial Study the sample problem on page 342. Also, review the table on page 343. What is the “about 25 joule” rule???

Molar Heat Capacity Example 1 from p. 342 #1 The molar heat capacity of tungsten is 24.2 J/K·mol. Calculate the heat energy needed to increase the temperature of 0.40 moles of tungsten from 285.0 K to 295.0 K.

Solution to Example 1 q = (0.40 moles)(24.2 J/K·mol)(295.0K -285.0K) q = nCΔT, so q = (0.40 moles)(24.2 J/K·mol)(295.0K -285.0K) q = 97 J

Molar Heat Capacity Example 2 from p. 342 #2 A sample of NaCl increased in temperature by 2.5 K when it absorbed 1.7 x 102 J of heat. How many moles of NaCl were heated if the molar heat capacity of NaCl is 50.5 J/K·mol?

Solution to Example 2 q = nCΔT, so solve for n n = q__ CΔT n = 1.7 x 102 J (50.5 J/K·mol)(2.5K) (ans. 1.3 mol)

Specific Heat The quantity of energy needed to raise the temperature of 1 gram of a substance one degree Kelvin (or Celsius). Symbol is cp Unit is J/g·K We use the equation q = mcpΔT Where m is mass in grams *Look at the sample problem on page 61.

Specific Heat Example 3 from p. 61 #1 Calculate the specific heat of a substance if a 35 g sample absorbs 48 J as the temperature is raised from 293. K to 313. K.

Solution to Example 3 q = mcpΔT, so solve for cp cp = __q__ mΔT Cp = 48 J________ (35g)(313K – 293K) (ans. 0.069 J/g·K)

Specific Heat Example 4 from p. 61 #4 How much energy in kJ must be transferred to raise the temperature of a 55 g sample of aluminum from 22.4 ˚C to 94.6 ˚C? The specific heat of aluminum is 0.897 J/g·K. (ans. 3.6 kJ)

Molar Heat Capacity and Specific Heat are Related Because molar mass is the mass of one mole, the following equation is true: M x cp = C, where M is molar mass Or, looking at units, (g/mol) x (J/g·K) = (J/K·mol)

Chapter 10 Causes of Change Section 2 Using Enthalpy

Molar Enthalpy Change We can calculate the change in enthalpy for one mole of a pure substance if we know that change in temperature that occurs. Molar enthalpy change, or ΔH = CΔT Notice that only applies to a change in kinetic energy (heating or cooling), and NOT chemical reactions or changes of state.

Molar Enthalpy Change Example from p. 346 #2 Calculate the ΔH of NaCl when it is heated from 0.0 ˚C to 100.0 ˚C. The molar heat capacity of NaCl(s) is 50.5 J/K·mol. (ans. 5.05 x 103 J/mol)

Using Enthalpy Changes For a reaction, ΔH = Hproducts – Hreactants If ΔH is positive, the reaction is endothermic If ΔH is negative, the reaction is exothermic

Endothermic Energy Diagram

Exothermic Energy Diagram

Which is endothermic? Exothermic?

Standard Enthalpy Change An enthalpy change that is measured when reactants in the standard states change to products in their standard states (25 ˚C and 1 atm pressure) Denoted as ΔH˚

Using Enthalpy Changes How much heat will be released if 1.0 gram of hydrogen peroxide (H2O2) decomposes in a bombardier beetle to produce a steam spray? 2 H2O2(l) −> 2H2O(l) + O2 (g), ΔH˚= -190 kJ (ans. -2.8 kJ)

And here he is….

Solution 1.0 g H2O x 1 mol H2O2 x -190 kJ = -2.8 kJ 34.0 g H2O2 2 mol H2O2 Notice that the value given for the standard enthalpy correlates to the molar ratio in the equation.