Unit 12 Thermochemistry

______________________ is the study of heat changes during chemical reactions.

Energy Capacity to do work or supply heat 2 Types of Energy ____________________________- stored energy ____________________________- energy of motion Energy is weightless, odorless, and tasteless

Energy Kinds of atoms and their arrangements in a substance determines the amount of energy stored in that substance.

Heat Energy Energy transferred from one object to another causing a ______________ change. It cannot be detected - only the changes by heat can be measured.

Heat Energy Thermometers measure the average kinetic energy, not ______________. Heat flows from a warmer object to a cooler object.

Heat Energy All chemical reactions and changes in physical states have either an ______________ of heat or a ______________ of heat.

Heat Energy ______________ - What we focus on. ______________ - everything else. Heat flow is given in relation to the System

System and Surroundings The ______________ includes the molecules we want to study (here, the hydrogen and oxygen molecules). The ______________ are everything else (here, the cylinder and piston).

Heat Energy Law of Conservation of Energy - energy cannot be ______________ nor ______________, it just changes from one form to another.

2 Processes of Heat Energy ______________ - a process in which the system absorbs heat. Value is ______________. Symbol - +H ______________ - a process in which the system releases heat. Symbol - -H

Units of Heat Calorie - food calorie calorie - quantity of heat required to raise the temperature of 1 gram of water 1°C Joule - more commonly used (J) 1 C = 1000 cal 4.184 J = 1 cal

Heat Capacity The amount of energy needed to ______________ the temperature of an object 1°C

Specific Heat Amount of energy needed to raise 1 gram of a substance 1°C q = -(m • C • T) q = heat change m = mass C = specific heat T = change in temperature

Calorimetry Measurement of heat changes ______________ - heat content of a system at constant pressure Heat released = Heat absorbed Calorimeter is the device used to measure the change

Calorimeter

qlost = qgained If q = m • C • T, then: (m • C • T)lost = (m • C • T)gained

How much heat is needed to warm 250 g of water (about 1 cup) from 22°C (about room temperature) to near its boiling point, 98°C? The specific heat of water is 4.18 J/g•ºC.

Large beds of rocks are used in some solar-heated homes to store heat Large beds of rocks are used in some solar-heated homes to store heat. Assume that the specific heat of the rocks is 0.082 J/g-K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0°C.

In the laboratory, a student heats a 100 g sample of metal to 101. 5ºC In the laboratory, a student heats a 100 g sample of metal to 101.5ºC. The student then adds the metal to a calorimeter that has 100 mL of water at 22.5C. Calculate the specific heat of the metal if the final temperature of the metal-water mixture is 38.7C.

Thermochemical Equations An equation that includes the heat change.

Reactant Heat as a reactant will give a positive q value. Heat as a reactant will be an ______________ process.

Product Heat as a product will give a negative q value. Heat as a product will be an ______________ process.

2H2O(l)  2H2(g) + O2(g) Hrxn = 572 kJ 572 kJ + 2H2O(l)  2H2(g) + O2(g)

Al2O3(s)  2Al(s) + 3/2 O2(g) Hrxn = -1676 kJ Al2O3(s)  2Al(s) + 3/2 O2(g) + 1676 kJ

Heat of Reaction Conditions are ______________ conditions. Hrxn 1 atm or 101.3 kPa and 25°C Hrxn

Heat of Combustion Complete burning of ______________ of a substance. Hcomb

Heat of Fusion Heat absorbed by one mole of a ______________ to change it to a liquid. Hfus

Heat of Solidification Heat released by one mole of a ______________ to change it to a solid. Hsolid

Hfus = -Hsolid Same process just in opposite direction.

Heat of Vaporization Amount of heat needed to change 1 mole of a ______________ to a gas. Hvap

Heat of Condensation Amount of heat needed to change 1 mole of ______________ to a liquid. Hcond

Hvap = -Hcond Same process just in opposite direction

Heat of Solution Change in heat when dissolving a ______________ in a solvent. Hsoln

Hess’s Law of Summation Hess’s law states that “If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”

If we know the individual reactions that make up the final equation, we can determine the Hrxn for the final equation.

Individual Steps C3H8 (g)  3C(graphite) + 4H2 (g) 3C(graphite) + 3O2 (g)  3CO2 (g) 4H2 (g) + 2O2 (g)  4H2O (l)

C3H8 (g)  3C(graphite) + 4H2 (g) 3C(graphite) + 3O2 (g)  3CO2 (g) 4H2 (g) + 2O2 (g)  4H2O (l) _______________________________ C3H8(g) + 5O2(g)  3CO2(g) + 4 H2O(l)

Standard Heats of Formation An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.

Standard Heats of Formation Standard enthalpies of formation, Hf, are measured under standard conditions (25°C and 1.00 atm pressure).

When we do not know the individual steps of a reaction, we can use the standard heats of formation to calculate H. H = nHf(products) - mHf(reactants) where n and m are the stoichiometric coefficients.

C3H8(g) + 5O2(g)  3CO2(g) + 4 H2O(l) H = [3(-393.5 kJ) + 4(-285.8 kJ)] - [1(-103.85 kJ) + 5(0 kJ)] = [(-1180.5 kJ) + (-1143.2 kJ)] - [(-103.85 kJ) + (0 kJ)] = (-2323.7 kJ) - (-103.85 kJ) = -2219.9 kJ