Thermochemistry. n Thermochemistry is the study of _________________ during chemical reactions.

Thermochemistry

Thermochemistry is the study of _________________ during chemical reactions.

Energy

Capacity to do ______________ or supply ______________

2 Types of Energy – ______________ - stored energy– ______________ - energy of motion

Energy is weightless, odorless, and tasteless

Energy

Kinds of ______________ and their arrangements in a substance determines the amount of ______________ stored in that substance.

Heat Energy

Energy ______________ from one object to another causing a temperature change.

It cannot be detected - only the ______________ by heat can be measured.

Heat Energy

Thermometers measure the average ______________, not heat.

Heat flows from a ______________ object to a ______________ 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 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 ______________ heat.– Value is ______________.– Symbol - +H

______________ - a process in which the system ______________ heat.– Value is ______________.– Symbol - -H

Units of Heat

Calorie - food 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 ______________ 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 ______________ 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. 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. The student then adds the metal to a calorimeter that has 100 mL of water at 22.5C. Calculate the specific heat of the metal if the final temperature of the metal-water mixture is 38.7C.

Thermochemical Equations

An equation that includes the ______________.

Reactant

Heat as a reactant will give a ______________ q value.

Heat as a reactant will be an ______________ process.

Product

Heat as a product will give a ______________ 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 standard conditions.– 1 atm or 101.3 kPa and 25°C

Hrxn

Heat of Combustion

Complete ______________ of 1 mole of a substance.

Hcomb

Heat of Fusion

Heat ______________ by one mole of a solid to change it to a liquid.

Hfus

Heat of Solidification

Heat ______________ by one mole of a liquid 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 ______________.

Hvap

Heat of Condensation

Amount of heat needed to change 1 mole of ______________ to a ______________.

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 = nHf(products) - mHf(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