Thermochemistry

The study of the transfer of heat energy

Let’s Revisit Intensive and Extensive Properties

• Restate in your own words:• Intensive Properties-

• Extensive Properties-

Definitions• ___________________—the study of heat or

thermal energy transfer and its application to chemical reactions and physical processes

• _______________—a measure of the total energy of a system

• _____________—the amount of heat that is required to change the temperature of that object by 1.00 degree Celsius (or Kelvin)

• _______________________________—the amount of heat energy needed to change the temperature of 1.00 gram of the substance by 1.00 degree Celsius

Different Types of Heat with Physical Processes• There is also a certain amount of energy associated

with phase changes.• There is a difference in the _____________ (the heat

that is associated with temperature changes) and the ___________ (the heat that is associated with a change in phase of a substance while the temperature remains constant)

• The heat associated with the phase change of solid to liquid is the latent heat of fusion (Hf). The heat required to change from a liquid to a vapor is called the latent heat of vaporization (Hv).

Heat with Temperature Change• The specific heat of a substance can be

determined experimentally by determining the quantity of heat transferred by a definite mass of the substance as its temperature rises or falls

• Specific heat (c) = quantity of heat transferred (q) mass of material (m) x temperature change (DT)

• Better known as:

Heating and Cooling Curve

Example 1: Temp Change

q = m c DT

How many joules of energy must be transferred from a cup of coffee to your body if the temperature of the coffee drops from 60.0 oC to 37.0 oC? Assume that the cup holds 250. mL and that the specific heat of coffee is the same as that of water, 4.18 J/goC.

Example 2: Phase Change

• q = m Hf for solid to liquid• q = m Hv for liquid to a vapor

Example 2:You have four ice cubes at 0oC. If each cube

has a mass of 9.63 g, how much heat is required to melt these ice cubes to liquid water at 0oC? The latent heat of fusion of ice is 333 J/g.

Heat Transfer

• When heat is transferred ______ the system, it is an endothermic process

• When heat is transferred ______ the system, the process is exothermic

• All energy transfers follow the first law of thermodynamics, the law of energy conservation. All of the energy transferred between a system and its surroundings must be _______________

• The heat transferred into (or out of) a system at a constant pressure is the _________________________

• Enthalpies are experimentally determined, and can be calculated for individual reactions.

Enthalpy of Formation

• One way to determine the DH of a reaction is to use a value called the enthalpy of formation (Ho

f)• The enthalpy of formation is defined as the energy needed

to form one mole of the compound from its elements• Example: The energy for H2 & O2 to form H2O• The values are always in units of kJ/mol• The Ho

f of an element is ______ kJ/mol because element cannot be formed from anything simpler than itself

• Enthalpies of formations are usually found in a reference table and are not something students memorize

• The enthalpy of an entire reaction can be calculated from the Ho

f of the reactants and products• The formula is:

DH = DHof (products) – DHo

f(reactants)

Example 3

DH = DHof (products) – DHo

f(reactants)

Given: DHof CO2(g) = -393.5 kJ/mol

DHof H2O(l) = -285.8 kJ/mol

DHof C5H12 (g) = -145.7 kJ/mol

Calculate the heat of the combustion reaction:

C5H12(g) + 8O2(g) 5CO2 (g) + 6H2O(l)

Graphing Enthalpy of Reactions• An ___________ reaction means that

the enthalpies of formation of the reactants is lower than the enthalpy of formations of the products

• An ____________ reaction that the enthalpies of formation of the products is lower than the enthalpy of formations of the reactants

Reaction Energy Diagrams

• Note that the energy of each reaction does not go directly from reactants to products

• There is an energy “hump” that must be overcome in order for a reaction to occur

• This energy barrier is also called the ____________ __________________________________________

• It is the amount of energy needed for a reaction to go from reactants to the transition complex (or activated complex) that must occur before the products can form. The energy associated with this activation step must be invested before the reaction will go to completion, no matter whether the net reaction is endothermic or exothermic.

Endothermic Pathway

• (a) = Energy of Activation (Ea)

• (c) = Enthalpy of reaction (DHrxn)

• Note that Enthalpy of reaction DHrxn is a ________________

Exothermic Pathway

• (a) = Energy of Activation (Ea)

• (c) = Enthalpy of reaction (DHrxn)

• Note that Enthalpy of reaction DHrxn is a ________________

Enthalpy Signs

A _________ enthalpy of reaction indicates that the reaction is endothermic, and the energy term can be written as if it is a reactant into the chemical equation. Any reaction that contains an energy term is known as a _____________________ equation

A _________ enthalpy value indicates that the reaction is exothermic. The energy term is written as a product in the thermochemical reaction. Note that when the exothermic enthalpy is written into the products, it does not carry its negative sign with it

The positive or negative sign in front of an enthalpy term simply indicates whether the reaction is endothermic or exothermic. By convention, the math is not performed with negative numbers.

Example 4

Carbon dioxide can be decomposed into carbon monoxide and oxygen.

2CO2 2 CO + O2

The DH for this reaction is 43.9 kJ• Is the reaction endothermic or exothermic?• If there is only 22.2 kJ of heat transferred,

how many moles of CO2 can be decomposed?

Measuring Heat Transfer

• _______________ is the process by which we measure heat transfer and its associated energy changes in the lab.

Example 5

• Suppose you heat a 55.0-g piece of silver in the flame of a Bunsen burner to 425.0oC and then you plunge it into a beaker of water. The beaker holds 600. g of water, and its temperature before you drop in the hot silver is 25.0oC. The final temperature of the silver and water mixture is 44.0oC. What is the specific heat of the silver? The specific heat of water is 4.18 J/goC.