Thermochemistry

The study of the energy changes that accompany physical, chemical or nuclear changes in matter

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Definitions: recall physics2

Energy (E): The ability to do work; measured in Joules (J)

Work: Amount of energy applied or transferred over a

distance Potential Energy (Ep):

Energy of object due to its position or composition; stored energy

Kinetic Energy (Ek): Energy of object due to its motion; movement

energy

Definitions: recall physics3

Thermal Energy (H): Total quantity of kinetic & potential energy within

a substance, Ep + Ek = H

Heat (q): The transfer of thermal energy from a warm

body to cooler body

Temperature: A measure of the average kinetic energy of

particles in a substance/object

Definitions: new ones4

Chemical System: The group of reactants & products being studied

Surroundings: The environment in which the chemical

reaction takes place; substance that is not part of the reaction

Open System Matter & Energy able to leave system

Closed System Energy able to leave system

Isolated System Neither matter nor energy able to leave system

System and Surroundings

can exchange energy and matter with surroundings

universe = system + surroundings

• the system is the sample being observed• the surroundings is everything else• interactions between a system and its surroundings involve exchange of

energy and matter

can exchange energy, not matter, with surroundings

cannot exchange matter or energy with surroundings

Three types of systems based on this type of exchange are:

CalorimetryMeasuring energy changes in a reaction

We measure the ΔT of the surroundings and calculate the thermal energy, q, lost or gained by the surroundings

-qsurroundings = +qsystem

q = mcΔT

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ΔT = Tfinal – Tinitial

Measurable properties of a system include:volume, mass, pressure, temperature, and specific heat capacity.

Calculating the Amount of Heat Entering and Leaving a System

A calculated property of a system includes heat (Q) that enters or leaves an object.

When heat enters a system, ΔT is positive and so is Q.

Specific Heat Capacity,(c);

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A 1.0 g sample of copper is heated from 25.0°C to 31.0°C. How much heat did the sample absorb?

LEARNING CHECK

The First Law of Thermodynamics:Energy is Conserved

The first law of thermodynamics states that:Energy can be converted from one form to another but cannot be created or destroyed.Since any change in energy of the universe must be zero,

ΔEuniverse = ΔEsystem + ΔEsurroundings = 0ΔEsystem = –ΔEsurroundings

• if a system gains energy, that energy comes from the surroundings

• if a system loses energy, that energy enters the surroundings

A 400 g iron rod is heated in a bunsen flame and then plunged into an insulated beaker containing 1.00 L of water (1000 g). The original temperature of the water was 20ºC. After the iron rod and the water have reached the same temperature, it is 32.8ºC. (specific heat capacity of iron is 0.45 J/gºC , and water is 4.18J/gºC) What was the original temperature of the iron rod?

USING THE FIRST LAW OF THERMODYNAMICS

EnthalpyOne way chemists express thermochemical changes is by a variable called enthalpy, H. The change in enthalpy, ΔH, of a system can be measured.It depends only on the initial and final states of the system, and is represented by

ΔH = ΔE + Δ(PV)

For reactions of solids and liquids in solutions, Δ(PV) = 0

If heat enters a system• ΔH is positive• the process is endothermic

If heat leaves a system• ΔH is negative• the process is exothermic

The Second Law of Thermodynamics

When in thermal contact, energy from hot particles will transfer to cold particles until the energy is equally distributed and thermal equilibrium is reached.

The second law of thermodynamics states that:

When two objects are in thermal contact, heat is transferred from the object at a higher temperature to the object at the lower temperature until both objects are the same temperature (in thermal equilibrium)

Definitions: new ones15

Enthalpy (H): another term for thermal energy Enthalpy Change (ΔH): the change in thermal

energy from reactants to products; this equals q under constant pressure:

ΔHsystem = qsystem ΔHsystem = Hproducts – Hreactants

Where qsurroundings is the measurable quantity Endothermic:

intake of energy, -qsurroundings & +qsystem & + ΔH Exothermic:

release of energy, +qsurroundings & -qsystem & - ΔH

Comparing Categories of Enthalpy Changes:

Enthalpy of Solution

The orange arrow shows the overall ΔH.

Three processes occur when a substance dissolves, each with a ΔH value.

1. bonds between solute molecules or ions break2. bonds between solvent molecules break3. bonds between solvent molecules and solute molecules or ions form

Sum of the enthalpy changes: enthalpy of solution, ΔHsolution

The ΔH for one phase change is the negative of the ΔH for the opposite phase change.

Heat must be added to or removed from a substance in order for the phase of the substance to change.

The ΔH for each phase change has a particular symbol.

For example, ΔHmelt is called the enthalpy of melting.

Thermochemical Equations and Calorimetry

Chemical reactions involve• initial breaking of chemical bonds (endothermic)• then formation of new bonds (exothermic)

ΔHr is the difference between the total energy required to break bonds and the total energy released when bonds form.

Thermochemical EquationsThermochemical equations include the enthalpy change.

The enthalpy term can also be written beside the equation.

Exothermic reaction:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + 890.8 kJ

Endothermic reaction:

N2(g) + 2O2(g) + 66.4 kJ → 2NO2(g)

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔHr = –890.8 kJ

N2(g) + 2O2(g) → 2NO2(g) ΔHr = +66.4 kJ

Enthalpy DiagramsEnthalpy diagrams clearly show the relative enthalpies of reactants and products.

• For exothermic reactions, reactants have a larger enthalpy than products and are drawn above the products.

• For endothermic reactions, the products have a larger enthalpy and are drawn above the reactants.

Molar Enthalpy Change ΔHn:

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Molar Enthalpy Change (ΔHn): enthalpy change associated with any change (physical, chemical or nuclear) in one mole of a substance; J/mol or kJ/mol

ΔHrxn = nΔHn

Common Molar Enthalpies22

Example: Ethanol is used to disinfect skin before

receiving a flu shot. How does this feel on your skin? The enthalpy of vaporization (ΔHvap)of ethanol is 38.6 kJ/mol. What is the enthalpy change (ΔHrxn) of this reaction if 1.0g of ethanol is rubbed on your skin?

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Determine ΔHcomb for 15.0 g of propane.

LEARNING CHECK

The molar mass of propane: 44.1 g/molTherefore, 15.0 g is 0.340 mol

ΔHcomb = ΔH°combΔHcomb = (0.340 mol) (–2219.2 kJ/mol)ΔHcomb = 755 kJ/mol

The symbol for a standard enthalpy change is ΔH°, read as "delta H standard" or, perhaps more commonly, as "delta H nought“. The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state (SATP (25° C and 100 kPa))

Expressing Molar Enthalpy Changes:1. As part of chemical equation:

2. Associated ΔH value:

3. Potential Energy Diagram:

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3) Potential Energy Diagram:26