CHAPTER 17THERMOCHEMISTRY

ENERGY

Energy is the capacity to do work or to supply heat.

Various forms of energy include potential, kinetic, and heat.

Chemical potential energy is energy stored in chemicals because of their compositions.

ENERGY

Heat (q) is a form of energy that always flows from a warmer object to a cooler object.

Thermochemistry is the study of the heat changes that occur during chemical reactions and physical changes of state.

ENERGY

Remember the law of conservation of energy?

In any chemical or physical process, energy is neither created nor destroyed.

All energy can be accounted for as work, stored energy, or heat.

HEAT CAPACITY

The SI unit of heat and energy is the joule (J).

The amount of heat it takes to change an object’s temperature by 1ºC is the heat capacity (J/ºC) of that object.

One calorie is the quantity of heat that raises the temperature of 1 gram of water 1ºC.

1 Calorie = 1000 calories and 1 Joule = 0.239 cal

EXAMPLE

If it takes 600.0 Calories to raise the temperature of a sample from 25.01ºC to 26.01ºC, what is the sample’s heat capacity in correct SI units?

SPECIFIC HEAT CAPACITY

The specific heat capacity (C), or specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 gram of the substance 1°C.

J/(g×°C)C = q OR q = mC△T

m △T

EXAMPLE

What would the temperature of a 1.25 kg piece of glass be if 1000.0 calories of heat were to be added to it at STP?

EXOTHERMIC/ENDOTHERMIC

A system is any specific part of the universe that attention is focused on.

The surroundings include everything outside the system.

The system and surroundings make up the universe.

EXOTHERMIC/ENDOTHERMIC

A process that absorbs heat from the surroundings is called an endothermic process.

Surroundings cool down

System heats upHeat change > 0 (positive)

EXOTHERMIC/ENDOTHERMIC

A process that loses heat to the surroundings is called an exothermic process.

Surroundings heat up

System cools down

Heat change < 0 (negative)

CALORIMETRY

Calorimetry is the accurate and precise measurement of the heat change for chemical and physical processes.

The enthalpy change (△H), is the heat change(q) for a process at constant pressure.

q = △H = mC△T

EXOTHERMIC/ENDOTHERMIC

To measure heat changes using calorimetry...1. Measure initial temperature of

water.2. Perform reaction inside insulated container.3. Record final water temperature.4. Use specific heat of water to

measure enthalpy change with the formula △H = mC△T.

Example Problem

The following acid-base reaction is performed in a coffee cup calorimeter:

H+(aq) + OH-(aq) --> H2O(l)

The temperature of 110 g of water rises from 25.0°C to 26.2°C when 0.10 mol of H+ is reacted with 0.10 mol of OH-. Calculate qwater Calculate ΔH for the reaction Calculate ΔH if 1.00 mol OH- reacts with 1.00 mol

H+

Solution

q = mCΔT where q is heat flow, C is the specific heat, m is

mass in grams, and Δt is the temperature change. Plugging in the values given in the problem: qwater = 4.18 (J / g·°C;) x 110 g x (26.6°C - 25.0°C) qwater = 550 J ΔHrxn = -(qwater) = - 550 J

We know that when 0.10 mol of H+ or OH- reacts, ΔH is - 550 J 1.00 mol x (-550 J/0.10 mol) = -5.5 x 103 J (or -55 KJ)

THERMOCHEMICAL EQUATIONS

A chemical equation that includes the amount of heat produced or absorbed during the reaction is the thermochemical equation.

The heat of reaction explains the heat released or absorbed during a chemical reaction (the change in enthalpy, △H).

THERMOCHEMICAL EQUATIONS

The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.

The heat absorbed by one mole of a substance while melting is the molar heat of fusion (△Hfus).

The heat lost when one mole of a substance freezes is called the molar heat of solidification (△Hsolid).

Molar heat of vaporization Molar heat of condensation The molar heat of solution (△Hsoln), is the heat change caused by dissolution of one mole of a substance.

Potentially Useful Information

For water… ΔHfus = 6.01 KJ/mol

ΔHvap= 40.7 KJ/mol

What about…• ΔHsolid

• ΔHcond

Phase Changes and Enthalpy

Sample Problem

Calculate the total enthalpy change when 42.0 grams of ice at -10oC is warmed to water at 20oC.

HESS’S LAW

Hess’s law of heat summation states that if you add two or more thermochemical equations to give a final equation, then you can also add the heat changes to give the final heat change.

HESS’S LAW

Did you read pages 527-529?Did you complete problem 35 on page 532?

Example Problem

Calculate the value of Ho/kJ for the following reaction using the listed thermochemical equations:

2 F2(g) + 2 H2O(l) 4 HF(g) + O2(g)

H2(g) + F2(g) 2 HF(g) Ho/kJ = -542 kJ2 H2(g) + O2(g) 2 H2O(l) Ho/kJ = -572 kJ

More Practice

http://lrc-srvr.mps.ohio-state.edu/under/chemed/qbank/quiz/bank4.htm

STANDARD HEATS OF FORMATION

The standard heat of formation (△Hf°) of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements with all substances in their standard states at 25°C.

For free elements △Hf°= 0 in their standard states.

Standard heat of reaction... △H°= △Hf°(products)-△Hf°(reactants)*Page 530 – standard heats of formation table

Example Problem

Calculate the heat of reaction (kJ) using the listed heats of formation:

CH4(g) + 4 F2(g) CF4(g) + 4 HF(g)

Hof CH4 = -74.86 kJ/mole

Hof CF4 = -925.0 kJ/mole

Hof HF = -271.1 kJ/mole