Measuring and Expressing Enthalpy Changes Prentice Hall Chapter 17.2 Dr. Yager.

Measuring and Expressing Measuring and Expressing Enthalpy ChangesEnthalpy Changes

Prentice Hall Chapter 17.2 Prentice Hall Chapter 17.2 Dr. YagerDr. Yager

ObjectivesObjectives

DescribeDescribe how calorimeters are used to how calorimeters are used to measure heat flow.measure heat flow.

ConstructConstruct a thermochemical equation. a thermochemical equation.

SolveSolve for enthalpy changes in chemical for enthalpy changes in chemical reactions by using heats of reaction.reactions by using heats of reaction.

A burning match releases heat to its A burning match releases heat to its surroundings in all directions. How much heat surroundings in all directions. How much heat does this exothermic reaction release? You does this exothermic reaction release? You will learn to measure heat flow in chemical and will learn to measure heat flow in chemical and physical processes by applying the concept of physical processes by applying the concept of specific heat.specific heat.

Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes.

In calorimetry, the heat released by the system is equal to the heat absorbed by its surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings.

CalorimetryCalorimetry

The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.

Constant-Pressure CalorimetersConstant-Pressure Calorimeters

The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system. The change in enthalpy is represented by ∆H.

Constant-Volume Calorimeters (∆U)Constant-Volume Calorimeters (∆U)

Calorimetry experiments can be performed at a constant volume using a bomb calorimeter.

∆H = ∆U + ∆(PV)

A small pebble is heated and placed in a foam cup calorimeter containing 25.0 mL of water at 25.0oC. The water reaches a maximum temperature of 26.6oC. How many calories of heat were released by the pebble?

qsys= ∆H = -qsurr = -m x C x ∆T

cal 40.- C1.6 x C g

cal 1.00 x 25.0g- q - ΔH

gssurroundin the to flowsheat C g

cal 1.00 C

OOsurr

OH2

,O

Thermochemical EquationsThermochemical Equations

In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product.

A chemical equation that includes the enthalpy change is called a thermochemical equation.

CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ

The heat of reaction is the enthalpy change for the chemical equation exactly as it is written.

CaO(s) + H2O(l) → Ca(OH)2(s) ∆H = -65.2kJ

Note: the sign changes because the heat is given off to the surroundings

17.2

Exothermic ReactionExothermic Reaction

Endothermic ReactionEndothermic Reaction

The production of iron and carbon dioxide from iron(III) oxide and carbon monoxide is an exothermic reaction. How many kilojoules of heat are produced when 3.40 mol of Fe2O3 reacts with an excess of CO?

Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) + 26.3 kJ

For every mole of Fe2O3, 26.3 kJ are given up to the surroundings . ∆H = -26.3 kJ

Heat produced = 26.3 kJ/mol x 3.40 mol = 89.3 kJ

H = -89.3 kJ

Heat absorbed by surroundings = 89.3 kJ

H = 89.3 kJ

The production of iron and carbon dioxide from iron(III) oxide and carbon monoxide is an exothermic reaction. How many kilojoules of heat are produced when 3.30 mol of CO reacts with an excess of Fe2O3?

Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) + 26.3 kJ

For every 3 moles of CO, 26.3 kJ are given up to the surroundings . ∆H = -26.3 kJ

Heat produced = 26.3 kJ x 3.30 mol CO = 28.9 kJ 3 mol CO

Heat absorbed by surroundings = 28.9 kJ

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

1.1. The change in temperature recorded by the The change in temperature recorded by the thermometer in a calorimeter is a measurement ofthermometer in a calorimeter is a measurement of

a.a. the enthalpy change of the reaction in the the enthalpy change of the reaction in the calorimeter.calorimeter.

b.b. the specific heat of each compound in a the specific heat of each compound in a calorimeter.calorimeter.

c.c. the physical states of the reactants in a the physical states of the reactants in a calorimeter.calorimeter.

d.d. the heat of combustion for one the heat of combustion for one

substance in a calorimeter.substance in a calorimeter.

1.1. The change in temperature recorded by the The change in temperature recorded by the thermometer in a calorimeter is a measurement ofthermometer in a calorimeter is a measurement of

a.a. the enthalpy change of the reaction in the the enthalpy change of the reaction in the calorimetercalorimeter..

b.b. the specific heat of each compound in a the specific heat of each compound in a calorimeter.calorimeter.

c.c. the physical states of the reactants in a the physical states of the reactants in a calorimeter.calorimeter.

d.d. the heat of combustion for one the heat of combustion for one

substance in a calorimeter.substance in a calorimeter.

2. For the reaction: 2. For the reaction: CaO(CaO(ss) + H) + H22O(O(ll) ) Ca(OH) Ca(OH)22((ss), ), H = H = 65.2 kJ. 65.2 kJ.

This means that 62.5 kJ of heat is __________ This means that 62.5 kJ of heat is __________ during the process.during the process.

• absorbedabsorbed

• destroyeddestroyed

• changed to masschanged to mass

• releasedreleased

3.3. How much heat is absorbed by 325 g of How much heat is absorbed by 325 g of water if its temperature changes from water if its temperature changes from 17.0°C to 43.5°C? The specific heat of 17.0°C to 43.5°C? The specific heat of water is 4.18 J/g°C.water is 4.18 J/g°C.

a.a. 2.00 kJ2.00 kJ

b.b. 3.60 kJ3.60 kJ

c.c. 36.0 kJ36.0 kJ

d.d. 360 kJ360 kJ

4. Which of the following is a thermochemical 4. Which of the following is a thermochemical equation for an endothermic reaction?equation for an endothermic reaction?

a.a. CHCH44((gg) + 2O) + 2O22((gg) ) CO CO22((gg) + 2H) + 2H22O(O(gg) + 890 kJ) + 890 kJ

b.b. 241.8 kJ + 2H241.8 kJ + 2H22O(O(ll) ) 2H2H22((gg) + O) + O22((gg))

c.c. CaO(CaO(ss) + H) + H22O(O(ll) ) Ca(OH) Ca(OH)22((ss) 65.2) 65.2

d.d. 2NaHCO2NaHCO33((ss) + 129 kJ ) + 129 kJ NaNa22COCO33((ss) + H) + H22O(O(gg) )

+ CO+ CO22((gg))

5. Oxygen is necessary for releasing energy 5. Oxygen is necessary for releasing energy from glucose in organisms. How many kJ of from glucose in organisms. How many kJ of heat are produced when 2.24 mol glucose heat are produced when 2.24 mol glucose reacts with an excess of oxygen?reacts with an excess of oxygen?

CC66HH1212OO66((ss) + 6O) + 6O22((gg) ) 6CO 6CO22((gg) + 6H) + 6H22O(O(gg) + ) +

2808 kJ/mol of glucose2808 kJ/mol of glucose

a.a. 4.66 kJ4.66 kJ

b.b. 9.31 kJ9.31 kJ

c.c. 1048 kJ1048 kJ

d.d. 6290 kJ6290 kJ

Measuring and Expressing Enthalpy Changes Prentice Hall Chapter 17.2 Dr. Yager.

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