Topic 5

• Thermochemistry• Thermodynamics• Energy• Heat• calorie/Calorie• Joule• Energy transfers occur btwn the

system and its surroundings

calorie is the amount of heat energy needed to raise the temperature of 1 g of water 1oC

Calorie is 1000 caloriesFood energy is measured in Calories

Fats are 9 Calories/g Carbs and proteins are 4 Calories/g

aka Law of conservation of energyEnergy in the universe is constant,

cannot be created or destroyedEnergy can be converted to different

forms

System transfers heat to its surroundings

Temperature of the surroundings increases

“feels hot”Potential energy of the system is

converted to heat energy that is released (system does not “cool off”)

System absorbs heat from the surroundings

Temperature of the surroundings decreases

“feels cool”Heat energy from the surroundings

is converted into potential energy in the system

1. Imagine an ice cube melting in your hand. Is the melting of ice endothermic or exothermic? Explain using the terms system and surroundings

2. Imagine warming your hands near a campfire. Is a campfire an endothermic or exothermic process? Explain.

Property of a system that explains heat flow between the system and its surroundings (constant P)

State function (middle steps don’t matter, just the beginning and end of a reaction)

• Defined as heat absorbed by the system during a physical or chemical change

• H is positive for endothermic rxns (because heat is absorbed by the system)

• H is negative for exothermic rxns (because heat is released by the system)

• Expressed in kJ or kJ/mol• Magnitude of H is directly proportional to

moles of reactants and products

Hrxn = heat of reaction of any chemical reaction

Hcomb = heat of combustion for combustion reactions (rxns with O2) only

Hfus = heat of fusion when a solid melts

• Hvap = heat of vaporization when a liquid vaporizes

• HBDE = bond dissociation energy or the heat required to break a bond

• Hf = heat of formation or the heat change when a compound is formed from its elements

• Hsols = heat of solution or the heat change when a solute dissolves in a solvent

How many kJ of heat are absorbed when 25.0 g of methane burn in air? Methane has a Hcomb of -802 kJ/mol.

If H is positive for the forward reaction, then it will be equal in size but opposite in sign for the reverse reactionH2O (s) H2O (l) H = +6.0 kJ/molH2O (l) H2O (s) H = -6.0 kJ/mol

The sign for H is positive or negative depending on the direction of energy flow

The sign does NOT indicate a positive or negative value for energy

What scientific law requires that the magnitude of the heat change for a forward and reverse reactions be the same with opposite signs? Explain.

H for phase changes from solid to liquid and liquid to gas are ALWAYS positive (endothermic/absorb heat)

H for phase changes from gas to liquid and liquid to solid are ALWAYS negative (exothermic/release heat)

Why is this? What is the sign of H for the process of sublimation? Deposition?

aka. measurement of heat flow Calorimeter measure heat flow Heat capacity (C) = amount of heat

required to raise the temperature of any object 1oC. Expressed in J/K or J/oC

We will more often use specific heat capacity (Cp) which is the capacity of 1 g of a substance

Water has a Cp of 1 cal/gK or 4.184 J/gK

Why is the Cp of water 1 cal/gK? What is the conversion between calories and joules?

What is the molar heat of combustion of liquid ethanol if the combustion of 9.03 grams of ethanol causes a calorimeter to increase in temperature by 3.54 K? The heat capacity of the calorimeter is 75.8 kJ/K.

If a reaction is carried out in a series of steps the H of the overall reaction is equal to the sum of the H’s for each individual step.

Useful for determining H for reactions that are difficult to measure directly, like sulfur trioxide…

The overall reaction is2S (s) + 3O2 (g) 2SO3 (g)

The reaction occurs in 2 measurable stepsS (s) + O2 (g) SO2 (g) Hrxn = -269.9 kJ

2SO2 (g) + O2 (g) 2SO3 (g) Hrxn = -196.6 kJ

To get the total H for the reaction, manipulate the equation steps like an algebraic equation.

Whatever you do to the reaction, you must also do to H.

Reaction that produces 1 mole of a substance from its constituent elements in their most stable thermodynamic state

To form 1 mole of HI, the equation looks like this:½ H2 + ½ I2 HI H = +25.94

kJ

Heat absorbed when 1 mole of a substance is formed from its elements in their standard states at 25oC and 1 atm.

There is a BIG table in the back of your book listing standard heats of formation.

Write the thermochemical equation associated with the standard heat of formation of AlCl3. What is the Ho

f for this equation?

The table of standard heats of formation includes elements, ions, and compounds.

The Hof of pure elements is always

zero.

Hess’s law allows us to calculate the Ho

rxn for just about any reaction.Breaking the overall reaction into the

formation reactions for both products and reactants and then putting them all together like this:Ho

rxn = nHof products – mHo

f reactants

Calculate the standard enthalpy change for the combustion of 1 mole of liquid ethanol. Tip: carefully watch the signs of Ho

f.

What is:1. The enthalpy of sublimation of solid

calcium?2. The heat of solution of gaseous ammonia?3. The bond dissociation energy of hydrogen

gas?4. The heat change when gaseous bromine

condenses to a liquid?Write chemical equations to illustrate your

answers.