Thermochemistry

Thermochemistry

• Recall “thermo” – heat energy

• “Chemistry”- the study of matter, so

• Thermochemistry is the study of heat energy within matter or the heat energy changes that accompany chemical reactions.

Heat & Temperature

• Heat is the sum of the kinetic energies of the particles in a sample of matter.

• Temperature is the average kinetic energy of the particles in a sample of matter.

Which has more heat?

• A cup of hot water at 70˚C?

• A bathtub full of hot water at 70˚C?

They both have the same temperature, but the bathtub has more heat.

Specific Heat

• Specific Heat is the amount of heat energy (measured in Joules, J or kJ) required to raise the temperature of 1 gram of a substance by 1 degree C.

• It is the measure of how efficiently a substance absorbs heat. Water is very efficient and therefore important in calorimetry, the process of measuring the amount of heat released or absorbed in chemical processes.

Specific Heat Calculationsalso known as Heat Capacity, Cp

Cp = __q______ m x ∆T

q= the quantity of heat absorbed or released You may also see “q” as an upper case Q

m= mass in grams

∆T= change in Temperature

• Can you use the equation to solve for Q?

for m?

for ∆T ?

q = Cp x m x ∆T

ANSWERSq = Cp x m x ∆T

m = __q______ Cp x ∆T

∆T = __q______ m x Cp

Let’s work a problem. (See page 497 in text)

• A piece of metal with a mass of 4.68 g absorbs 256 J of heat when its temperature increases by 182 C.

1. What is the specific heat of the metal?

2. Identify the metal.

• 0.300 J/g x ⁰C

• Strontium (see table on page 495)

More Practice Problems

Page 498

12.If 335 g water at 65.5 ⁰C loses 9750 J of heat, what is the final temperature of the water?

13.The temperature of a sample of water increases from 20.0 ⁰C to 46.6 ⁰C as it absorbs 5650 J of heat. What is the mass of the sample?

ANSWERS12.Did you solve for ∆T? ∆T = __q______ m x Cp ∆T = __9750J_____ 335g x 4.184J/g x ⁰C ∆T = 6.956137097What was the FINAL temp.? Did it lose or gain heat according to the

problem?

65.5- 6.96 = 58.5⁰C

13. Did you solve for mass, m? m = __q______

Cp x ∆T m = 5650J__________

4.184J/g x 26.6

= 50.766g rounded to 50.8g

The quantity of heat transferred during temperature change depends on:

1. Nature of material2. Mass of matter3. Amount of temperature change

Thermochemistry equations

• This is an Exothermic reaction because the

4 Fe(s) + 3 O2(g) → 2Fe2O3(s) + 1652kJ

heat energy amount is on the product side of the equation.

• In an Endothermic reaction the heat energy amount is on the reactant side of the equation.

1652kJ + 2Fe2O3 (s) → 4 Fe(s) + 3O2(g)

Enthalpy

• Enthalpy is the heat content of a system at constant pressure.

The symbol is H.

The change in enthalpy for a reaction called the enthalpy heat of reaction & is

represented as:

∆H (rxn) = H(final) - H(initial)

OR

∆H (rxn) = H(products) - H(reactants)

CHANGES OF STATE (pages 502-505)

Molar Heat (enthalpy) of Vaporization, (∆Hvap)

Hint: vapor = gas so when a substance changes state from gas to liquid or liquid to gas, use the Heat of Vaporization values

Molar Heat (enthalpy) of Fusion, (∆Hfus)

Hint: fusion=molecules fused together as in solid, so when a substance changes state from solid to liquid or liquid to solid use the Heat of Fusion values.

Refer to Table 16-6 page 502

• Table 16-6 shows the standard enthalpies of Vaporization and Fusion

Note that the units are in

kJ/molewhich means when given mass, you will

need to calculate the mole value for that substance

Practice Problems

• Page 504 #20, #21, #22

Solution #20-

What is given in the problem?

Mass= 25.7 g of methanol (CH3OH)

Is this substance melting or freezing?

See chart on page 502. The Heat of fusion for methanol is 3.22kJ/mol.

The Heat of Vaporization is 35.2kJ/mol.

Which do you use?

Answer

• 32.05 g CH3OH = 1 mole

25.7g x 1mole x 3.22kJ = 2.58kJ/mol

32.05g mole

Solution #21

• Is this substance melting or freezing? Condensing or boiling?

• What substance is it?

• How many grams?

More Problems

• In class together:

• Page 881, Problems 3-8

• Page 525, Problems 78-85

You may work with a partner.

Show me your work as I come around to

check your answer.

Homework questions

HW -Answer questions in text page 524

(46, 47), 49, 52-55, 57-60

Note again

In Endothermic reactions heat is absorbed and the enthalpy value is on the reactant side of the equation

In Exothermic reactions, heat is released as a product and the enthalpy value is on the product side of the equation.

QUESTION?

• Is a combustion reaction an ENDO or EXOthermic reaction?

Is heat gained, absorbed OR

Is heat lost, released in a combustion reaction?

Where would the ∆H appear, on the reactant side or the product side?

Combustion is the same as what biological process?

• Would you expect the ∆H value to be a positive value or a negative value?

Why?

QUESTIONS???

• Answer questions 23-27 on page 505 of text.