Thermochemistry(UNIT 2)

Grade 12 Chemistry SCH4U0

What is THERMOCHEMISTRY?

THERMOCHEMISTRY is THERMOCHEMISTRY is the study of the the study of the energy changes that energy changes that accompany physical or accompany physical or chemical changes in chemical changes in matter.matter.

Energy Transformations

Heat vs. Temperature Heat (q) is the amount of

energy transferred between substances(Units: Joules (J), kJ, kJ/mol)

Temperature (T) is the measure of the average kinetic energy (energy of motion) of the particles in a sample of matter(Units: oC, oK, oF)

Heat and Energy Changes What are the products when methane

combusts in air? CH4 + 2O2 CO2 + 2H2O + energy Energy that is released from this

CHEMICAL SYSTEM to the SURROUNDINGS is called thermal thermal energy (heat)energy (heat).

Since the molecules have greater kinetic energy, temp. of surroundings increases

System and Surroundings

The systemsystem is a well-defined part of the universe singled out for study.

The surroundingssurroundings is the remainder of the universe.

In a closed systemclosed system energy, but not matter, can be exchanged with the surroundings.

In an open systemopen system , both energy and matter can flow into our out of the system.

In an isolated systemisolated system (ideal system) neither matter nor energy can move in or out.

Exothermic and Endothermic Processes

Heat (q) is a form of energy transfer.

Units: 1 calorie (cal) = 4.184 J

q > 0 Heat is transferred from surrounding to system. Process is endothermic.

q < 0 Heat is transferred from system to surroundings. Process is exothermic.

System Surroundingsqq

A basic calorimeter )

Endothermic = absorbing energy

Law of conservation of energy = release and absorption of energy must be equal

Surroundings everything else

Exothermic = releasing energy

System with can as

boundary

Heat and Temperature Change

How is heat transferred related to the change in temperature of a system with mass m?

q = specific heat (c) m T

The specific heat (c) of a substance is the amount of heat required to raise the temperature of 1 gram by 1C.

Al 0.90 J/gC H2O 4.18 J/gC

Example Question 1 When 600 g of water in an electric kettle is

heated from 20 C to 85 C to a make a cup of Rupinder’s favorite tea, how much heat flows into the water?

Given: m = 600 g, ΔT = 85C – 20 C = 65 C

c = 4.18 J/gC (from Table 1, p.301) Required: q (amount of heat transferred) Solution: q = mcΔT = (600)(4.18)(65) =

163 kJ

Let's try this question now

If 150.0 grams of iron at 95.0 °C, is placed in an insulated container containing 500.0 grams of water at 25.0 °C, and both are allowed to come to the same temperature, what will that temperature be? The specific heat of water is 4.18 J/g °C and the specific heat of iron is 0.444 J/g °C)

Energy and Enthalpy

Chemists give a special symbol, ΔH(delta H) to the heat change in a reaction

This heat changeheat change is called ENTHAPLY

ΔH (enthalpy) = energy absorbed or released to the surroundings when a system changes from reactants to products.

ΔHsystem = - Qsurrounding

HEATHEATHEATHEATHEATHEATHEATHEAT

HEATHEATHEATHEATHEATHEATHEATHEAT

Molar Enthalpy ΔHx: the enthalpy change associated

with a physical, chemical or nuclear change involving 1 mol of a substance

(x – is the letter used to indicate the type of change that is occurring)[Units: kJ/mol]

ΔHvap ΔHsolΔHfr ΔHcomb (See Table1 p.306)

Types of Molar Enthalpies

Example: COMBUSTION: (ΔHcomb ) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) ΔHcomb

= - 890kJ

The amount of energy involved in a change depends on the quantity of matter undergoing that change. So, twice the mass of methane will release twice as much energy into the surroundings.

Thus, ΔH = n ΔHcomb (ΔHcomb is obtained from a reference

source)

Enthalpies of ReactionH is an extensive property, so H depends on the amounts of reactants and products.

What is H for the combustion of 11.0 g of CH4 in excess oxygen?

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) H = -802

kJ

= -550 kJ11.0 g CH44

4

CHg04.16CHmol

4CHmol1kJ802

Example Question (p.309)

In a calorimetry experiment, 7.46 g of KCl is dissolved in 100.0 mL of water at an initial T of 24.1 C. The final T of the solution is 20C. What is the molar enthalpy of solution of KCl?

Calorimetry of Physical and Chemical Changes

Three simplifying assumptions often used in calorimetry:

i) No heat is transferred between calorimeter and the outside environment

ii) Any heat absorbed or released by the calorimeter material is negligible

iii) A dilute aqueous solution is assumed to have a density and specific heat equal to that of water

Homework Questions

p. 308 # 1 - 3 p. 310 # 4 p. 311 # 7, 9