Notes from Mondays class

5.1 Changes in Matter and Energy

Thermochemistry – The study of the energy changes that accompany physical or chemical changes in matter.

-energy can be transferred or converted onto other forms.

Eg:

The Energy of the Universe is ________________

Heat and energy Changes (p298)

What energy changes are involved in lighting a match?

In this example we can define:

Chemical system:

Surroundings:

Types:

Heat (Q) -

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-

Temperature (T) –

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Calorimetry: Measuring Energy Change (p300)

q = m c D T

specific heat capacity: quantity of heat needed to raise the temperature of a gram by 1 °C

ice: 2.0 J/(g°C)

Water: 4.18 J/(g°C)

Steam: 2.01 J/(g°C)

Aluminum: 0.900 J/(g°C)

Calculating Quantity of Heat (q)

When 600mL of water in an electric kettle is heated from 20°C to 100°C how much heat flows into the water:

Heat Transfer and Enthalpy Change (p302)

In any system there are Kinetic and Potential energies

Enthalpy (H) – total internal energy of a substance at a constant pressure.

Scientist have not found a way to measure the sum of all these energies

We study the changes in enthalpy (DH): the difference in enthalpies of reactants and products during a change.

-enthalpy change is equal to the change of energy to the surrounding

- energy is either added or removed from the surroundings

Types of enthalpy change:

Endothermic change –

Exothermic Change –

Ex: In a calorimetry experiment 7.46 g of potassium is dissolved in 100mL of water at an initial temperature of 24.1°C. The final temperature of the solution is 20.0°C. What is the molar enthalpy of solution of potassium chloride?

Representing Exothermic Reactions (5.3)

Method 1: use a thermochemical equation- balanced chemical equation that indicates the amount of heat released

H2(g) + 1/2 O2(g) ® H2O(l) + 285.8kJ .

Method 2: thermochemical with DH values

H2(g) + 1/2 O2(g) ® H2O(l) DH rxn = -285.8 kJ

Method 3: Molar Enthalpies

: The energy change associated with the reaction of one mole of a substance. Measured at SATP

DH°_f = -285.8 kJ/mol H2O(l)

Other types:

Method 4: Enthalpy diagram

Stoichiometry and Thermochemical Equations

Ex. Aluminum reacts readily with chlorine gas to produce aluminum chloride. The reaction is highly exothermic.

2 Al(s) + 3Cl2(g) ® 2AlCl3(s) DHo = -1408kJ

a) What is the enthalpy change for 1 mole AlCl3?

b) What is the enthalpy change when 1.00 kg of Al reacts with excess Cl2?