Thermochemistry

Energy is the capacity to do work

• Thermal energy is the energy associated with the random motion of atoms and molecules

• Chemical energy is the energy stored within the bonds of chemical substances

• There are also other forms of energy, such as nuclear, potential, and electrical

Heat is the transfer of thermal energy between two bodies that are at different temperatures.

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.

Temperature = Thermal Energy

900C400C

greater thermal energy6.2

Which one has greater thermal energy?

Law of Conservation of Energy:

• Energy can not be created or destroyed. It can only change form.

• What types of energy are involved when you eat a burger?

Thermochemistry is the study of heat change in chemical reactions.

The system is the specific part of the universe that is of interest in the study.

open

mass & energyExchange:

closed

energy

isolated

nothing

SYSTEMSURROUNDINGS

6.2

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.

Endothermic process is any process in which heat has to be supplied to the system from the surroundings.

2H2 (g) + O2 (g) 2H2O (l) + energy

Here’s a video of this reaction: Can you tell energy is a product?

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

6.2

energy + H2O (s) H2O (l)

Exothermic and Endothermic

• Exothermic

• Chemical Thermal

• Releases heat!

• Endothermic

• Thermal Chemical

• Absorbs heat!

Firewood burning Ice melting

Product: 127 kJ/mol

Reactants: 520.5 kJ/mol

Reactants: 139.3 kJ/mol

Products: 720 kJ/mol

H = H (products) – H (reactants)

H = heat given off or absorbed during a reaction

Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.

Exothermic: negative ΔH Endothermic: positive ΔH

Reaction Endo or Exo?

N2(g) + O2 (g) 2NO (g) ∆H = 168 kJ/mol

2H2(g) + O2 (g) 2H2O (l) ∆H = -571 kJ/mol

2 P(s) + 5 Cl2(g) 2 PCl5(s)   ∆H = -880 kJ/mol

H2O(l) H2O(g) ∆H = 44 kJ/mol

Endo!

Exo!

Exo!

Try It!

Endo!

The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.

Heat (q) absorbed or released:

q = mct

t = tfinal - tinitial

6.4

I Do: How much heat is given off in Joules when an 869 g iron bar cools from 940C to 50C? Is this process endothermic or exothermic?

q = mct

6.4

We Do: 71.5g of aluminum metal is heated from 20°C to 50°C in order to increase its malleability. What amount of heat is absorbed by the aluminum?

Is this process endothermic or exothermic?_____________

q = mct

You Do: A 2.0-gram sample of graphite cools from 10°C to 0°C. How much heat was released from the graphite sample?

Endothermic or exothermic? _______________

q = mct

Determining Endo/Exothermicity

• To determine if a reaction is exothermic or endothermic, we measure the temperature of the surroundings.

CalorimetryCalorimetry is the quantitative measurement of the heat

required or evolved during a chemical process.

For example, if the energy from an exothermic chemical reaction is absorbed in a container of water, the change in temperature of the water provides a measure of the amount of heat added.

The enthalpy of solution (Hsoln) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.

Hsoln = Hsoln - Hcomponents

6.6

Which substance(s) could be used for melting ice?

Which substance(s) could be used for a cold pack?

LiCl and CaCl2

NaCl, KCl, NH4Cl, NH4NO3