SCH 4U
• ENERGY CHANGES AND RATES OF REACTION
The Energy of Physical, Chemical and Nuclear Reactions
• Thermodynamics - the study of energy and energy transfer
• Thermochemistry - the study of energy involved in chemical reactions
Energy (symbol: E – unit: J joules)
Law of Conservation of Energy
- total energy of the universe is constant
- energy can neither be created nor destroyed Δ universe=0 𝐸- energy can be transferred from one substance to another
- energy can be converted into various forms
System - part of the universe that is being studied and
observed (reactants & products)
Surroundings - everything else in the universe
- part of the universe that is likely to be affected by
energy changes in the system
Universe = System + Surroundings Δ𝐸universe= Δ𝐸system+Δ𝐸surrounding = 0
Any change in the system is accompanied by an equal
and opposite change in the surroundings. Δ𝐸system =−Δ𝐸surrounding
Heat (symbol: Q – unit: J joules) - transfer of kinetic
energy (Ek) spontaneously from warmer to cooler
objects
Temperature (symbol: T – unit: K Kelvin, °C
Celsius degrees) TK = T°C + 273.15
- measure of the average kinetic energy of the
particles that make up a substance or system
Celsius scale – relative to water (0°C water melting
point, 100°C water boiling point)
Kelvin scale – absolute scale (O K is temp. when a
substance has no kinetic energy)
Enthalpy (symbol: H – unit: J joules)
- total internal energy of a substance at a constant pressure
- hard to measure; includes (1) energy of physical state, (2)
energy in bonds, (3) energy in nucleus; includes:
- moving electrons within atoms
- vibration of atoms connected by chemical bonds
- rotation and translation of molecules
- nuclear potential energy of protons and neutrons in atomic
nuclei
- electronic potential energy of atoms connected by chemical
bonds
- easily measure and study change in enthalpy (ΔH) in a
reaction
- enthalpy change of a process is equivalent to its heat change
at constant pressure
-chemical bonds are sources of stored energy
-breaking a bond is a process that requires energy
- creating a bond is a process that releases energy
Endothermic Reaction:
- system absorbs E
- more energy needed to break bonds than released by creating bonds
- change in enthalpy is positive (+)
Exothermic Reaction:
- system releases E
- more energy released by creating bonds than needed to break bonds
- change in enthalpy is negative (-)
Representing Enthalpy Changes
• ΔHrxn enthalpy of reaction (depends on
temperature and pressure)
• ΔH°rxn standard enthalpy of reaction at SATP
(25°C, 100 kPa)
(1) Thermochemical Equation – balanced equation that indicates enthalpy change
(2) Separate Expression
(3) Enthalpy Diagram
1.Physical changes
- Small change in enthalpy (tens of kJ/mol) - Change in intermolecular forces between molecules as they change phase o ΔHvap enthalpy of vaporization (liquid gas) o ΔHcond enthalpy of condensation (gas liquid) o ΔHmelt enthalpy of melting (solid liquid) o ΔHfre enthalpy of freezing (liquid solid)
o ΔHsoln enthalpy of solution (solid aqueous – dissolving)
2. Chemical changes
- Moderate change in enthalpy (hundreds of kJ/mol)
- Change in intramolecular bonds within molecules as reactants break apart to form products o ΔHcomb enthalpy of combustion
o ΔHneut enthalpy of neutralization
o ΔHf enthalpy of formation
3. Nuclear changes
- Enormous change in enthalpy (billions of kJ/mol)
- Change in nuclear binding energy holding together the nucleus of an atom o A significant amount of mass of the reactants is actually converted to energy
o Einstein: E=mc2, where m=mass and c=speed of light (3.0x108m/s) so a tiny mass is equivalent to a significant amount of energy
Mass Defect o difference in mass between a nucleus and its nucleons (particles found in the nucleus)
o caused by the energy associated with the strong force that holds a nucleus together
o the higher the nuclear binding energy, the more stable the nucleus
Nucleus + Nuclear binding energy Nucleons
