sch 4u energy changes and rates of reaction. the energy of physical, chemical and nuclear reactions...

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