chem - energetics

define temperature

the average kinetic energy of molecules (K)

define heat

the amount of energy exchanged due to a temperature difference between two substances (J)

define thermochemistry

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

define a chemical system

a set of reactants and products under study, usually represented by a chemical equation

define enthalpy

the amount of energy or heat content of a substance

where is energy stored in a substance? what energy is included?

in the chemical bonds. includes kinetic and potential energy and is not measured directly but changes are measured (J)

define standard enthalpy change of reaction. what are standard conditions?

the diff between the enthalpy of the products and the enthalpy of the reactant

SC: 298K and 1 × 10⁵ Pa

what is the equation for change in enthalpy?

change in H = H_products - H_reactants

if change in H is negative, what type of reaction is it?

exothermic

if change in H is positive, what type of reaction is it?

endothermic

what does it mean about the substance when the enthalpy is high?

less stable

what does E represent? is this reaction endothermic or exothermic?

E represents change in energy - reaction is exothermic

are reactants or products more stable?

products

name two examples of exothermic reactions. what are three characteristics about exothermic products?

• combustion reactions

• neutralization reactions

have less stored energy

more stable

have stronger bonds

what is the equation for energy? what are the units for each?

Q=mc(change in temperature)

m = mass in g

c= specific heat capacity of substance JK^-1g^-1

T= change in temperature in K

how is energy of surroundings related to energy of the system

q_surroundings = -q_system

what are four examples of kinetic energy

• electron motion in an atom

• vibrations of atoms connected by chemical bonds

• rotation of molecules

• translation of molecules; motion of molecules through space

what are four examples of potential energy?

• forces of attraction between molecules

• chemical bond energy present within the molecule

• energy associated with electrons

• forces holding nuclear particles together

what does Hess’ Law state?

the value of change in H for any reaction that can be written in steps equals the sum of the values for change in H for each of the individual steps

What are the two rules for Hess’ Law?

• if the rxn is reversed, H sign is flipped

• if the rxn is multiplied by a factor, H is multiplied by same factor

define the standard enthalpy change of formation (H⁰_f)

the enthalpy change that results when one mole of a compound is formed from its element at 298K and 1 × 10⁵ Pa and at their standard states.

how do you calculate change in H using bond enthalpies?

H = BE_broken - BE_formed

define average bond enthalpy

the amount of energy required to break one mole of bonds in the gaseous state averaged across a range of compounds containing that bond

what is the Born-Haber Cycle?

elements from standard states through multiple steps to form the solid compound

What are the 3 steps of the Born-Haber Cycle?

1) elements to gases (standard enthalpy change of atomisation)

• Na_(s) —> Na_(g)

• Cl_2(g) —> 2Cl^._(g) —> breaking bond energy, not state change

2) gas atoms to anions (electron affinity)

• Cl^. —> Cl^-

3) metal atoms to cations (ionization energy)

• Na —> Na⁺

define lattice enthalpy

the energy required to convert 1 mole of the solid compound into gaseous ions

draw the Born-Haber Cycle

give the equation of the Born Haber cycle

H⁰_atom + EA + IE = H⁰_f + H⁰_LE

define enthalpy of atomisation

the energy change required to change one mole of atoms from their standard states to their gaseous states

define ionization energy

minimum energy required to remove one mole of electrons from one mole of atoms or positive ions in the gaseous state