Heat flow
the direction/heat flowing from hot systems to cold systems
Specific heat
the energy required to raid the T of one gram of a substance one degree celsius
Tempurature
a measure of the average kinetic energy of molecules in a substance or system
What happens to T during phase change
remains constant till change is fully completed
When phases are changing what type of energy do they gain?
Potential Energy (PE), to break bonds
when ice melts PE is gained
equation for changing the temp of a substance (no phase change)
q=mc∆ t (∆ t = temp change, final temp - initial temp)
Equation for phase change with moles
g = ∆ H x mol (q in kJ)
∆ Hfus = 6.01 kJ/mol
∆ Hvap = 40.7 kJ/mol
Equation for phase change with grams (m)
q = L x m (q in J)
Lfus = 334 J/g
Lvap = 2,260 J/g
Calorimetry
Measurement of the amount of heat evolved (released) or absorbed in a chemical reaction
System
the substance whose change we are focused on
Surroundings
everything besides the system, usually water in an insulated thing, problem heat loss
professionals use bomb calorimeter
Calorimeter
instrument used to measure amount of heat released or absorbed. in a calorimeter, usually assumed that the water is the entire surroundings.
Enthalpy (H)
the heat content of a system (how much heat could be released)
Enthalpy of reaction (∆ H)
Heat of reaction, how much heat is absorbed or released in reaction
Endothermic
needs heat, takes it in for reaction, positive ∆ H (from liquid to gas)
Exothermic
giving off/releasing energy, negative ∆ H
Bond Dissociation Energy (BDE)
Energy required to break bond in reactants (endo) or form bonds in products (exo)
Heat of formation (Standard Enthalpy of Formation or ∆ Hfus)
∆ H of formation is the heat gained or lost when one mol of compound is formed from its elements under standard conditions (25˚ C and 1 atm NOT STP)
product coefficient is one, others may be fractions
Heat of combustion (∆ Hcomb)
Energy change when one mole of a substance is completely burned in oxygen at standard conditions (25˚ C and 1 atm NOT STP)
substance that is burned has the coefficient of one, others may be fractional
Hess’ Law
The total energy change in a chemical reaction will be the same if the reaction happens in one step or several steps
Lab based problem sequence
heat change of surroundings (usually water) use q = mc∆T, assume c for solutions is same as water and that the lab apparatus absorbs no heat
heat change of the system. assume no heat loss, so basically opposite sign as #1
Moles of specific chemical being studied (with combustion, moles burned so initial mass - final mass)
Heat of reaction (enthalpy change) heat of system/moles of system (#2/#3)