Pre unit 9 test: energetics

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

1. 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
2. heat change of the system. assume no heat loss, so basically opposite sign as #1
3. Moles of specific chemical being studied (with combustion, moles burned so initial mass - final mass)
4. Heat of reaction (enthalpy change) heat of system/moles of system (#2/#3)