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

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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)

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∆ Hfus = 6.01 kJ/mol

∆ Hvap = 40.7 kJ/mol

Equation for phase change with grams (m)

q = L x m (q in J)

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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

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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)

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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)

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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)