Chemistry - Chap 8 - Heat flow

Rules to calculating ∆H°

- elements in their standard states can be omitted - coefficients of the products and reactants in the thermochemical equation must be taken into account.

conversion Joules to cal

1 cal = 4.184 J

Specific heat of water

4.18 J/g x °C

heat capacity of the coffee cup calorimeter is that of the water

C(cal) = m (water) x c (water) = m(water) x 4.18 J/g x °C q(reaction) = -m(water) x 4.18 J/g x °C x ∆t

Thermodynamics

Deals with all kinds of energy effects in all kinds of processes

Bond Enthalpy vs Enthalpy of formation

Enthalpy of formation is more exact.

Relating enthalpy, H, to energy, E

H = E + PV ∆H = ∆E + ∆(PV) * gases obey ideal gas law ∆(PV) = ∆n(gas)RT *∆n is the change of moles when the reaction takes place ∆H = ∆E + ∆n(gas)RT Difference between ∆E and ∆H is typically less than 1%

units for measuring heat flow

Joules (J)

Bond enthalpy

The enthalpy change that takes place when breaking 1 mol of a given bond in the molecules of a gaseous species.

Calculation of ∆H°

The standard enthalpy change, ∆H°, for a given thermochemical equation is equal to the sum of the standard enthalpies of formation of the product compounds minus the sum of the standard enthalpies of formation of the reactant compounds. ∆H° = ∑∆H°ƒ (products) - ∑∆H°ƒ (reactants)

Rule 3 of thermochemistry

The value of ∆H for a reaction is the same whether it occurs in one step or in a series of steps

thermochemical equation

a chemical equation with ∆H included NH₄NO₃ (s) -> NH₄⁺ (aq) + NO₃⁻(aq) ∆H=+28.1kJ

calorimeter

a device used to measure heat-flow

bomb calorimeter

a more versatile calorimeter used for high temperatures and gases. The bomb is a heavy metal surrounded by water and placed in a insulated outer container. q(reaction)= -q(calorimeter)

enthalpy

a type of chemical energy referred to as "heat content". it is a state variable

Whenever the bond involves two different atoms (e.g. O-H) the value listed is ________

approximate rather than exact. It represents and average of two or more different species.

heat flow is measured with a __________

calorimetera

State properties

depend only on the state of the system not the way the system reached that state. Composition, temperature and pressure.

Enthalpy of formation ∆H°ƒ

equal to the enthalpy change when one mole of a compound is formed at a constant pressure of 1atm and a fixed temperature of 25˚ from elements in their stable states at that pressure and temperature. Enthalpy of an element in its stable state is taken to be 0

Surroundings

exchange energy with the system and make up the rest of the universe

Two types of energy in thermodynamics

heat (q) and work (w)

enthalpy heat flow relationship

heat flow at constant pressure is equal to the difference in enthalpies between the products and reactants

q is positive when _________

heat flows into the system from surroundings

q is negative when _____

heat flows out of the system into surroundings

q and w are positive when

heat or work enters the system from surroundings

q and w are negative when

heat or work moves from system to surroundings

Work

includes all forms of energy EXCEPT heat

As the order of bonds increases from single to double to triple the bond enthalpy ______

increases

There are ______ bonds in the reactants than there are in the products

more

When a chemical bond forms, enthalpy is always ______

negative

Bond enthalpy is always _____

positive

relationship between heat flow of a system and heat flow of a calorimeter.

q (reaction) = -q (calorimeter) q (reaction) = -C(cal) x ∆t

heat flow equation

q = C x ∆t

heat flow (for pure substance of mass m)

q = m x c x ∆t

exothermic

q<0. Heat flows from the reaction system into the surounds. e.g. Combustion of Methane

endothermic process

q>0 Heat flows from the surroundings into the system. e.g. Ice melting

Bonds in the reactants are ______ than those in the products

stronger

Specific heat

the amount of heat required to raise the temperature of 1 gram 1°C. It is an intensive property

Heat capacity (C)

the amount of heat required to raise the temperature of a system 1°C

Rule of thermochemistry

the of ∆H is directly proportional to the amount of reactant or product

Hess's law

the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process

System

the part of the universe in which attention is focused.

coffee-cup calorimeter

two nested foam cups partially filled with water. the heat given off by a reaction is absorbed by the water. if you know the mass of water, it's specific heat and temperature change you can calculate the heat flow , q, for the reaction

Law of conservation of energy

∆E (system) = ∆E(surroundings)

Calculating ∆E with q and w

∆E = q + w

change in enthalpy equation

∆H = H(products) - H(reactants)

Rule 2 of thermochemistry

∆H for a reaction is euqal in magnitude but opposite in sign to ∆H for the reverse reaction H₂O (s) -> H₂O(l) ∆H=+6.00kJ H₂O(l) -> H₂O(s) ∆H=-6.00kJ

State property equation

∆X=X₂-X₁