Textbook Ch.8: Thermochemistry Flashcards
System
the part of the universe in which attention is focused
Surroundings
exchange energy within the system, make up the rest of the universe
State of a system
composition, temperature, and pressure
State properties
depend ONLY on the state of the system, not on the way the system reached the state
q
the variable for heat flow (magnitude of heat flow)
- expressed in joules and kilojoules
- q is positive when heat flows into the system from the surroundings
- q is negative when heat flows out of the system into the surroundings
Endothermic reaction
a reaction in which heat flows from the surroundings into the reaction system
- q is positive (q>0)
- Temperature of the surroundings DROPS
- Ex: melting ice
Exothermic reaction
a reaction in which heat flows from the reaction system into the surroundings
- q is negative (q<0)
- Temperature of the surroundings RISES
- Ex: combustion of methane
- Reaction evolves heat
Calorie
the amount of heat required to raise the temperature of 1g of water 1°C
- 1 cal = 4.194 J
- 1 kcal = 4.184 kJ
What equation gives the relationship between magnitude of heat flow (q) and the temperature change (∆t)
q = C * ∆t
q = magnitude of heat flow C = heat capacity ∆t = tfinal - tinitial
Heat Capacity (C)
the amount of heat required to raise the temperature of the system 1°C and has the units J/°C
What equation gives the relationship between heat flow (q), temperature change (∆t), and the mass (m)
q = m * c * ∆t
Specific Heat (c)
the amount of heat required to raise 1g of the substance 1°C
- It is like density or melting point because it is an intensive property that can be used to identify a substance or determine its purity
Calorimeter
a device used to measure the heat flow in a reaction
- If the reaction is exothermic, the heat flow in the calorimeter is positive because heat flows from the reaction to the calorimeter
- If the reaction is endothermic, the heat flow in the calorimeter is negative because the calorimeter gives up heat to the reaction mixture
reaction = - Ccalt
Coffee-cup calorimeter
two foam cups partially filled with water and have a thermometer inserted
Heat capacity of the coffee-cup calorimeter = heat capacity of the water
CcalWater * water = water * 4.18 J/g°C
q reaction = -water * 4.18 J/g°C * ∆t
Bomb calorimeter
more versatile than a coffee-cup calorimeter because it can be used with gases, and reactions in which the products reach high temperatures
Ccal = 9.33 kJ/°C
- All heat given off by the reaction is absorbed by the bomb calorimeter
q reaction = -calorimeter
reaction = -Ccal * ∆t
Enthalpy
a type of chemical energy sometimes referred to as “heat content”
- The heat flow for the reaction system = the difference in enthalpy (H) between the products and reactants
- In exothermic reactions, the enthalpy is negative
- In endothermic reactions, the enthalpy is positive
- the enthalpy of a substance is a state property
Thermochemical equation
a chemical equation that shows the enthalpy relation between products and reactions
How do you interpret thermochemical equations?
- Sign of ∆H (enthalpy) indicates whether the reaction is endothermic (+∆H), or exothermic (-∆H)
- The coefficients represent the number of moles
- The phases of all species must be specified
Rules of Thermochemistry
- The magnitude of ∆H is directly proportional to the amount of reactant or product
- ∆H for a reaction is equal in magnitude but opposite sign to ∆H for the reverse reaction (AKA heat evolved = heat absorbed)
- The value of ∆H for a reaction is the same whether it occurs in one step of in a series of steps (Hess’s Law)
Heat of fusion
the heat absorbed when a solid melts (solid –> liquid)
Heat of vaporization
the heat absorbed when a liquid vaporizes (liquid –> gas)
Standard molar enthalpy of formation ΔHf
enthalpy change when one mole of the compound is formed at a constant pressure of 1 atm and a fixed temperature of 25°C
The enthalpy of formation of an element in its stable state at 25°C and 1 atm is taken to be:
zero
The standard enthalpy of change for a given thermochemical equation is equal to:
the sum of the standard enthalpies of formation of the compounds minus the sum of the standard enthalpies of formation f the reactant compounds
ΔH = ΣΔH(products) − ΣΔH(reactants)
If ΔH is a large negative number:
the reaction gives off a lot of heat
If ΔH is positive
heat must be absorbed for the reaction to occur
Bond enthalpy
ΔH when one mole of bonds is broken in the gaseous state
ALWAYS POSITIVE
A reaction is expected to be endothermic if:
- The bonds in the reactants are stronger than in the products
- There are more bonds in the reactants than in the products
Bond enthalpy is ____ for a multiple bond than a single bond
larger
Thermodynamics distinguishes between:
two types of energy: Heat (q) and work (w)
Work
all forms of energy except heat
Law of conservation of energy
energy (E) can neither be created nor destroyed; it can only be transferred between system and surroundings
1st Law of Thermodynamics
in any process, the total change in the energy of a system, ΔE, is equal to the sum of the heat (q), and the work (w) transferred between the system and the surroundings
ΔE = q + w
q and w are positive when:
heat or work enters the system
q and w are negative when
heat or work goes to the surroundings
