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)
