Chemistry- Thermochemistry Flashcards
(52 cards)
1
Q
thermochemistry
A
study of the transfer of energy as heat that accompany chemical reactions and physical changes
2
Q
calorimeter
A
- measures the energy absorbed or released as heat in a chemical or physical change
- is a closed system
3
Q
temperature
A
a measure of the average kinetic energy of the particles in a sample of matter
4
Q
joule
A
- SI unit of heat
- there are 1000 J in a KJ
- there are 4.18 Joules in a calorie
5
Q
calorie
A
- unit of heat
- amount of energy it takes to raise 1 g of water by 1 C
6
Q
heat
A
- energy transferred between samples of matter due to difference in temperature
- flows from high to low temperature
- heat lost by the system is equal to the heat gained by the surroundings
7
Q
specific heat
A
- amount of energy required to raise the temperature of one gram of a substance by one Celsius or kelvin
- if specific heat is high, the material retains heat well
- if specific heat is low, it is a good conductor
8
Q
specific heat of water
A
- gas: 1.87 J/g*k
- liquid: 4.18 J/g*k
- solid: 2.06 J/g*k
9
Q
formula for energy lost or gained
A
- q = MCpT
- when q is positive, energy is absorbed
- when q is negative, energy is released
10
Q
heat of vaporization
A
energy required to vaporize a substance
11
Q
heat of fusion
A
energy required to melt a substance
12
Q
system
A
part of the universe being studied
13
Q
surroundings
A
the rest of the universe
14
Q
open system
A
a system that freely exchanges energy and matter with its surroundings
15
Q
closed system
A
a system that exchanges only energy with its surroundings
16
Q
isolated system q
A
a system that does not exchange energy or matter with its surroundings
17
Q
first law of thermodynamics
A
energy can be converted from one form to another with the interaction of heat, work, and internal energy, but it cannot be created nor destroyed
18
Q
enthalpy change
A
- amount of energy absorbed by a system as heat
- can be positive or negative
19
Q
formula to calculate enthalpy change
A
Hproducts - Hreactants
20
Q
activation energy
A
- minimum energy required to transform the reactants into an activated complex
- always positive (energy is put in to break bonds)
21
Q
activated complex/ transition state
A
- transitional structure that results from an effective collision and that persists while old bonds are breaking and new bonds are forming
- high energy, unstable, can’t be isolated
- bonds have broken but have not yet formed
22
Q
exothermic reaction
A
- reaction that released heat or energy
- Hproducts < Hreactants
- temp. of surroundings increase
- A + B –> AB + heat
- H is negative
23
Q
Endothermic reaction
A
- reaction that absorbs heat or energy
- Hproducts > Hreactants
- temp. of surroundings decrease
- A + B + heat –> AB
- H is positive
24
Q
enthalpy of reaction
A
- the quantity of energy transferred as heat during a chemical reaction
- also called heat of reaction
25
thermochemical equation
an equation that includes the quantity of energy released or absorbed as heat during the reaction
26
components of thermochemical equation
- H is proportional to the coefficients
- coefficients represent number of moles
- quantity of energy depends on the amount of reactants and products
- if the direction is reversed, flip the sign of H
27
molar enthalpy of formation
enthalpy change that occurs when one mole of a compound is formed from its elements in their standard state at 25 C and 1 atm
28
standard state of an element
- state of matter the element exists in at 298 K and 1 atm pressure
- elements in their standard state have a H = 0 KJ/mol
29
Unstable and stable compounds
- EXOTHERMIC compounds tend to be more STABLE
| - compounds with POSITIVE H tend to be UNSTABLE and react or decompose VIOLENTLY
30
Hesse's Law
- basis for calculating enthalpies of a reaction
| - overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process
31
General principles for combining thermochemical equations
- if the reaction is reversed, the sign of H is also reversed
- if the coefficients are multiplied, the value of H should be multiplied by the same factor
- substances that are not present in the desired thermochemical equation should cancel
32
state function
- Hesse's Law states that enthalpy difference between reactants and products is independent of pathway
- examples: enthalpy, altitude
- distance is NOT a state function
33
reactions in nature
- tend to proceed in a direction that LOWERS ENERGY STATE
| - a majority of chemical reactions are EXOTHERMIC
34
entropy (s)
- a measure of the degree of randomness of the particles (molecules) in a system
- decreasing entropy requires energy
- an increase entropy is represented by a + S (energetically favorable, releases energy)
35
lowest to highest entropy
solid, aqueous, liquid, gas
36
second law of thermodynamics
- the state of entropy of the entire universe, as a close isolated system, will always increase over time
- changes in the entropy in the universe can never be negative
37
what determine whether a reaction spontaneously
entropy and Gibb's free energy
38
phase changes effect on entropy
entropy increased from solid to gas
39
making a solution effect on entropy
almost always involves an increase in entropy
40
temperature effect on entropy
entropy increases as temperature increases
41
pressure effect on entropy
entropy decreases as pressure increases (applies to gases only)
42
number of moles effect on entropy
- entropy increases as # of moles increases (if moles of products > moles of reactants)
- not reliable
43
size of atoms/ molecules effect on entropy
entropy increases as size increases
44
processes in nature are driven in two directions
least enthalpy and toward entropy
45
free energy (g)
- function that relate the enthalpy and entropy factors at a given temperature and constant pressure
- combined enthalpy-entropy function
- natural process tend to lower the free energy of a system (less energy, more stable)
46
free energy change
the difference between the change in enthalpy and the product of the kelvin temperature and entropy change
47
free energy change formula
G = H - TS
48
spontaneous reaction
- when G is negative
| - energy is released
49
H = - , S = +
- G is always negative
| - temperature does not matter
50
H = - , S = -
- G is negative at lower temperatures
| - ex. water freezing
51
H = + , S = +
- G is negative at higher temperatures
| - ex. boiling water
52
H = + , S = -
G is never negative
