Topic 5A- Thermochemistry

Question 1

energy

Answer 1

ability to do work or transfer heat

-energy used to move object= work
-energy used to increase temp= heat

Question 2

kinetic energy

Answer 2

energy an object gains by motion

-eg bike going down a hill

Question 3

potential energy

Answer 3

energy an object gains from its position or chem composition

-E el

-smaller separation, greater repulsion (more work) , higher E el
-greater separation, less repulsion (less work) , lower E el

Question 4

units of energy

Answer 4

-Joules
-also calorie

-1 cal= 4.184 J

Question 5

system and surroundings

Answer 5

-system: includes molecules, the reference point

surroundings- everything else

Question 6

work equation

Answer 6

W= f x d
force and distance

Question 7

heat flows from

Answer 7

warmer to cooler objects

Question 8

conversions of energy

Answer 8

-energy can be converted from one type to another

-eg- the cyclist has potential energy at top of hill then turns into kinetic when going down

Question 9

first law

Answer 9

• total energy is constant
  -if system loses energy, it must be gained by surroundings
  -if surroundings lose energy, it will be gained by systemi

Question 10

if energy is lost to surroundings what will the ^E be? Gained from surroundings?

Answer 10

loss- negative

gain- positive

Question 11

internal energy

Answer 11

-sum of kinetic energy and potential energies of all components of the system (E or U)

Question 12

change of internal energy

Answer 12

-change in internal energy is the final energy of system minus the initial energy of the system

-^E= Efinal - Einital

-if Efinal > Einitial, the system absorbed energy from the surroundings, endergonic, pos ^E

-if Efinal < Einitial, system released energy to surroundings, exergonic, neg ^E

Question 13

how does energy being exchanged between system and surroundings work?

whats its formula

Answer 13

-its exchanged as heat or work

-^E= q + w

Question 14

if ^sys is pos whats the surr? vis versa

Answer 14

^sys= -^surr
-^sys= ^surr

Question 15

what do + and - mean in terms of q, w, ^E (system is the reference)

Answer 15

q= + gain heat, - lose heat
w= + work done on sys, - work done by sys
^E= + energy gain, - energy loss

Question 16

when heat is absorbed by system from surroundings (heat enters to something cold) what is it

Answer 16

endothermic

Question 17

when heat is released by system to surroundings (heat exiting to surroundings)

Answer 17

-utilized more
-exothermic

Question 18

why do we not know internal energy of a system

Answer 18

eg- if something is hot it will cool to room temp, and if we have ice it will melt to room temp which ends up being the same so we can tell

Question 19

what is the internal energy of a system dependent on

Answer 19

-depends on present state of the system, not by the path

-state function

-so only depends on initial and final

Question 20

what isnt a state function

Answer 20

-q and w

-the battery will always be the same from start to finish whether

Question 21

enthalpy and its formula

Answer 21

internal energy plus the product of pressure and volume

^H= q (heat)

-at constant pressure, the change in enthalpy is the heat gained or lost: products- reactants

Question 22

three truths about enthalpy

Answer 22

1. enthalpy depends on amount
2. ^H forward= ^reverse but opposite in size
3. ^H depends on state of prod and reactants- not the path

Question 23

calorimetry

Answer 23

• we dont know exact enthalpy of reactants, so we measure ^H through calorimetry which is the measurement of heat flow

-calorimeter measures the q surr so we can then measure the sys

Question 24

heat capacity

Answer 24

-amount of energy required to raise the temp of a substance by 1 K (1 C degree)

J/g-K or J/m-k

-al heat capacity is low (could grab it out of oven)

-Fe is much higher (burn out of oven)

Question 25

^H formula

Answer 25

q= m x Cs x ^T -determine if endo or exo and then apply sign at end qsys= ^Hrxn

Question 26

bomb calorimetry

Answer 26

-constant volume -carried out in a sealed bomb -measures the ^E not ^H -difference is very small

Question 27

hess law

Answer 27

-if a reaction is carried out in a series of steps, ^H overall will be equal to the sum of all indiv steps ^Hrxn=^H1 + ^H2 + ^H3..... -depends only on initial state and final state of products

Question 28

enthalpies of formation

Answer 28

^Hf is the enthalpy change where a compound is made from its elements in there elemental forms -in bar

Question 29

tips for hess law

Answer 29

- "calculate the ^H for the formation" -just break apart what its asking for -make the equation you want -watch stoich mols -may have to switch around equation to cross out what you need - might need to switch signs if switching -need to multiply by mols -^Hf= + + + +

Question 30

standard heat of combustion formula

Answer 30

sum if products- sum of reactants -formula + O2 and ends in CO2 and H2O -multiply mols by the ^Hf on data sheet

Question 31

if heat capacity is given in J/mol or J/g what does that determine

Answer 31

your m in the formula will be mass or moles depending on this