Thermochemistry

Question 1

what is thermochemistry?

Answer 1

deals with the change in energy that occurs during a chemical reaction

Question 2

chemical equations v thermochemical equations

Answer 2

• they are different

- thermochemical must have enthalpy value

Question 3

what is an enthalpy value?

Answer 3

tell chemist about the reaction participants and about energy change. enthalpy is defined as the amount of energy change per set amount of substance

Question 4

to use enthalpy in mol what must be present

Answer 4

chemical formula

Question 5

incomplete combustion

Answer 5

• limited O2
• products CO and H2O
• all product gas

Question 6

complete combustion

Answer 6

• O2
• products CO2 and H2O
• products gas

Question 7

how to balance combustion reactions normally?

Answer 7

CHO

Question 8

how to balance when oxygen is present in the fuel or reactant?

Answer 8

need to subtract O atom total in the products by the number of O atoms in the chemical reactant (not O2) before you balance O2

Question 9

if you have to halve or double the stoichiometry you must…. the enthalpy

Answer 9

halve or double

Question 10

how do you reverse a thermochemical equation?

Answer 10

stoich is the same but enthalpy value becomes positive

Question 11

how does the state of water impact the enthalpy value?

Answer 11

H2O to liquid releases heat

H2O to gas needs heat

Question 12

what are the thermochemical reaction types?

Answer 12

endothermic or exothermic

Question 13

endothermic?

Answer 13

• absorb thermal energy from surrounding environment

- prac will indicate a decrease in temp

Question 14

exothermic?

Answer 14

• release thermal energy to environment

- prac indicates an increase in temp

Question 15

what is activation energy needed for?

Answer 15

to break bonds

Question 16

what happens in an endothermic energy profile?

Answer 16

• reactants have lower energy than products

- energy is absorbed

Question 17

what happens in an exothermic energy profile?

Answer 17

• reactants have higher energy content than products

- there is some energy absorption for bonds to break

Question 18

what happens at the transition state?

Answer 18

atoms begin to be rearranged

Question 19

how is enthalpy calculated from an energy profile?

Answer 19

H(product)-H(reactant)

• positive in endo
• negative in exo

Question 20

how is energy for a set thermochemical equation calculated? do you need a negative or positive sign?

Answer 20

• get enthalpy from data book
• multiply by number of mole of the fuel
• no sign because not saying whether exo or endo
• if in grams, convert to mol

Question 21

how to calculate the amount of substance from energy?

Answer 21

(the number of mol x the energy)/ enthalpy that correlates to mol

Question 22

what is specific head capacity?

Answer 22

ability to hold onto heat energy

Question 23

what is the density of water?

Answer 23

1ml is 0.997g

Question 24

what is the SHC equation?

Answer 24

energy= SHC x mass x change in temp

Question 25

how do you use an experiment to calculate enthalpy?

Answer 25

use water’s capacity to hold onto energy to determine the chemical’s energy

Question 26

how to find energy produced by burnt fuel?

Answer 26

E = (SCH of water) x mass of water x change in temp of water

this gives energy in J which should be converted to KJ

Question 27

how is the enthalpy of the burnt fuel calculated?

Answer 27

energy/amount (mol or mass)

Question 28

calculate enthalpy with mass

Answer 28

KJ of energy/mass

• KJ from SHC eqn
• mass from initial-final mass of fuel

Question 29

calculate enthalpy with mole

Answer 29

• calculate mol with mass and molar mass

- divide energy KJ by mol

Question 30

calculate % efficiency

Answer 30

experimental enthalpy/ theoretical enthalpy x 100

Question 31

calculate heat loss

Answer 31

100%-% efficiency

Question 32

calculate mass lost

Answer 32

mass x heat loss

Question 33

calculate amount of fuel needed to reach 100%

Answer 33

100%/ your efficiency x quantity or mass of fuel or chemical