C Unit 2.1 Thermochemistry Revamp

Question 1

What is the principle of conservation of energy?
(2-way)

Answer 1

• Energy cannot be destroyed or created
• only transferred from one form to another

Question 2

4 types of chemical reaction observations?
(Know 1/4)

Answer 2

• Colour change
• Temperature change
• Combustion
• Precipitate (change of state)

Question 3

If endothermic, △H = ?

Answer 3

Positive

Question 4

If exothermic, △H = ?

Answer 4

Negative

Question 5

Draw a reaction profile of an endothermic reaction

Answer 5

Stage is yours

Question 6

Draw a reaction profile of an exothermic reaction

Answer 6

Stage is yours again

Question 7

How to tell if a products’ of a chemical reaction is more stable?
(2-way)

Answer 7

• The more exothermic (more negative)
• ΔH value is

Question 8

What does the θ in ΔH_fθ mean?

Answer 8

Measured under standard conditions

Question 9

Define bond enthalpy (ΔH)?
(4-way)

Answer 9

• Enthalpy required to break
• a covalent x-y bond into
• x and y atoms
• all in gas phase

Question 10

Define average bond enthalpy?
(3-way)

Answer 10

• Avg. value of enthalpy required
• to break a given type of covalent bond
• on molecules of a gaseous species

Question 11

Formula for calculating average enthalpy change?

Answer 11

ΔH = reactants - products

Question 12

The “polarity” of reactants?
Hence, what it means for when calculating average enthalpy change?

Answer 12

• Endothermic
• When adding em, they all positive

Question 13

The “polarity” of products?
Hence, what it means for when calculating average enthalpy change?

Answer 13

• Extothermic
• When adding em, they all negative

Question 14

Before understanding Hess’ law, what are the 3 types of enthalpy change to acknowledge?

Answer 14

• Enthalpy change of reaction (ΔH_r)
• Enthalpy change of combustion (Δ_cH^θ)
• Standard molar enthalpy change of formation (Δ_fH^θ)

Question 15

Define standard molar enthalpy change of formation? (Δ_fH^θ)
(3-way)

Answer 15

• When 1 mole of product formed
• from its constituent elements
• in their standard state under standard conditions

Question 16

“Polarity” and units of standard molar enthalpy change of formation?

Answer 16

• Negative
• kJ mol^-1

Question 17

Define standard enthalpy change of combustion? (Δ_cH^θ)
(3-way)

Answer 17

• When 1 mole of substance
• completely combusted in oxygen
• under standard conditions

Question 18

“Polarity” and units for standard enthalpy change of combustion?

Answer 18

• Also negative
• kJ mol^-1

Question 19

Define enthalpy change of reaction? (ΔH_r)
(2-way + ())

Answer 19

• Enthalpy change in a reaction
• between n° of moles shown in rxn eqn
• (any chemical rxn contrast to “other two”)

Question 20

What is Hess’ Law?
(3-way)

Answer 20

• Total enthalpy change for a reaction
• is independent of the route taken
• from the reactants to the products

Question 21

How to calculate the enthalpy change of reaction from enthalpy change of formation?
(5-way, practice on this “question”)

CaO(s) + H₂O(l) -> Ca(OH)₂(s)
ΔHf CaO = -635.1 kJmol^-1
ΔHf H₂O = -285.8 kJmol^-1
ΔHf Ca(OH)₂ = -986.1 kJmol^-1

Answer 21

• Draw the enthalpy cycle
• Arrows from “elements” towards eqn
• Must go from reactants - elements - products
• Sign changes based on direction & moles changes values
• Total em up, VOILA!

Answer = -65.2 kJmol^-1

Question 22

How to calculate the enthalpy change of reaction from enthalpy change of combustion?
(5-way, practice on this “question” too)

3C(s) + 4H₂(g) -> C₃H₈(g)
ΔHf C = -394 kJmol^-1
ΔHf H₂ = -286 kJmol^-1
ΔHf C₃H₈ = -2219 kJmol^-1

Answer 22

• Draw the enthalpy cycle
• Arrows go from eqn towards “elements”
• Must go from reactants - elements - products (likewise to “formation”)
• Sign changes based on direction & moles changes values (likewise-likewise)
• Total em up, EUREKA!

Answer = -107 kJmol^-1

Question 23

Difference between temperature and heat:
Define temperature
(2-way)

Answer 23

• Measurement of the kinetic energy of the molecules
• in a system, independent of quantity

Question 24

Difference between temperature and heat:
Define heat
(3-way)

Answer 24

• Measure of the total energy
• in a given amounts of substance
• depends on quantity

Question 25

Before gaining the main eqn for calculating enthalpy change of simple chemical reactions, what’s the eqn for heat energy (q)?

Answer 25

q = mcΔT

Question 26

Define c
(Heat energy eqn)
(Also data booklet)

Answer 26

Specific heat capacity of water
(4.19Jg^-1K^-1)

Question 27

Should the change in temperature be in kelvins or celsius?
(… lil what?)

Answer 27

The power of asking

Question 28

After heat energy eqn, what next to gain ΔH?
(3-way)

Answer 28

• Joules to kJ (/1000)
• then ΔH = -q/n
• Turns into kJmol^-1

Question 29

Hence, final eqn for enthalpy change (ΔH) for simple chemical reactions?

Answer 29

ΔH = (-mcΔT)/n

Question 30

What apparatus is used to find the enthalpy change?

Answer 30

Calorimeter

Question 31

What are the 3 assumptions to be made in order to find ΔH with the apparatus?
(Perhaps know 1/3)

Answer 31

1. All heat transferred to the solution (none lost to surroundings)
2. The (_aq) sol. has same heat capacity as water
3. Density of the solution is 1gcm^-3 (that of water - means that mas = vol in cm³)

Question 32

How has a bomb calorimeter been modified to improve accuracy?
(3 things…. likewise man)

Answer 32

1. Lid to reduce heat loss
2. Motorized stirrer, constant and prevents inconsistency
3. Insulated container also preventing loss of heat

Question 33

What will be used to find ΔH using ΔH eqn?

Answer 33

… a graph

Question 34

Great work

Answer 34

:D