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Unit A > Module 1, thinking energy > Flashcards

Module 1, thinking energy Flashcards

1
Q

Thermochemistry

A

Thermochemistry is the study of energy changes (energy produced or absorbed) by a chemical system during a chemical reaction
* Energy is always conserved, just converted from one form to another

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2
Q

Kinetic energy

A

Kinetic energy: energy of motion from moving electrons and also moving/vibrating atoms
“heat energy”

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3
Q

Potential energy

A

Stored energy in the form of intermolecular bonds (between molecules) and intramolecular bonds (between atoms within the molecule)
“bond energy”

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4
Q

What type of system is ideal for studying energy changes in thermochemistry?

A

An isolated system is required (neither matter nor energy can enter or leave)
However it is impossible to create an isolated system
* Usually takes place in a closed system (matter cannot enter of leave, but energy can enter or leave)

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5
Q

What is the enthalpy change (∆𝐻) in a chemical reaction

A

The change in potential energy of the reactants compared to the potential energy of the products
* Also referred to as the net energy for a reaction
𝐻 (products) - 𝐻 (reactants)
(J or KJ)

( J or KJ )

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6
Q

Exothermic Reaction vs Endothermic Reaction

A

Exothermic: Produces or releases energy to the surroundings, ∆𝐻 will be negative because the chemical system is losing energy.
Examples:
* combustion reactions
* cellular respiration
Endothermic: Gains or absorbs energy from the surroundings, ∆𝐻 will be positive because the chemical system is gaining energy.
Examples:
* ice packs
* photosynthesis

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7
Q

When one mol of glucose is burned 2802.5 kJ of energy is released calculate the quantity of energy released to a person by eating 5.00 g of glucose in a candy
C₆H₁₂O₆(s)

A
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8
Q

What is the molar mass of ammonium phosphate?

A
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9
Q

Convert a mass of 1500 g of calcium carbonate to a chemical amount.

A
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10
Q

Convert a reacting amount of 3. 46 𝑚𝑚𝑜𝑙 of sodium sulfate into mass in grams.

A
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11
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12
Q
  • All combustion reactions in an open system will produce __________
  • All combustions reactions in a closed system will produce _________
A
  • All combustion reactions in an open system will produce gaseous water
  • All combustions reactions in a closed system will produce liquid water
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13
Q

Calorimetry

A

The science of measuring the change in heat associated with a chemical reaction

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14
Q

Calorimeter

A

Isolated system (insulated cup) used to measure the heat energy released or absorbed by surrounding it with a liquid like water and measuring the temperature change (thermometer)

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15
Q

Thermal energy

formula

A

Total kinetic energy of the entities of a substance.
Q=mc∆t

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16
Q

(m) = _______
(c) = _______
(∆t) = _______
(Q) = _______

A

(m) = mass
(c) = specific heat capacity
(∆t) = temperature change
(Q) = quantity of thermal energy transferred

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17
Q

Specific Heat Capacity

A

The quantity of energy required to raise the temperature of a unit mass of a substance by one degree celsius or one kelvin

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18
Q

3 main methods of communicating the enthalpy change for both endothermic and exothermic reactions

A
  1. ∆𝐻 notation
  2. Thermochemical equation
  3. Potential Energy Diagram
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19
Q

∆H Notation

A

The enthalpy change/energy term is written as a separate expression at the end of a balanced chemical equation
* ∆𝐻 is always negative for an exothermic reaction
CH₄(g)+2O₂(g)→CO₂(g)+H₂O(g) ∆H=-802.5 KJ
* ∆𝐻 is always positive for an endothermic reaction
2Fe₂O₃(s)+3C(s)→2Fe(s)+3CO₂(g)
∆H=+467.9 KJ

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20
Q

Thermochemical Equation

A

A balanced chemical equation that includes the enthalpy change/energy term as a reactant or product
* Exothermic: The enthalpy change/energy terms is always written on the product side because energy is being released.
CH₄(g)+2O₂(g)→CO₂(g)+H₂O(g)+802.5KJ
* Endothermic: The enthalpy change/energy term is always written on the reactant side because energy is being absorbed.
2Fe₂O₃(s)+3C(s)+467.9KJ→2Fe(s)+3CO₂(g)

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21
Q

Potential Energy Diagram

A

A graphical representation of the change in potential
energy as a chemical reaction progresses

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22
Q

Determine the change in thermal energy when 115 mL of water is heated from 19.6°C to 98. 8°C

A
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23
Q

What is the gain in thermal energy if a cup (250 𝑚𝐿) of tap water is increased in temperature from 15°C to 95°C

A
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24
Q

If you heat 1L of water from 5. 0°C to 97. 0°C, What is the gain in thermal energy of the water?

A
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25
Q

To test the efficiency of an energy transfer, a chemist supplies 30.0 kJ of external energy to a simple calorimeter. If 150 𝑚𝐿 of water in the calorimeter is heated from 20. 6°C to 52. 8°C, what is the percent efficiency of the calorimeter?

A
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26
Q

Molar Enthalpy change

A

(∆ᵣ𝐻) is a measurement that indicates the potential energy change for only one mole of a specific chemical undergoing a particular reaction. Measured in units of joules/mole (J/mol) or kilojoules/mole ( kJ/mol)
The subscript letter just represents a type of reaction

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27
Q

∆𝐻=𝑛∆ᵣ𝐻

A

∆𝐻 is the enthalpy change (J of kJ)
n is the amount of moles for a specific chemical species (mol)
∆𝐻 is the molar enthalpy for a specific chemical species (J/mol or kJ/mol)
(r is for any generic reaction, c is for combustion reaction, f is formation reaction, so/ is for a dissolving/solution reaction, etc.)

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28
Q

The superscript “o” indicates what

A

SATP. (25°C and 100kPa).

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29
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30
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31
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32
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33
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34
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38
Q

Explain the difference between an exothermic reaction and an endothermic reaction in terms of enthalpy change.

A
  • Exothermic reactions release energy resulting in a decrease in enthalpy. The initial energy will be higher than the
    final energy. (combustion)
  • Endothermic reactions absorb energy resulting in an increase in enthalpy. Initial energy will be lower than the final energy. (photosynthesis)
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44
Q

Predict the change in enthalpy due to the combustion of 10.0 g of propane used in a camp stove
To calculate ∆𝑐𝐻 you need to know the chemical amount (n) and the molar enthalpy of combustion (∆𝑐𝐻𝑚). You can calculate the chemical amount from the mass and molar mass. The molar enthalpy of combustion of
propane (producing water vapor) is -2043.9 kJ/mol.

A
45
Q

Predict the enthalpy change due to the combustion of 10.0 g of butane in a camp heater. The molar enthalpy of combustion to produce water vapor is -2657.3 kJ/mol.

A
46
Q

Ethanol is often added to gasoline as a renewable component that reduces harmful emissions. The mixture is known as gasohol. In a research laboratory, the combustion of 3.50 𝑔 of ethanol in a sophisticated calorimeter causes the temperature of 3.63 𝐿 of water to increase from 19.88°C to 26.18°C. Use this evidence to determine the molar enthalpy of combustion of ethanol.

A
47
Q

Write the symbols for the following terms.
a) enthalpy of formation
b) enthalpy of decomposition
c) molar enthalpy of formation
d) molar enthalpy of decomposition

A

a) ∆𝑓𝐻
b) ∆𝑑𝐻
c) ∆𝑓𝐻𝑚
d) ∆𝑑𝐻𝑚

48
Q

Predict the enthalpy change for the combustion of every 100 g of methane in a natural gas water heater.

A
49
Q

Methanol is one type of fuel that is used in fondue heaters. In an experiment using a simple tin can calorimeter, 2.98𝑔 of methanol was burned to raise the temperature of 650𝑔 of water by 20.9°C. Using this evidence, calculate the molar enthalpy of combustion of methanol (to produce water as a vapour).

A
50
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51
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52
Q

What is Hess’ Law

A
  • Hess’s Law is a way to calculate the enthalpy change of a chemical reaction by adding all the individual enthalpy changes of individual reactions together to get a of all desired net reaction
  • Hess’s law is based off the idea that the same reactants can have different and multiple pathways to get to the same end products
53
Q

Two rules of Hess’ Law

A

● If you reverse a chemical equation, the sign of the enthalpy change
(ΔrH) changes.
● If you multiply or divide the coefficients of a chemical equation by a constant factor, the ΔrH is altered by the same factor.

54
Q

3 steps for Hess’ Law

A

Step 1: Create or identify the desired overall reaction.

Step 2: Adjust the given reactions to match the overall reaction:
- Make sure that the chemicals in the intermediate reactions are on the same side as in the overall reaction. If needed, flip the reaction (this changes the sign of ΔH).
- Adjust the number of moles so that the amounts of substances match the overall reaction. If you multiply or divide the reaction, do the same to ΔH.

Step 3: Subtract/add the remaining reactants and products to yield the net equation
-Chemicals on the same side of the reaction arrow add up
-Chemicals on opposite sides of the reaction arrow subtract/cancel out

55
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56
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57
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58
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59
Q
A

+232.4 kJ/mol

60
Q
A

+8.45MJ

61
Q
A

659g

62
Q

Molar Enthalpies of Formation formula

A

∆𝑟𝐻° = Σ𝑛△𝑓𝑃𝐻𝑚° − Σ𝑛△𝑓𝑅𝐻𝑚°

63
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64
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65
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66
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67
Q

predict the standard molar
enthalpy of reaction for the first reactant listed in each equation.

A
68
Q

predict the standard molar enthalpy of reaction for the first reactant listed in each equation.

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69
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70
Q

Calculate the molar enthalpy of combustion.
Methane: 𝐶𝐻₄(𝑔) + 2𝑂₂(𝑔) → 𝐶𝑂₂(𝑔) + 2 𝐻₂𝑂(𝑔)

A
71
Q

Calculate the molar enthalpy of combustion.
Propane: 𝐶₃𝐻₈(𝑔) + 5 𝑂₂(𝑔) → 3 𝐶𝑂₂(𝑔) + 4 𝐻₂𝑂(𝑔)

A
72
Q

Calculate the molar enthalpy of combustion.
Octane: 𝐶₈𝐻₁₈(𝑔) + 12.5 𝑂₂(𝑔) → 8 𝐶𝑂₂(𝑔) + 9 𝐻₂𝑂(𝑔)

A
73
Q

kinetic energy vs potential energy

A

The main difference between potential and kinetic energy is that one is the energy of what can be and one is the energy of what is. In other words, potential energy is stationary, with stored energy to be released; kinetic energy is energy in motion, actively using energy for movement

74
Q

All reactants are in ? motion . Therefore, before a reaction, all reactants have ? energy . All chemicals will also have some type of ? energy due to the bonds that are holding the particle together.

A

All reactants are in random motion. Therefore, before a reaction, all reactants have kinetic energy. All chemicals will also have some type of potential energy due to the bonds that are holding the particle together.

75
Q

A chemical reaction must involve a _________ between reactants. When
particles _______, they temporarily stop moving and therefore, have no
________ _______ anymore. When all reactants collide and form one entity, this is known as the _________ ________. The ________ _______ occurs at maximum _________ ________ (all _______ energy is converted into _______ energy stored in chemical bonds needed to hold the complex together momentarily) and is extremely unstable due to its _____ _______ energy.

A

A chemical reaction must involve a collision between reactants. When
particles collide, they temporarily stop moving and therefore, have no
kinetic energy anymore. When all reactants collide and form one entity, this is known as the activated complex. The activated complex occurs at maximum potential energy (all kinetic energy is converted into potential
energy stored in chemical bonds needed to hold the complex together
momentarily) and is extremely unstable due to its high potential energy.

76
Q

An effective collision requires sufficient energy before the reaction can occur. This required energy is called the _______ _______ (Ea) and all reactions need to overcome this energy barrier before the reaction can occur. The energy required to overcome the ________ _______ is obtained from the _________ ________ of the _________. Remember that at higher temperatures, molecules have _______ _________ ________.

A

An effective collision requires sufficient energy before the reaction can occur. This required energy is called the activation energy (Ea) and all reactions need to overcome this energy barrier before the reaction can occur. The energy required to overcome the activation energy is obtained from the kinetic energy of the reactants. Remember that at higher temperatures, molecules have more kinetic energy.

77
Q

what is an energy pathway

A

An energy pathway describes the relative changes in the potential energy of the chemical system as the chemical reaction takes place (moves from reactants, to activated complex, to products) and can be represented by a potential energy diagram

78
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79
Q

Explain the energy changes that occur at different points during the reaction progress, referring to potential and kinetic energy.

A

As the reactant molecules approach each other, their combined kinetic energy is at a maximum and the relative potential energy is at a minimum. As they collide and “climb” the energy barrier, kinetic energy is converted to potential energy. Provided they have enough kinetic energy in the first place, they will surmount the barrier. At the top of the energy profile, the potential energy of the system will be at a maximum and the kinetic energy will be at a minimum. At the end of the reaction the system will have less kinetic and more potential energy than it did originally.

80
Q

Draw an enthalpy change diagram for the combustion of hydrogen to form liquid water. Include actual enthalpy of formation values. Use whole-number coefficients in the chemical reaction equation.

A
81
Q

Assuming a one-step reaction, draw an energy pathway diagram. Label the reactants, products, enthalpy change, activation energy, and activated complex.

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82
Q
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83
Q
A

a. Exothermic
b. H = -20kJ
c. 60 kJ
d. 80kJ

84
Q
A

Unit A (5 decks)

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