Module 4 - Drivers of Reaction

Question 1

Explain the difference between Entropy and Enthalpy.

Hint: Order vs Energy

Answer 1

Entropy is the state of randomness/disorder of particles and molecules in an element or compound. Increases from solid > liquid > gas.

Enthalpy is the total energy absorbed or released per mole of a reactant or product, to break or form bonds.

Question 2

Explain the difference between Endothermic reaction and Exothermic reaction.
State whether ΔH is positive or negative for either one.

Answer 2

An exothermic reaction is where energy is released, whereas an endothermic reaction occurs when energy is absorbed.
Endothermic Reaction: ΔH is positive
Exothermic Reaction: -ΔH is negative

Question 3

What occurs at the molecular level with lower heat capacities? (Bonds, energy ∝ heat capacity)

Answer 3

At a molecular level: Whichever substance has the lower heat capacity, takes less energy to break/form intermolecular and intramolecular bonds.

Question 4

State the formula for the quantity of heat absorbed/released.
(Q=…)

Answer 4

Q = mass x specific heat capacity x ΔTemperature

Q=mcΔT

Question 5

Define what Specific Heat capacity is.
(Include SI Units for g)

Explain this at the molecular level

Answer 5

Amount of heat required to increase the temperature of a substance (1g) by exactly 1K. (Measured J/g/K)

The strength of the intermolecular and intramolecular bonds determine how easy/difficult to break or form the bonds for the compound/element.

Question 6

State the formula for heat released/absorbed per mole.

ΔH = ___

Answer 6

ΔH = Total heat released or absorbed (Q) / no. of moles (n)

Question 7

State 2 examples of Endothermic reactions.
Explain the chemical process involved in each.
(H2O and AgCl)

Answer 7

1) Ice melting - As heat is absorbed by ice, the hydrogen bonds are broken and turns into water.
2) Silver Chloride - Sunlight enters the substance, causing it to decompose as bonds between Chlorine and Silver are broken.

Question 8

Explain what Molar Heat of Combustion is, the reaction and the 2 products it forms

What should the ΔH sign be? (+ or -) Explain.

Answer 8

ΔH (Exothermic reaction) = Thermal energy released when one mole of the substance undergoes complete combustion (positive), producing CO2 and H2O

ΔH of combustion = -ve since stronger reactant bonds are broken and energy is released.

Question 9

Define what Molar Enthalpy of Dissolution is.

Answer 9

ΔH = Amount of energy absorbed/released when one mole of solute dissolves in an aqueous or liquid solution.

Question 10

Explain the dissociation process of ionic compounds in aqueous solutions.

Include the molecular explanation as well, using water as an example. (Dipole-Dipole)

Answer 10

Definition: Ionic compounds dissolved in aqueous solutions, separates into cations and anions.
This is an ion-dipole attraction.

Molecular Level: Positive ends of the ‘H- molecules are attracted to the anion molecules of the substance. Negatively charged ends of the ‘O-molecules’ are attracted to the cation molecules of the dissolved substance.

Question 11

State what Calorimeter is and its purpose in an enthalpy, chemical reaction.
Include 1 common example.

Answer 11

A device used to measure the heat change that occurs during a chemical or physical change.

A common example - Polystyrene Cups (Cheap, easy insulators).

Question 12

Explain what Bond Energy is, & units.

State the formula for calculating the change in bond energy (ΔH).

Answer 12

The required amount of energy to break or form bonds. (kJ/mol).

Calculation:
ΔH = ΣReactants - ΣProducts
(+ve from Endothermic, -ve from Exothermic)

Question 13

Distinguish the differences and similarities between the enthalpic processes of:
Photosynthesis and Respiration
(Hint: Glucose: C6H12O6)

Answer 13

Photosynthesis: Endothermic reaction (Absorbs light energy).
6CO2 + 6H2O –> C6H12O6 + 6O2.

Respiration: Exothermic reaction (Human Body process)
C6H12O6 + 6O2 –> 6CO2 + 6H2O.

Question 14

State the 4 factors that determine when a positive entropy change occurs.

Answer 14

1) Temperature increases.
2) Substance changes from solid to liquid to gas.
3) Solid dissolves in solvent.
4) Number of moles increases.

Question 15

Explain what Standard Entropy is.

State the formula to calculate Change in Entropy.

Answer 15

Standard Entropy: Entropy/Randomness of one mole of the substance in its standard state (at 298K).

ΔS = Sum of products entropy - Sum of reactants entropy

Question 16

Explain what Gibbs Free Energy is.

If ΔG = -ve, spontaneous/non-spontaneous?
If ΔG = +ve, spontaneous / non-spontaneous?

Answer 16

Gibb’s Free energy change: Measure of the net result of energy and entropy drives for a reaction.

If ΔG = Negative, reaction is spontaneous.
If ΔG = Positive, reaction is non-spontaneous.

Question 17

List the 4 steps in calculating Change in Gibb’s Free Energy.

Answer 17

1) Calculate ΔH
2) Calculate ΔS
3) Convert ΔS into kJ (÷ 1000)
4) ΔG = ΔH - (ΔS x Temperature)

Question 18

Explain how higher/lower Bond Energies determine the stability or reactivity of a molecule.

Answer 18

The weaker/lower a molecule’s bond energy is, the more reactive it is since its bonds are easier to break and form.
The stronger/higher bond energy is, the more stable that molecule is since it takes higher amounts of energy to break or form the bonds this molecule has.

Question 19

Distinguish between a Forward and Reverse reaction.

Answer 19

Forward Enthalpy = Reaction will go as written, and is spontaneous, so it provides and gives off its own energy.

Reverse Enthalpy = Reaction tends to go in the opposite direction, and is not spontaneous.

Question 20

When Enthalpy is - and Entropy is +, what is the spontaneity?

Answer 20

Always spontaneous reaction.

-ΔH and -ΔS = Spontaneous

Question 21

When Enthalpy is - and Entropy is -, what is the spontaneity?

Answer 21

Only spontaneous at low temperatures.

Question 22

When Enthalpy is + and Entropy is +, what is the spontaneity?

Answer 22

Only spontaneous at high temperatures.

Question 23

When Enthalpy is + and Entropy is -, what is the spontaneity?

Answer 23

Non-spontaneous reaction.

+ΔH and -ΔS = Non-Spontaneous

Question 24

Explain what it means for a reaction to be spontaneous.

Answer 24

Spontaneous:
Does not require any ONGOING energy input. (May need an initial ‘nudge’ of energy). Produces and releases its own’s energy, causing the reaction to continue occurring. Exothermic and occurs in the forward direction.

Question 25

Explain what it means for a reaction to be non-spontaneous.

___thermic & ____ direction?

Answer 25

Non-Spontaneous:
Requires a constant amount of energy for the reaction to occur/maintain reacting, as it does not produce its own energy. Generally endothermic and reaction is in the reverse direction.