SCH4U - Introduction to Thermochemistry

What is the definition of Energy, and what is it measured in (2)?

• ENERGY: the ability to do work

• JOULES

What is the definition of Work, and what is it measured in (2)?

• WORK: the amount of energy transferred by a force over a distance

• JOULES

Define Potential Energy (1)

• The energy of a body or system due to its position or composition 

Define Kinetic Energy (1)

• The energy of an object due to its motion  

Define Thermochemistry (1)

• The study of thermal energy (heat) changes that accompany physical or chemical changes in matter

Define Thermal Energy:

• The total quantity of kinetic and potential energy in a substance

  
  

Define Heat (1)

• The transfer of thermal energy from a warm object to a cooler object

  
  
  

Define Temperature

• A measure of the average kinetic energy of entities in a substance

  
  
  

What does the Law of Conservation of Energy state?

• ENERGY CANNOT BE CREATED OR DESTROYED (main idea)

• Energy is always CONSERVED

• Energy can only be CONVERTED from one form to another

Define a Chemical System (1)

• A group of reactants and products being studied

  
  

Define “Surroundings” in regards to chemical systems

• All matter that is not part of the system

Define an “Open System” in regards to chemical systems and provide an example

• Matter AND Energy are free to enter and leave the system

• A chemical reaction in an unsealed beaker

Define a “Closed” system in regards to chemical systems and provide an example

• Energy can enter and leave the system, but NOT matter

• A glow stick

Define an “Isolated system” in regards to chemical systems

• an ideal system in which BOTH matter and energy cannot enter or leave the system

  
  
  

Define an Exothermic Reaction

• A chemical reaction that RELEASES energy into the surroundings

• EXAMPLE: The combustion of methane

CH₄ (g) + 2 O₂ (g) → CO₂ (g) + 2 H₂O (l) + energy

Define an Endothermic Reaction

• A reaction that ABSORBS thermal energy from the surroundings

• EXAMPLE: The synthesis of nitric oxide from its elements

N₂ (g) + O₂ (g) + energy → 2 NO (g)

Define Calorimetry

• The experimental process of measuring the thermal energy change in a chemical or physical change

What is a calorimeter?

• A device used to measure thermal energy changes in a chemical or physical change

Define the “Specific Heat Capacity” of a substance

• The quantity of thermal energy required to raise the temperature of 1 g of a substance by 1^oC

  
  
  

Recall the specific heat capacity chart

What are some calorimetry assumptions we must make?

• Any thermal energy transferred from the calorimeter to the outside environment is negligible

• Any thermal energy absorbed by the calorimeter itself is negligible

• All dilute aqueous solutions have the same density (1.0 g/mL) and heat capacity (4.18 J/(g.^oC) as water

In regards to the formula q = mcΔT, what does q represent?

• q: the total amount of thermal energy absorbed or released by a chemical system

In regards to the formula q = mcΔT, what does m represent?

• m: the mass of the substance

In regards to the formula q = mcΔT, what does c represent?

c: the specific heat capacity of the substance

In regards to the formula q = mcΔT, what does ΔT represent? (1)

ΔT: the change in temperature (ΔT = T_final - T_initial)

If q has a negative value, what does that mean?

• it means that the system transfers thermal energy to its surroundings

• The change is EXOTHERMIC

• The temperature of the water in the calorimeter INCREASES

If q has a POSITIVE value, what does it mean?

• it means that the system absorbs thermal energy from its surroundings

• The change is ENDOTHERMIC

• The temperature of the water in the calorimeter DECREASES

q_system = -q_? (1)

• q_system = -q_surroundings

What is the definition of enthalpy and what letter do we represent it with? (2)

• The total amount of thermal energy in a substance

• H

Define Enthalpy Change, state its symbol and units (3)

• the energy released to or absorbed from the surroundings during a chemical or physical change

• (ΔH)

• kJ (kilojoules)

ΔH_system = q_? (1)

• ΔH_system = q_system

If ΔH is LESS than zero, the reaction is:

• EXOTHERMIC

If ΔH is GREATER than zero, the reaction is:

• ENDOTHERMIC

Define Molar Enthalpy, state its symbol and main units (3)

• The enthalpy change associated with a physical, chemical or nuclear change involving 1 mol of a substance (kJ/mol)

• (ΔH_r)

• kJ/mol

Can molar enthalpy be measured in kJ/mol as well?

• Yes

Recall the table of the 6 different types of molar enthalpy (ΔH_r) change:

What is a thermochemical equation?

• A chemical equation that describes the thermal energy change (change in heat) of a reaction

What are the 2 ways thermal energy change can be expressed in thermochemical equations?

• Within the equation as a distinct term

• At the end of the equation with a positive or negative sign describing the scenario

What is a Potential Energy Diagrams?

• A graphical representation of the energy transferred during a physical or a chemical change

How do we draw a Potential Energy Diagram for an Exothermic Reaction?

• the reactants have MORE potential energy than the products

How do we draw a Potential Energy Diagram for an Endothermic Reaction?

• the reactants have LESS potential energy than the products

When we get an answer for q, do we change its’ sign in verbal form?

• No

• You only do it if the question specifically asks for the opposite of what happened (ex. asking for the amount of energy ABSORBED but the sign was negative originally, , indicating an exothermic reaction)

Recall Qasid’s Rule

• Exothermic, temp increases, -q (-kJ/kJ/mol)

• Endothermic, temp decreases, +q (+kJ/kJ/mol)

What is the first thing you should do when you see a thermochem question?

• Always work backwards/reverse - engineer. Understand what you are looking for first, ont build up.

Do we add masses if given 2 or more?

• If given a solid and a water, do not. Treat them separately. Water for q = mcdeltaT, solid for n = m/M

• If given 2 waters, YES.