Thermochemistry

Thermochemistry

The study of heat released or absorbed during a chemical reaction

Conversion of energy

First law of thermodynamics: Law of conservation of energy:

• Energy can be converted from one form to another but energy cannot be created nor destroyed 

What is a system

What your studying (eg chemical reaction)

Whats the surroundings

Anything else other than the system (eg beaker)

System will exchange energy with the WHAT (if energy is released by the system it will be absorbed by the WHAT and vice versa)

System will exchange energy with the SURROUNDINGS (if energy is released by the system it will be absorbed by the SURROUNDINGS and vice versa)

Internal energy (U) - WHAT 

Total energy of the system 

It is impossible to know the exact value of Ur or Up but we can determine or monitor the change in U WHAT equation

1st law: energy of universe is constant

ΔUniverse = ΔUsystem + ΔUsurroundings

Two possible ways a system can exchange energy 

ΔUsystem = Heat + Work 

Heat increases work WHAT

Heat increases work DECREASES

Inversely proportional

ΔUsystem internal energy is constant when

1. Given a reaction 

2. A given amount of reactnat’s given 

Heat 

• energy transferred results from temperature difference

• Endothermic

• exothermic

• (+) when energy is absorbed by system

• (-) when energy is released by system

Work

• Force over distance 

• Reaction involves gas:

          - Expansion 

           - Compression 

Both heat and work can change depending on WHAT 

Both heat and work can change depending on EXPERIMENTAL CONDITIONS  

Heat (q) is positive when WHAT

Heat is ABSORBED by the system

Heat (q) is negative when WHAT

Heat is RELEASED by the system

Work (w) is positive WHEN

Work done ON the system

Work (w) is negative WHEN

Work done BY the system

What are the three factors affecting Pressure-Volume (PV) work

1. The work done by expansion and compression 

2. Change in volume 

3. External pressure → constant 

What is the work for expansion

• Vf > Vi 

• Work is done BY the system 

• W < 0 (negative) 

What is the work for compression 

• Vf < Vi 

• Work is done ON the system 

• W > 0 (negative) 

What is the equation for work

W = -Pexternal x ΔV

Heat:

• Endothermic WHAT heat ( q WHAT 0) 

• Exothermic WHAT heat ( q WHAT 0) 

• Heat is not equal to WHAT 

Heat:

• Endothermic ABSORBS heat ( q > 0) 

• Exothermic RELEASES heat ( q < 0) 

• Heat is not equal to TEMPERATURE 

What 2 factors affect hoe much an objects temperature changes when heat is added

1. The substance (know how easy or difficult it is to change the temp of the substance) 

2. Amount of substance (more substance means longer time) 

What is a calorimetry experiment

Measures the heat released or absorbed during a chemical reaction

What is specific heat capacity

The amount of heat that must be added to 1g of substance to raise its temperature by 1K

What is the specific heat capacity equation

q = mCΔT

Δ1°C WHAT Δ1K

Δ1°C = Δ1K

Enthalpy (H):

• a WHAT function (only depends on the initial and final position doesn’t depend on the pathway)

• Heat released or absorbed in a reaction or process occurring at a WHAT

• qp(constant pressure) = WHAT

Enthalpy (H):

• a STATE function (only depends on the initial and final position doesn’t depend on the pathway)

• Heat released or absorbed in a reaction or process occurring at a CONSTANT PRESSURE

• qp(constant pressure) = ΔrH

ΔrH > 0 = WHAT 

Endothermic 

ΔrH < 0 = WHAT 

EXOTHERMIC 

Hess’s Law:

• The total enthalpy change during a reaction is the WHAT whether the reaction occurs via one step or multiple steps 

• The ΔrH for the overall reaction is the WHAT of ΔH values for each step 

Hess’s Law:

• The total enthalpy change during a reaction is the SAME whether the reaction occurs via one step or multiple steps 

• The ΔrH for the overall reaction is the SUM of ΔH values for each step 

Standard states:

• The most WHAT form of an element or compound under the given conditions

• If the substance is a gas, 1 WHAT

• If the substance is a solution 1 WHAT 

Standard states:

• The most STABLE form of an element or compound under the given conditions

• If the substance is a gas, 1 BAR

• If the substance is a solution 1M 

Standard Enthalpy of formation ΔfH°:

• The change in enthalpy during the formation of HOW MANY mole of a substance from its constituent, with all substances in their WHAT states

• Values are known for for large number of molecules 

• ΔfH° of an ELEMENT in STANDARD STATE is WHAT 

Standard Enthalpy of formation ΔfH°:

• The change in enthalpy during the formation of 1 mole of a substance from its constituent, with all substances in their STANDARD states

• Values are known for for large number of molecules 

• ΔfH° of an ELEMENT in STANDARD STATE is ZERO 

Bond dissociation enthalpy (ΔBDEH)

• The energy required to WHAT of bonds 

• Everything is in the WHAT phase

Bond dissociation enthalpy (ΔBDEH)

• The energy required to BREAK ONE MOLE of bonds 

• Everything is in the GAS phase

ΔBDEH → positive for bond WHAT

ΔBDEH → Negative for bond WHAT 

ΔBDEH → positive for bond BREAKING

ΔBDEH → Negative for bond FORMING 

ΔBDEH looks at WHAT (energy required to WHAT bond) - WHAT (energy WGAR to form the bond) 

ΔBDEH looks at REACTANTS (energy required to BREAK bond) - PRODUCTS (energy RELEASES to form the bond) 

ΔfH looks at WHAT - WHAT (looks at the whole reaction)

ΔfH looks at PRODUCTS - REACTANTS (looks at the whole reaction)