1. INTRODUCTION
Thermochemistry: the branch of chemistry that focuses on the heat released or absorbed in a chemical reaction.
Many chemical reactions involves changes in energy as well as changes in matter.

2. COLLISION THEORY
Chemical reactions involve the transformation of reactants into products that results in a chemical change.
During this transformation, atoms are rearranged through bond breaking and bond making – this can only occur if there is sufficient energy present for the reaction to proceed.
Collision Theory: for a chemical reaction to occur …
The reactant particles must collide
The colliding must have sufficient energy for the reaction to proceed.
The minimum energy required for a chemical reaction to proceed is called the activation energy (Ea)
The activation energy is required to begin the process of breaking bonds between atoms to start the reaction (remember: bond breaking requires energy, bond making releases energy). If reactant particles collide with energy less than the activation energy, bonds won’t begin to break and the reaction will not proceed.

3. ENERGY CHANGE IN REACTIONS
Chemical reactions are accompanied by the gain or loss of energy, often in the form of heat.
When a reaction proceeds, it either absorbs energy from or releases energy to its surroundings.
Definitions:
System: the molecules that are reacting (products + reactants)
Surroundings: everything else around the reacting molecules

4. ENERGY CHANGE IN REACTIONS
Exothermic Reaction: releases energy into the surroundings
Energy of reactants greater than energy of products.

5. ENERGY CHANGE IN REACTIONS
Endothermic Reaction: absorbs energy from the surroundings
Energy of reactants less than energy of products.

7. QUESTIONS
Explain how you would be able to tell if a reaction was exothermic.
Explain what is meant by the term “activation energy” and suggest why it is required to initiate a chemical process.
State whether the following statements describe endothermic or exothermic reactions:
When two chemical mix, the temperature rises.
A solid burns brightly and releases heat, light and sound.
When two chemical mix, their temperature drops.
Two chemicals will only react if you heat them continually.
Plants take in light energy for photosynthesis.

8. ENTHALPY
Enthalpy (H): a measure of internal energy (heat energy) in a system at constant volume and pressure.
Energy is not stored in a chemical as heat so enthalpy cannot be measured directly. Instead we measure the change in enthalpy.
Change in Enthalpy (ΔH): a measure of how much heat energy is lost or gained during a chemical reaction.
ΔH indicates whether a chemical reaction has gained or lost energy to the surroundings.
+ ΔH = endothermic reaction
‒ ΔH = exothermic reaction
Enthalpy: the “heat content” of a system or the potential of a system to create heat

9. ENDOTHERMIC
EXOTHERMIC

10. MEASURING ΔH
ΔH can be calculated experimentally by measuring the change in temperature recorded during a reaction (ΔT).
ΔH of a system is proportional to the change in temperature of the system.
The measurement of heat changes in a chemical reaction is referred to as calorimetry.
Calorimetry Video

11. MEASURING ΔH Q Q = m x c x ΔT ΔH = n m x c x ΔT ΔH = n
Q is the energy (heat) released or absorbed in the reaction (unit – J)
m is the mass of the “surroundings” (unit – g)
c is the specific heat capacity of the solution (for water this is J.g-1. °C-1)
ΔT is the temperature change of the solution
n is the number of moles of the substance (unit – mol)
ΔH = Q n
Q = m x c x ΔT
ΔH =
m x c x ΔT n
Q = calculate how many joules of energy …
DeltaH = calculate the enthalpy change, molar enthalpy, molar heat of solution

12. MEASURING ΔH Q Q = m x c x ΔT ΔH = n m x c x ΔT ΔH = n Calculating Q:
“Calculate how much energy is released …”
“Calculate how many joules of energy …”
“Calculate the number of joules …”
Calculating ΔH:
“Calculate the enthalpy change …”
“Calculate the molar enthalpy …”
“Calculate the molar heat of solution …”
Sign of Q
- Q = heat released
+Q = heat absorbed.
Q = calculate how many joules of energy …
DeltaH = calculate the enthalpy change, molar enthalpy, molar heat of solution

13. ENTHALPY OF NEUTRALISATION (ΔHneut)
Neutralization Reaction: acid + base → salt + water
Enthalpy of neutralization is the heat change occurring when:
One mole of aqueous hydrogen ions (H+) from an acid, and
One mole of aqueous hydroxide ions (OH-) from a base,
React together to produce one mole of water (H2O)
H+ + OH- → H2O
A neutralization reaction is an exothermic reaction. The enthalpy of neutralization for a reaction is the energy released per mole of water
ΔHneut will always be negative
Enthalpy: the “heat content” of a system or the potential of a system to create heat

14. MEASURING ΔH
When 5.2g of sodium nitrate was dissolved in cm3 of water, the temperature dropped by 4.2°C.
Calculate the quantity of heat energy absorbed (Q) by the NaNO3.
Calculate the heat absorbed per mole (ΔH) of NaNO3.
Note: 1cm3 = 1mL = 1g
Calorimetry Video

15. Na2CO3(s) + aq  2Na+(aq) + CO32-(aq)
MEASURING ΔH
An experiment was conducted in which 5.19g of sodium carbonate was dissolved in 75.0g of distilled water.
Na2CO3(s) + aq  2Na+(aq) + CO32-(aq)
A temperature increase of the system of 3.80°C was observed. Calculate ΔH of the reaction.