Read all the information in the link Thermochemistry  at Wikipedia. As you read this information, either make notes or be sure to pay attention to the following.

A system consists of a container in which a reaction is occurring and the surroundings. Note the image of the calorimeter. Chemicals are placed in the interior of the vessel where the reaction will take place. The air around the vessel will be the surrounding.

A chemical reaction may either give off heat (an exothermic reaction) or absorb heat (an endothermic reaction). The heat involved is the result of chemical bonds either breaking or being formed. View the next two videos:

Collision Theory & Reactions Part 1 | The Chemistry Journey | The Fuse School

Collision Theory & Reactions Part 2 | The Chemistry Journey | The Fuse School

The important thing to take away from these videos it that exothermic reactions have a negative value and endothermic reactions have positive values. This sounds confusing. Reread the paragraph describing the system.  If the reaction inside the chamber is exothermic, it is losing heat to the surrounding. Hence a negative value. Likewise if the reaction in the chamber absorbs heat from the surrounding, the heat is increased in the chamber. Hence a positive value.

Terms to know:

Redox – an abbreviation for reduction and oxidation reactions.

Enthalpy – is the amount of energy given off (exothermic) or absorbed (endothermic) in a reaction. It comes from the Greek word that means “heat inside”.

Entropy – is a measure of disorder in a chemical system as defined above.

Free energy – is the amount of work that a system can perform.

Calories – is a unit to measure the amount of heat energy. See Calorie.

Joule – is another unit to measure energy.

Standard State – This refers to the given conditions when a measurement is made. This means stating the temperature, pressure, and quantity at which the reaction is observed. Common Standard state conditions are 25o C temperature, 1 atmosphere (101.3 kiloPascals kP) pressure, one mole of chemical (1 molar when a solution is used).

Heat of Formation – The enthalpy change when one mole of a compound is formed from its elements. The following reaction between carbon and hydrogen to form methane will demonstrate this.

Methane heat forma

The enthalpy of any reaction is the difference between the sum of the enthalpies of the products minus the sum if the enthalpies of the reactants. An equation for this is:

Delta H formula

where ∑ is the sum of enthalpy for all products and all reactants.

In a heat of formation reaction at standard conditions for the formation of a compound, the elements have an enthalpy of zero.   So the enthalpy of -74.8 for CH4 in this reaction is ascribed to the formation of the CH4.   This is the case for all heats of formation for compounds. There are tables of Heats of formation available. Some online sources are: Standard enthalpy change of formation (data table)  and Table of the Heats of Formation of Common Compounds.   You have to scroll down to see the table.   The student can Google Table of Heats of Formation and find additional tables.

The heat of formation for compounds can be used to calculate enthalpy for other reactions.   The balanced equation is important as it determines the number of moles that react or are formed.

The following reaction will illustrate these calculations. The enthalpy values were obtained at Wikipedia . Scroll down to the Miscellaneous Compounds table

Propane reax

This is a reaction between propane and oxygen. This reaction is an redox reaction that is also a combustion reaction because the butane burns.

The first step is to balance the chemical reaction.
There are 3 C on the reactant side so the CO2 will need a coefficient of 3 to balance carbon. There are 8 H on the reactant side so the H2O will need a coefficient of 4 to balance H.
There are 10 O on the product side so the O2 will need  coefficient of 5 to balance O

Propane reax bal

To calculate the enthalpy of this reaction one needs to consult a Table of Heats of formation. The heat of formation enthalpy values were obtained at Wikipedia and for the the compounds in the chemical equation are:

C3H8(g) = -104.7 kJ/mol       O2g = 0 kJ/mol       CO2g = – 393.5 kJ/mol       H2Ol = – 285.8 kJ/mol

The heat of reaction equation is:
∑H reaction = [ 3(heat of formation for CO2g) + 4(heat of formation for H2Ol) ] – [ (heat of formation for C3H8(g)) + 5(heat of formation for O2g)]

∑H reaction = [ 3(- 393.5 kJ/mol) + 4(- 285.8 kJ/mol)] – [(-104.7 kJ/mol) + 5(0 kJ/mol]

∑H reaction = [ -1180.53 kJ/mol + (-1143.2 kJ/mol] – [ -104.7 kJ/mol] = – 2219.0 kJ

Without going into details, energy released (or given off) from this reaction can be used to for work. Such as heating water or a home.

What has been presented is just an introduction. An upper level high school or a freshman college level chemistry course will discuss more details regarding thermochemistry.