Enthalpy of Formation and Enthalpy of Reaction

∆Hr is the enthalpy for the entire reaction, EXACTLY as it is written.  If you change the coefficients or states, you will also change the heat absorbed or evolved.


∆Hf is the enthalpy of formation.  It is the heat absorbed or given off for the synthesis of ONE mole of specific substance in a specific state.  Remember this is ONLY for a SYNTHESIS reaction.  Because it is always for 1 mole, the units of ∆Hf will always be kJ/mole. The ∆Hf for a free element in its state a standard conditions will always equal 0 kJ.

The wonderful thing about heat of formation is that you can use tables to look up specific substances in given states, then use the ∆Hf to find the ∆Hr.  The heat of the entire reaction is equal to the sum of all the heat from the product side minus the heat from the reactant side.  Remember that in science, ∆ means change or difference and we always use FINAL - INITIAL.

Here's an example:

First, you need a balanced reaction with states.  Remember that ∆Hf is given in kJ/mole, so you will need to multiply by the number of moles in the balanced reaction (the coefficients).  You need states to choose the correct value from the table.

The enthalpy of the reaction therefore is:

By using the tables, ∆Hr = -89.3 kJ

Try another example

Hess' Law of Heat Summation

Hess' Law of Heat Summation states that the enthalpy of a net reaction is equal to the sum of the individual steps.  The reason this works is that enthalpy is a state function.  All that matters is where you start and where you end; it is path independent.

One way of using this concept is to add up several equations, and by canceling out the extraneous substances.  Think of this process like putting a jig-saw puzzle together.  Try to find what matches best to start with, then add the other reactions as needed.  Remember that you final equation must look EXACTLY like the reaction you were given.  The states and coefficients must be the same.

 In this example you are given 3 reactions to put together- a,b, and c.  You want your final answer to look EXACTLY like the original equation.  To do this, you need to start with a, reverse b (remember to make the negative positive), then multiply c by 1/2 and add.  Remember that whatever you do to the reaction you must do the same thing to the ∆Hr.

Law of Conservation of Energy

In thermochemistry we decide on the boundaries of the problem.  In other words, we define the system.  We could define a system as only a piston, as the engine, the car, the car in the garage or the entire universe.  Obviously, the smaller the system, the easier the problem will be.  Anything not IN the system is considered surroundings. If energy is lost to a system (exothermic), that means that the surroundings have gained the energy (endothermic).

There are 3 basic types of systems:

• open systems allow for the exchange of both matter and energy
• closed systems allow energy to move but not matter
• isolated systems cannot exchange matter nor energy with their surroundings

Now the Law of Conservation of Energy makes more sense- 

The amount of energy in the universe is constant, 

therefore energy is neither created nor destroyed, just converted.  

That means that energy could be lost to a system, BUT it will be gained by the surroundings.  Energy can also convert from one type to another.  For instance mechanical energy could be converted into heat energy through friction.

We use the term internal energy to mean the total amount of energy in a system- kinetic, potential, heat, work, ...  Although we can't calculate the exact amount of internal energy, it is possible to determine the CHANGE in internal energy or ∆E.  

In science we always calculate a change (∆) by subtracting the FINAL - INITIAL values. We must define the way we subtract because it will determine the sign of the value.  The the final value is larger than the initial value, the change will be positive- for Energy that means ∆E>0 or endothermic.  If the final value is smaller than the initial value, the change will be negative- ∆E<0 or exothermic.

The change in internal energy to a system is the sum of the the heat (q) added to the system and the work (w) done on a system.  

∆E = q + w

While the math is very simple, the problem is interpreting the signs of the values of heat and work.  Refer to the chart for hints.  Remember compressed or decreased volume is positive (work done ON the system), expand or increase volume is negative (work done BY the system).

Exothermic & Endothermic

Thermochemistry is the study of energy changes in a chemical reaction.  First, you must understand a few basic definitions about energy from a physics standpoint.  Energy is defined as the ability to perform work. Work is a force applied through a distance, and force is defined as a push or a pull.  Therefore energy is the ability to make something move through a distance.  In physics, we are generally thinking about making an object move- like a ball rolling down a hill or dropping a rock.  For chemistry, we are concerned about the constant movement of atoms, ions and molecules.

Energy is measure in 2 different units.  A Joule (J) is defined as a Newton*meter.  A Newton is the metric unit of force.  Remember Sir Isaac Newton's 2nd law of motion states that a Force=mass*acceleration (F=ma).  In SI units, a Newton (N) = mass (kg) * acceleration (m/s^2).  So if work is defined as a force applied through a distance, a Joule is a Newton*meter.

Another unit of energy is the calorie (cal).  A calorie is defined as the amount of energy needed to raise the temperature of 1.0 g of water 1 C.  Temperature is the average amount of Kinetic Energy (E of motion) is a substance.  So a calorie is basically the amount of energy we need to add to 1g of water to make it's molecules move faster, so that it raises the temperature of the water 1 C.

Since we have 2 units, we need a conversion to change one unit to another.

 1 calorie = 4.184 Joules

You've probably heard about calories before in relation to food.  These are actually Calories, with a large C.  Sometimes manufacturers and advertisers mix them up, but a food Calorie is actually a kilocalorie.

If a system LOSES energy, it is called EXOTHERMIC.  Since heat is leaving the system, it will feel hot when you touch it.  The energy is leaving the system and going into your finger.

If a system GAINS energy, it is called ENDOTHERMIC.  Since heat is being added to the system, it will feel cool when you touch it.  The energy is leaving your finger and entering the system.

Steps for Balancing Redox by 1/2 Reactions

Oxidation & Reduction

Redox reactions are reactions in which particles change charge by either losing or gaining electrons.  Whether a particle loses are gains is determined by its electronegativity, or attraction for shared electrons.  We've actually used this before in single replacement reactions.  A more active metal or nonmetal can replace a less active metal because it can either take or force another element to take electrons.

Reduction occurs when a particle gains electrons.  In other words by gaining negative charges, its charge is reduced.  Oxidation means a particle has lost electron, therefore its charge will become more positive.  There are several pneumonics you can use to remember this.

Thus far we have been balancing reaction only by mass.  We could do this because all the substance were written in neutral form.  Now we are using ions so we also have to balance a reaction by charge.

If the electrons are produced, they are being lost to the particle- they are no longer attached.  If the electrons are a reactant, they are being stuck on the particle- they've been gained.

Net Ionic Equations

Water is a very polar molecule, meaning it has a partial positive charge on the hydrogen end and a partial negative charge on the oxygen end.  This is caused by the unequal sharing of electrons by the hydrogen and oxygen atoms.

Because water is polar, it will dissolve most ionic compounds.  Since ionic compounds are composed of a positive ion (cation) and a negative ion (anion), the opposite charged end of a water molecule will be attracted and break a large crystal into smaller pieces.  This is called hydration. If the molecules are completely broken into their ions by water, it is called dissociation.

For instance, table salt (NaCl) will completely dissociate in water. Every single molecule will be broken apart into ions and kept apart by the water molecules.

NaCl (aq) --> Na+ (aq) + Cl- (aq)

In chemistry, STRONG means that every molecule will dissociate when dissolved in water.  WEAK means that is partially dissociates, or that only some of the particles will dissociate while others will remain in neutral/molecular form.

Strong acids, strong bases and strong electrolytes will always dissociate when dissolved in water.  Weak acids, weak bases and weak electrolytes may or may not dissociate.

In a molecular or complete reaction, all substances are shown in neutral (molecular) form.

In an ionic reaction, the substances that all always dissociate (strong) are broken into their ions.

Because lead (II) nitrate, sodium chloride and sodium nitrate all all soluble salt (strong electrolytes), they will all completely dissociate in water.  They will not exist as molecules, but as ions.  Remember you can't lose any mass in a reaction!  The Law of Conservation of Matter still applies!

 Notice that sodium and nitrate are EXACTLY the same on the reactant and product sides?  They are spectator ions.  Spectator ions do not change in a reaction.  When you cancel the spectator ions, you're left with the net ionic reaction.

This shows us what is really changing in a reaction- the driving force for the reaction to proceed.

Oxidations Numbers

Oxidation numbers are the "effective charge" a particle has in a molecule or ion.  While all atoms WANT to have a full outer shell, and they TRY to lose or gain electrons.  We all know that you don't always get what you want.  Sometimes 2 non-metals are forces to share electrons.  Both WANT to gain electrons, but the more electronegative element will get the electrons most of the time.  In other words, they don't share equally.  Oxidation numbers tell us what the charge really is in a particle situation.  Manganese can form a +2, +4, +5 and even +7 charge depending on what other atoms are around to take its electrons.  While sulfur wants to gain 2 electrons and form a -2 charge, it is very common for oxygen to grab its electrons and sulfur is left with a +6 charge.  It now has a full outer shell because its lost ALL its valence electrons.

Follow these rules to determine the oxidation number of an ion-

You can calculate the charge of an ion by using the entire compound (must =0) or a polyatomic ion (must = the charge given).  Here are 2 ways to calculate the charge of sulfur in sulfuric acid.

No matter which method you use, the oxidation number of sulfur in sulfuric acid is +6.