Thermochemistry

In our studies thus far we have written simple equations, predicted the product of simple reactions, balanced equations and performed quantitative calculations using balanced chemical equations. But there is one important characteristic of a chemical reaction which we have not yet included in a chemical reaction, the heat associated with the chemical reaction.

We have seen several reactions which we could classify as violent. Recall the reaction between aluminum and bromine. Although the reaction was slow to start, once it began it proceeded rapidly. We saw bromine vapor billowing out of the beaker and pieces of aluminum glowed red hot as they danced about on the surface of the liquid bromine. The reaction of potassium in water was another violent reaction. Both of these reactions produced considerable heat. Heat is what we will focus on in Chapter 6.

Let's look at two reactions to set the stage for this discussion.

As an introduction to this chapter we will look at two chemical reactions which clearly demonstrate the important characteristics we are going to discuss. The first reaction is between magnesium (Mg) and hydrochloric acid (HCl(aq)).

Lets watch as the two reactants are mixed together.

The reaction is;

Mg(s) + 2HCl(aq) ----> MgCl2(aq) + H2(g)

Before we mixed the reactants the reaction containers were tested to determine their initial temperature. When we mixed the two reactants we saw bubbles forming and the students in the front row indicated the beaker was warmer after the reaction began compared to before mixing the reactants. Heat was given off during the chemical reaction.

(Click on the picture on the right to view the reaction.)

 

The second reaction is between barium hydroxide and ammonium chloride. Both of these reactants are white crystalline solids. The two white solids are mixed together in a erlenmeyer flask. Before the reaction begins the two solids are at room temperature as is the flask. When the two solids are mixed the first observation is the two solids form a slurry, as a liquid is formed. This is very interesting, but not surprising when you are told water is a product of the reaction. The second observation occurs when the flask is picked up. The temperature has fallen when the two reactants are mixed to a very low temperature. The reaction is;

Ba(OH)2.8H2O(s) + 2NH4SCN(s) ---> Ba(SCN)2(aq) + 2NH3(g) + 10H2O(l)

As the two white solids are mixed we observed how the temperature of the reaction changed. The temperature is falling-it is getting cooler. Notice also the change which has occurred in the flask. The two solids have changed and we now have a liquid. If we could smell the reaction vessel it would smell of ammonia. Heat is a reactant, it is absorbed by the reactants.

(Click on the picture on the right to view the reaction.)

We will use the terms system and surroundings to help focus on how heat flows. System is that portion of the universe we single out for study and which is bounded by some boundary real or imaginary. The surrounding is everything else.

If in a chemical change or reaction, energy is released, the temperature of the system will get warmer than the surroundings, than heat will flow from the system to the surroundings. When we touch the system it feels hot. In the first experiment magnesium reacted with hydrochloric acid. Heat was produced in the reaction and it flows from the system (the reaction) to the surroundings. This is an example of an exothermic reaction.

Mg(s) + 2HCl(aq) ----> MgCl2(aq) + H2(g) + heat

In the second example the reaction occurred in the beaker. That was the system. Had we touched the reaction container (the system) is would have felt cool. Heat would be removed from our hand, which is at a higher temperature than the system. Heat must flow from the surrounding to the system. When an endothermic change or reaction occurs heat is absorbed by the system. In order for that to happen the temperature of the system is cooler than the surroundings and when we touch the system it feels cool. As the reaction occurred heat flowed from the surroundings to the system.

heat + Ba(OH)2.8H2O(s) + 2NH4SCN(s) ---> Ba(SCN)2(aq) + 2NH3(g) + 10H2O(l)

So what have we discovered in these two examples? These two reactions were chosen because the particular change I wanted you to note was very evident. In the case of the reaction between magnesium and hydrochloric acid the reaction produced heat. Heat was released when the reaction began. In the case of the barium hydroxide and ammonium thiocyanate instead of getting hotter the reaction mixture became cooler.

The study of the energy changes when a reaction occurs is called thermochemistry. Thermochemistry is part of thermodynamics,the study of heat, energy, and work and and their transformations. To begin our study of thermodynamics (and we will encounter thermodynamics many different times during this year) we are going to begin by covering thermochemistry.

As an introduction to thermochemistry I need to define several important terms so that we might better understand the two reactions we observed earlier.

Those terms are energy, temperature, heat and work.

Because energy is not tangible, as are material objects, it become difficult to define it completely. Energy is the capacity to do work or to transfer heat.

When we discuss energy two forms of energy come to mind potential energy and kinetic energy. Potential energy

U = mgh: g = 9.8 m sec-2

is energy stored in an object by virtue of its position. A book held above my head has more potential energy than a book held at my side near my waist. The amount of potential energy an object has depends on the mass of the object and its height above the earth's surface. If I drop the book the potential energy is converted to kinetic energy. Kinetic energy is energy of motion. The magnitude of the kinetic energy of an object depends on its mass and velocity.

Ek = 1/2 mv2

The heavier and faster an object is going the more work it can do on whatever. If we substitute the SI units for mass and velocity into the kinetic energy equation we have the correct units for energy. The SI unit for energy is the joule (J) (1 kg-m2s-2 ). An object with a mass of 1 kg traveling at a velocity of 1 m sec-1 has a kinetic energy of 1 J. A 100 watt light bulb produces 100 J of energy for every second it operates. (When the City of Stillwater sends a bill for the energy used at your house it includes an electrical bill for energy used in the form of electricity and the units used are kilowatt-hours. A single kilowatt-hour is equivalent to 1000 watts-3600 seconds or 3.6 x 106 watt-sec or 3.6 x 106 joules. A BTU is the amount of energy required to raise the temperature of one pound of water one degree Fahrenheit.)

In the first reaction we saw, between magnesium and hydrochloric acid energy was given off in the form of heat. No work was done by the reaction, with the exception of pushing back the atmosphere. So no useful work. The appears to have been energy present in the reactants which is liberated when the products are formed, as heat when the reactants were combined.

In the second reaction energy was absorbed when the reaction occurred. Again no useful work was accomplished.

If we had the proper equipment we could have measured the temperature of both reactions. Temperature is a measure of the degree of hotness or coldness of an object. If I indicate the temperature of a sample of water is 95 C we know the sample is very hot. If another sample of water has a temperature of 1 C, we know the sample will feel cool when we touch it.

Heat is energy that is transferred as a result of a temperature difference. Heat always flows from a warmer object to a cooler object. Heat causes a change in temperature. So when we 'heat' an object it get hotter.

Lighting a bunsen burner and placing it beneath a beaker filled with water causes the temperature of the water to increase. Heat flows from the warmer flame of the bunsen burner to the cooler water. We can measure this temperature change using a thermometer. If we have two beakers, a 100 mL beaker and a 25 mL beaker each filled with water at the same initial temperature, and an equal amount of heat is added to each beaker we will find the water in the larger beaker to have a lower temperature compared to the temperature of the water in the small beaker. The temperature change depends on the amount of heat added to the water. Heat depends on the amount of matter present. When the bunsen burner is removed, the source of heat, the water in the beaker will cool, as heat flows from the warmer water in the beaker to the cooler air of the room, and return to room temperature. I have described heat as flowing or being transferred and this may be misleading. Heat is not matter, it is not contained in matter. Heat is a way to exchange energy. We will use the terms system and surroundings to help focus on how heat flows. System is that portion of the universe we single out for study and which is bounded by some boundary real or imaginary. The surrounding is everything else. If in a chemical change or reaction, energy is released and the temperature of the system will get warmer than the surroundings, than heat will flow from the system to the surroundings. When we touch the system it feels hot. In the first experiment potassium permanganate reacted with glycerine. Heat was produced in the reaction and it flows from the system (the reaction) to the surroundings. This is an example of an exothermic reaction.