1 Chapter 11 Thermochemistry

2 Energy Thermochemistry - concerned with heat changes that occur during chemical reactions Energy - capacity for doing work or supplying heat –weightless, odorless, tasteless –if within the chemical substances- called chemical potential energy

3 Gasoline contains a significant amount of chemical potential energy Heat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them. –only changes can be detected! –flows from warmer  cooler object

Exothermic and Endothermic Processes Essentially all chemical reactions, and changes in physical state, involve either: –release of heat, or –absorption of heat

Studying heat changes…

The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed. –All the energy is accounted for as work, stored energy, or heat.

7 A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 o C. –Used except when referring to food –a Calorie, written with a capital C, always refers to the energy in food –1 Calorie = 1 kilocalorie = 1000 cal.

Imagine two college freshman who find themselves in that most difficult (yet prevalent) of all college situations: trying to pull an all-nighter with no food anywhere around. Being well versed in the college rituals, they do what all such unfortunate souls do: order pizza at 3:00 am! Now, our two coeds are both the same size and weight, but there is a significant difference: Student 1 eats two slices, and starts to feel full. Student 2 eats two slices and feels like she is just getting started. After 3 slices, student 1 is stuffed, while it takes almost 6 slices to fill student 2. It is obvious to the observer that these two students do not have the same capacity for pizza. We might even say that student 2 has a larger pizza capacity!

9 The calorie is also related to the joule, the SI unit of heat and energy –named after James Prescott Joule –4.18 J = 1 cal Heat Capacity - the amount of heat needed to increase the temperature of an object exactly 1 o C

10 Specific Heat Capacity - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 o C (abbreviated “C”) –often called simply “Specific Heat” Water has a HUGE value, compared to other chemicals

11 Specific Heat For water, C = 4.18 J/(g o C) C = 1.00 cal/(g o C) Thus, for water: –it takes a long time to heat up, and –it takes a long time to cool off!

12 To calculate, use the formula: q = mC  T m is the mass in grams heat is abbreviated as “q”  T = change in temperature C = Specific Heat - units are either J/(g o C) or cal/(g o C)

Problem: The temperature of a piece of copper with a mass of 95.4g increases from 25.0 o C to 48.0 o C when the metal absorbs 849 J of heat. What is the specific heat of copper?

Calorimetry Calorimetry - the accurate and precise measurement of heat change for chemical and physical processes. The device used to measure the absorption or release of heat in chemical or physical processes is called a Calorimeter

Calorimetry Foam cups are excellent heat insulators, and are commonly used as simple calorimeters For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system Thus, q =  H = m x C x  T  H is negative for an exothermic reaction  H is positive for an endothermic reaction

16 C + O 2  CO 2 Energy ReactantsProducts  C + O 2 C O 2 395kJ kJ

17 Energy ReactantsProducts  Heat content of the system decreases…  H is <0

18 In terms of bonds C O O C O O Breaking this bond will require energy. C O O O O C Making these bonds will give off energy. In this case making the bonds gives off more energy than it takes to break them.

19 CaCO 3  CaO + CO 2 Energy ReactantsProducts  CaCO 3 CaO + CO kJ CaCO kJ  CaO + CO 2

20 Energy ReactantsProducts   H is > 0 Heat content of the system increases…

21 Chemistry happens in MOLES An equation that includes energy is called a thermochemical equation CH 4 + 2O 2  CO 2 + 2H 2 O kJ 1 mole of CH 4 releases 890 kJ of energy. When you make 890 kJ you also make 2 moles of water

22 CH O 2  CO H 2 O kJ If grams of CH 4 are burned completely, how much heat will be produced? g CH g CH 4 1 mol CH kJ = 571 kJ

23 CH O 2  CO H 2 O kJ How much energy will be produced from the burning of methane with 55 L of O 2 ? How many grams of water would be produced with 506 kJ of heat?