Thermochemistry and Energy Water freezing and boiling at the same time, really?!

O Thermochemistry is the study of energy changes that occur during chemical reactions and phase changes (changes of state)

The Nature of Energy O Energy is the ability to do work or produce heat O Heat (q or Q) is energy; flows due to temperature differences (always hot to cold)

System E E E E E E E E E E

Law of Conservation of Energy O Energy cannot be created nor destroyed; only converted into different types.

Kinetic vs. Potential Energy O Two main types of energy – kinetic and potential O Kinetic – energy of motion O Potential – energy due to position or energy stored in chemical bonds O Chemical potential energy - the energy stored in a substance because of its composition O Example: gasoline

Kinetic vs. Potential Energy O Two main types of energy—kinetic and potential O Kinetic—energy of motion O Potential—energy due to position or energy stored in chemical bonds O Chemical potential every—the energy stored in a substance because of its composition O Example: gasoline

Temperature vs. Heat O Temperature is a measure of the motion in a sample O Temperature is a measurement of heat. O Heat is the total energy of molecular motion, dependent upon amount, size, and type of particles. Heat is energy.

Units of Heat O calorie - the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius O Calorie – nutritional calorie; O 1 Calorie = 1000 calories = 1 kilocalories (kcal) O Joule – SI unit of heat O 1 calorie = J

Converting Energy Units Calorie/calorie/kilocaloriecalorie/Joule 1 C = 1000 cal = 1 kcal 1 cal = J

Example 1 A cereal has 155 nutritional Calories per serving. How many calories, kilocalories and Joules is this?

Example 2 A person on a diet consumed 1350 Calories in one day. How many calories, kilocalories and Joules is this?

System and Surroundings O Universe = system + surroundings O System – the specific part of the universe you wish to study. In chemistry this is your chemical reaction/physical process O Surroundings – Everything else in the universe O When heat is transferred it can flow in or out of the system

Endothermic vs. Exothermic O An Exothermic process is one that releases heat to its surroundings (feels warm) O Energy products < Energy reactants O An Endothermic process is one that absorbs heat from the surroundings (feels cold) O Energy products > Energy reactants

Exothermic Process Potential E products < E reactants

Endothermic Process Potential E products > E reactants

Q and heat flow O Exothermic process, heat is released, q is negative (-) O Endothermic process, heat is absorbed, q is positive (+)

Specific Heat = C s O Specific heat of a substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius. O Unit for specific heat is J/g o C

O Each substance has a different specific heat O Water = J/g o C O Gold = J/g o C O Copper = J/g o C O The lower the specific heat the lower the amount energy is required to raise its temperature.

Calculating Heat Released and Absorbed O Q = mC s (  T) O Q = Heat (Joules or kilojoules) O m is mass (gram) O  T is temperature change (°C) T final -T initial O C s is specific heat at a constant pressure (J/g°C)

Example 3 If the temperature of 56.6g of ethanol increases from 45.0°C to 80.0°C, how much heat has been absorbed by the ethanol? Specific heat of ethanol =2.44J/g°C

Example 4 A 4.00g sample of a substance was heated from 274K to 314K and absorbed 32J of heat, what is the specific heat of the substance?

Example 5 If 98000J of energy are added to 6200g of water, what will the change in temperature of the water be? Specific heat of water is J/g°C