Chapter 11 Notes Energy and Heat Heat Capacity and Specific Heat

Thermochemistry This chapter deals with studying heat changes that occur during chemical reactions, which is defined as thermochemistry. This guy is enjoying the sun; the sun is an example of thermochemistry in action.

A few thermo terms… Energy-the capacity to do work or supply heat Chemical Potential Energy-stored energy in the bonds of chemical compounds Heat (q)-energy that transfers due to a difference in temperature (remember we already defined temperature as the measure of how much energy something has)

Measuring Heat There are two units of measurement we will discuss when calculating heat (q). Calories- a calorie is the amount of heat it takes to raise one gram of water by one degree celsius Note that a food calorie (Cal) is different than this version, and has a value of 1000cal (or 1 kcal)

Measuring Heat Though you may be more familiar with calories as a unit, we will usually use Joules to measure, since it is the SI unit of heat and energy Joules = 1 calorie Typically, heats of reaction are expressed in kilojoules (kJ)

Exothermic reactions When heat is transferred from the system to the surroundings during a reaction, it is called exothermic. Combustion is exothermic. Only YOU can prevent forest fires.

When heat is transferred from the surroundings to the system during a reaction, it is called endothermic.When heat is transferred from the surroundings to the system during a reaction, it is called endothermic. Endothermic reactions

Heat and Reactions When written as part of a reaction, heat can be thought of as a product in exothermic reactions and a reactant in endothermic reactions.

Fe 2 O 3 + 2Al  Al 2 O 3 + 2Fe + energy

Heat Capacity How much heat it takes to raise the temperature of an object by 1 o C is called its heat capacity. What variables affect heat capacity? –Amount of substance –Type of substance

Specific heat capacity (a.k.a.) specific heat Scientists do not generally like to compare heat capacities, because there are two different variables, SO they compare 1g of substances to come up with the specific heat capacity. As you can see, scientists like Einstein do not like two variables!

Specific heat The amount of energy required to raise the temperature of 1g of substance by 1 o C is its specific heat. The formula for specific heat is: q=(m)(C p )(  T) Heat = mass x specific heat x change in temp.

Practice Problem #1 When 435J of heat is added to 3.4g of olive oil, it raises the temperature from 21 o C to 85 o C. What is the specific heat of olive oil?

Practice Problem #2 A 1.55g piece of stainless steel absorbs 141J of heat when its temperature is increased by 178 o C. What is the specific heat of the stainless steel? Mr. Huggins’s morning coffee increases the temperature of his travel mug. Chemistry in action!

Practice Problem #3 How much heat is lost when a 250.0g sample of mercury’s temperature is lowered by 52 o C? C Hg =0.14 J/g o C Mercury was once used in the hat making industry, but the vapors reacted with employee nervous systems and drove them insane. Hence the phrase, mad as a hatter.