Heat

Heat

Homework

• Read 15.2– Assessment 16-22

• Read p. 530 (Thermochemical Equations for changes of state)– Assessment Question 27

Kinetic Theory of Matter (KTM)Kinetic Molecular Theory

• All matter is made of tiny particles that are in constant motion

• The rate of motion determines – the temperature of the substance– the state of matter of the substance

Heat• Heat is defined as the total amount of kinetic

energy (so it depends on the amount of matter present)

• When energy is transferred from one object to another, it is known as Heat

Temperature

• Temperature is defined as the average amount of kinetic energy

Energy and Phase Changes

endothermic – a chemical reaction that requires energy be put in

- energy is a reactant

(more E needed to break existing bonds in the reactants than is released when the new bonds form in the products)

sunlight + 6CO2(g) + H2O(l) C6H12O6(aq) + 6O2(g)

•Photosynthesis is endothermic

•Cooking is endothermic

• exothermic- a chemical reaction that releases energy

- energy is a product

(more E is released than is required to break bonds in the initial reaction)

•Reacting Zn and HCl is exothermic

Zn(s) + HCl (aq) ZnCl2(s) + H2(g)

•Reacting Na and Cl is exothermic

http://www.chemeddl.org/collections/whats_this/wt20080215/index.html

Heat Calculations• Heat (q)

Energy transferred from an object at a higher temperature to an object at a lower temperature. (heat lost = -heat gained)

q = mcT q=mHfus

q=mHvap

Heat Calculations• A 10.0g sample of iron at 50.4oC is cooled to

25.0oC in 50.0g of water. Calculate the amount of heat lost by the iron.

ciron= 0.449 J/goC

• A 2.1g ice cube at –8.0oC melts completely and warms to 12.5oC. How much heat was required?

Hfus ice = 334 J/g

cice = 2.03 J/goC

cwater = 4.18J/goC

Some Helpful Constants

specific heats:gold = 0.129 J/g°Caluminum = 0.897 J/g°Cwater = 4.184 J/g°Ciron = 0.449 J/g°Csilver = 0.135 J/g°Cice = 2.03 J/g°CHfus water = 334 J/gHvap water = 2260 J/g

Practice1. How much heat is required to raise the temperature of 68.0g of

aluminum from room temperature 24.0°C to 80.0°C?

2. A beaker containing 150.0g of water at 80.0°C is cooled to 50.0°C. How much heat is lost by the water?

3. A 75.0g chunk of ice at –15.0°C is warmed to 0.0°C, completely melted, then warmed to 25.0°C. How much energy did this process require?

4. If a 1.55g piece of stainless steel at 30.0°C absorbs 141.0 J of heat, what will its final temperature be?

5. An unknown sample of metal has a mass of 0.625g. It is heated from 25.0°C to 55.0°C with 2.42 J of energy. What is the specific heat of this metal? Identify the metal.