Chemistry – Oct 12, 2017 P3 Challenge – Do Now (on slips of paper today) 1. Convert 792 mmHg to kPa 2. Convert 287 K to C

Chemistry – Oct 12, 2017 Objective – Assignments Agenda – Energy and Heat Capacity Solutions and Solubility Assignments Pressure-Temperature Worksheet due Oct 17 Heat Worksheet due Oct 17 Agenda – Energy Heat Heat Capacity

What is Energy? Energy = Ability to do work Law of Conservation of Energy: Energy is neither created nor destroyed. It can only be transformed from one form to another or transferred from one system to another. Total energy of a system = Kinetic Energy + Potential Energy Kinetic energy – Energy of motion Potential energy – Stored energy; Energy of position

Kinetic Energy Types Thermal energy – Energy of random motion of atoms Electromagnetic energy – Energy of light Sound energy – Energy of motion of atoms due to pressure differences Electrical energy – Energy of the flow of electrons

Potential Energy Types Gravitational potential energy – Energy of position within Earth’s gravity Electrical potential energy – Energy of separation of electric charge (voltage) Chemical potential energy – (Bond energy)Energy contained within chemical bonds Nuclear potential energy – Energy contained within the nucleus

Heat Heat is the transfer of thermal energy either to or from a system. Heat is given the symbol, Q. Positive heat transfers heat to a system. (Endothermic process) Temperature of substance increases Negative heat releases heat to the surroundings. (Exothermic process) Temperature of substance decreases Each substance absorbs/releases at a characteristic rate called it’s heat capacity. A substance with a high heat capacity requires a large amount of energy to change its temperature.

Endothermic process Energy E Final state Initial state An endothermic process is one that absorbs heat. You must heat an endothermic process to make it occur. Energy is a reactant. Q, heat is positive The system gets hotter. Surroundings gets colder. T = Tfinal – Tinitial is positive Tf > Ti Energy E Final state Initial state

Exothermic process Energy Initial state Final state E An exothermic process is one that releases heat. An explosion is an exothermic process (easy way to remember) Energy is a product. Q, heat is negative The system gets colder. Surroundings gets hotter. T = Tfinal – Tinitial is negative Tf < Ti Energy Initial state Final state E

Specific heat capacity, c Substance c in J/g K c in cal/g ◦C Aluminum 0.900 0.215 Bismuth 0.123 0.0294 Copper 0.386 0.0923 Brass 0.380 0.092 Gold 0.126 0.0301 Lead 0.128 0.0305 Silver 0.233 0.0558 Tungsten 0.134 0.0321 Zinc 0.387 0.0925 Mercury 0.140 0.033 Alcohol(ethyl) 2.4 0.58 Water 4.186 1.00 Ice (-10 C) 2.05 0.49 Granite .790 0.19 Glass .84 0.20 If the heat capacity is per gram of material, it is the specific heat capacity, c. Specific heat capacity is the amount of energy needed to raise the temperature of 1 gram of substance by 1 degree Celsius. Typical units are cal/g◦C or J/gK. ◦C and K are interchangeable for these units. Problems may use either Joules or calories. Given a choice, use Joules form. Note water’s extremely high value 1 Food Calorie = 1 kcal (note the capital C)

Measuring heat, q For any change in temperature from Tinitial to Tfinal, ΔT = Tfinal – Tinitial For a temperature increase, ΔT is positive. For a temperature decrease, ΔT is negative. Heat exchanged during the temperature change is given by: Where q is the heat, m is the mass of the substance, c is the specific heat capacity of the substance and ΔT is the temperature change.

Heat calculations q = m = c = ΔT = Ex: How much heat is needed to raise 2.50 g of water from 25 ◦C to body temperature of 37 ◦C? What mass of aluminum can be heated from 5°C to 45°C by using 275 J of heat? (Specific heat capacity of Al = 0.900 J/g°C)? q = m = c = ΔT =

Heating Curve of Water Heat and temperature are not the same. Phase changes occur at a constant temperature, but still require heat to happen. Not all phase changes require the same amount of heat It takes more energy to boil than to melt. It takes more heat to warm water than ice or steam the same amount. The slopes of the ice and steam lines are greater than the line for water reflecting water’s higher heat capacity.

Exit Slip - Homework Exit Slip: How much heat is needed to warm a 450 g copper pan from 25°C to 150°C ? What’s Due? (Pending assignments to complete.) Pressure-Temperature Worksheet due Oct 17 Heat Worksheet due Oct 17 (Note: The specific heat of iron is 0.460 J/gK or 0.101 cal/gC ) What’s Next? (How to prepare for the next day) Start reviewing for Test 2 Oct 24