02 – Energy Changes in one substance

1. Unit 03 - Thermodynamics 02 – Energy Changes in one substance

2. Calculating Energy Changes in One Substance: • When energy is given off in the form of heat, this value can be measured or calculated • Recall that the base unit for energy is the joule (J) • If the amount of heat given off by a substance is to be measured, its specific heat capacity must also be known. • Specific heat capacity is defined as the amount of heat required to raise 1 g of a substance 1˚C. Therefore the units for a substance’s specific heat capacity is J/g˚C. • The specific heats of several substances will be provided for you (Page 42 in your text has a few)

3. Calculating Energy Changes: • There is a WAY COOL FORMULA we can use to calculate the amount of energy absorbed or released from a substance when the temperature changes. It is: Q = mc∆t • Q = energy (J) absorbed or released • m = mass of the substance (g) • c = specific heat capacity (J/g°C) • ∆t = change in temperature (°C) • tfinal - tinitial

4. Calculating Energy Changes: • Please note: • When using the Q = mc∆t formula: • If heat is absorbed, Q will be positive. • If heat is released, Q will be negative. • The value of Q itself will always be positive. You need to understand mentally if Q is representing heat being absorbed or heat energy being released; the negative sign is simply representing if it’s absorbed or released

5. Example 1: • What quantity of heat is required to raise the temperature of 100.g of liquid water 10.°C?

6. Example 2: • What quantity of heat is released when 420.0g of lead cools from 85°C to 25°C? The specific heat capacity of lead is 0.13J/g°C.