Energy Changes in One Substance

Heating and Cooling Substances Have you ever noticed that it takes practically no time at all for a metal pan to get really hot? At the same time, water often takes a fairly long time to change in temperature.

Heating and Cooling Substances This is because different materials absorb (or release) heat at different rates The rate of absorbing or releasing heat is called the Specific Heat Capacity

Calculating Energy Changes in One Substance 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 in your refrence booklet

Heat capacities GENERALLY: Metals have low heat capacities Solids have low heat capacities Liquids have fairly high heat capacities Gases have very large heat capacities

Just so you know… McDonald’s produces one of the (thermodynamically speaking) most efficient coffee cups They cup is double walled Basically it is a cup within a cup with a gap in between The gap is filled with cardboard and AIR. Air has a high specific heat = lots of energy needs to be absorbed in order to change in temperature This means it takes a long time for the cup to increase in temperature saving your hand from getting burned.

Calculating Energy Changes in One Substance: When the amount of energy in a system changes, it can be measured. 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.

Calculating Energy Changes: There is a formula we can use to calculate the amount of energy absorbed or released by a system 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) t final - t initial

Calculating Energy Changes: Please note: When using the Q = mc∆t formula: If heat is absorbed by the system, Q will be positive. If heat is released by the system, 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

Example 1: What quantity of heat is required to raise the temperature of 1.00*10 2 g of liquid water 10.0°C? What was the energy change in the surroundings?

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.