Objectives Students will recognize that energy exchanged between

Objectives • Students will recognize that energy exchanged between the system and surroundings might only cause a temperature change. • Students will understand how energy is related to temperature change for a substance.

Temperature Change • Last week, we learned that processes that change a substance (phase change, chemical reaction) had a value represented by ΔH. • To find the amount of energy exchanged, we could solve: q = n ΔH • Although there is a formula, in most cases, we solved for q using unit analysis. Either way, the Δ H value was utilized.

Temperature Change • But, what if the only change that occurs is a temperature change? • In this case, we have to focus on a new concept – specific heat capacity, or just specific heat.

Specific Heat • Think about going to the beach. • How does the sand feel at noon? How about the water? • How does the sand feel at 10 pm? How about the water? • Why?

Specific Heat • Water has a relatively high specific heat. u It takes a lot of energy to raise the temperature of water. u Due to this, the temperature of the water doesn’t change quickly upon heating or cooling. • Sand, cement, metals, etc. have relatively low specific heats. u It doesn’t take much energy to raise the temperature of these materials. u Due to this, these materials heat up and cool down quickly.

Specific Heat Specific heat capacity (or specific heat), c, is a measure of how much energy is needed to raise the temperature of an object OR how much energy must be released to lower the temperature of an object. We can use specific heats to determine how much energy is needed to raise the temperature of an object OR to determine how much energy is released when the object cools down.

Units for energy calorie (cal) – defined as the amount of energy needed to raise the temperature of 1 g of water by 1°C. How much energy is needed to raise the temp. of 5 g of water by 1°C? How much energy is needed to raise the temp. of 5 g of water by 10°C?

Units for energy Calorie (Cal) – this is a dietary unit. It is actually equal to 1 kilocalorie. 1 Cal = 1 kcal = 1000 cal Joule (J) – this is the SI unit for energy. We will be using this unit primarily. 1 cal = 4. 184 J

q = mcΔT q = energy exchanged (J or k. J) m = mass (g or kg) c = specific heat (J/g°C) ΔT = Tf – Ti, change in temperature (°C)

q = mcΔT A 22. 0 g sample of water cools from 35. 0°C to 21. 0°C. What is q? Do we know the specific heat of water? For H 2 O(l), c = 4. 184 J/g°C (You will need to memorize this value. Specific heats of other substances will be provided. )

q = mcΔT A 22. 0 g sample of water cools from 35. 0°C to 21. 0°C. What is q?

q = mcΔT A 22. 0 g sample of water cools from 35. 0°C to 21. 0°C. How much heat is released? The question indicated the direction of the heat flow. Our answer should be positive. 1290 J are released

More examples 1. The specific heat of ethanol is 2. 46 J/g∙°C. Find the energy required to raise the temperature of 193 g of ethanol from 19 o. C to 35 o. C. 2. When a 120 g sample of aluminum absorbs 9612 J of energy, its temperature increases from 25 o. C to 115 o. C. Find the specific heat of aluminum. Include the correct units.