Topic: Matter & Energy Aim: How can the amount of energy gained or lost by a system be calculated? Do Now: What Celsius temperature is the equivalent of 500K? HW:

Heat Energy measured in Joules or calories Usually use Joules in here

Heat Equation Measures how much heat energy is absorbed or released in a system when a substance is warmed or cooled Ref. Table T Q = mCΔT Q = energy (Joules) m = mass C = specific heat capacity (given on Table B for water) ΔT = change in temperature (Tf – Ti) THIS EQUATION CAN ONLY BE USED WHEN TWO TEMPERATURES ARE GIVEN IN THE PROBLEM!!!

Physical Constants for Water Focus on Specific Heat Capacity first

Specific Heat Capacity Amount of heat needed to raise the temperature of 1g of a substance by 1oC Cwater = 4.18J/g.oC (on Table B) This means that if I have 1g of water, I have to add 4.18J of energy to it to raise the temperature 1oC. Considering small size of H2O molecule, this # is high WATER RESISTS CHANGES IN TEMP!!

Practice Problems What is the total number of joules of heat energy absorbed by 15 grams of water when it is heated from 30°C to 40°C? What is the total # of Joules lost when 10g water at 80 degrees is cooled to 60 degrees? What is the total # of Joules of heat absorbed when the temp of 200g water is raised from 10 to 40 degrees? How many Joules of heat are absorbed in raising the temp of 10g of water by 15 degrees?

Practice Problems A sample of water is cooled from 45 degrees to 35 degrees by the removal of 84 Joules of heat. What is the mass of the water? A sample of water is heated from 10 to 15 degrees by the addition of 126 Joules of heat. What is the mass of the water?

Practice Problems 50 grams of water absorbs 4200 Joules of heat. What is the change in temperature? What is the temperature change if 80 grams of water absorbs 1680J of energy? When a 500g sample of water at 19 degrees absorbs 84000J of heat, the final temperature of the water will be…? The specific heat of ethanol is 2.46 J/goC.  Find the heat energy required to raise the temperature of 193 g of ethanol from 19oC to 35oC.

Other Heat Problems What if the substance is changing phase? What if the question states that temperature is constant? YOU CANNOT USE Q=mCΔT!!! BECAUSE THERE IS NO ΔT!!

Heat of Fusion amount of energy required to melt or freeze (crystallize) 1 gram a solid, Hf. Table T for equation, Table B for Hf value Q = mHf NOTICE – THERE IS NO TEMP. CHANGE LISTED – BECAUSE THE TEMP. REMAINS CONSTANT DURING A PHASE CHANGE! DO NOT FORGET THIS! YOU HAVE BEEN WARNED! Now, let’s do some practice problems.

Sample Problems How much energy is required to completely melt 31.5 g of H2O at its melting point of 0 °C? How much energy must be removed to freeze 53.1 g of H2O at 0 °C? How much heat energy is absorbed when 10 g ice melts to form liquid water at constant temperature?

Heat of Vaporization amount of energy required to evaporate (vaporize) or condense 1 gram of a liquid, Hv. Table T for equation, Table B for Hv value Q = mHv NOTICE – THERE IS NO TEMP. CHANGE LISTED – BECAUSE THE TEMP. REMAINS CONSTANT DURING A PHASE CHANGE! BY NOW, YOU SHOULD SEE THAT THIS IS IMPORTANT! Shall we practice? We shall.

Sample Problems How many joules of energy are required to completely vaporize 49.5g of H2O at its boiling point? 80.1 g of H2O exists as a gas at 100°C. How much energy must be removed to condense the steam into liquid? How much heat is absorbed when 70 g of water is completely vaporized at its boiling point?

Sample Problems What is the total # of joules of heat energy absorbed when 10 grams of water is vaporized at its boiling point? How much heat in joules is released when the temperature of 10 grams of water is changed from 15.5 degrees Celsius to 14.5 degrees Celsius? How much heat energy is absorbed when 55g of ice melts to form liquid water?