Energy in Chemical & Physical Changes
Thermochemistry
• Study of changes that accompany chemical reactions and phase changes
• The Universe is considered to be made of 2 parts:
1. System: part that contains the reaction or process
2. Surroundings: everything else
ENERGY• defined as the ability to do work or transfer
heat energy.2 types of energy
1.Potential Energy (PE): Energy at rest due to the position of an object; chemical potential energy is the energy stored in a substance’s bonds.
2.2. Kinetic energy (KE): Energy of the motion of particles in a substance and is directly proportional to temperature. As temperature increases, KE also increases.
Law of Conservation
• Law of Conservation of Energy states that energy is neither created nor destroyed, just changed in form
C8H18 + O2 H2O + CO2 + Energy
• Stored PE converts to 25% work and 75% heat (ENERGY)
Exothermic Reactions• HOT PACK• An exothermic reaction is when the system
releases energy; heat flows out of a reaction and the surroundings get warmer. They have a NEGATIVE H.
• H products < H reactants 4Fe + 3 O2 2 Fe2O3 + 1625 kJ OR 4Fe + 3 O2 2 Fe2O3 H = - 1625 kJ
Endothermic Reactions• COLD PACK• An endothermic reaction is when the system
absorbs energy; heat flows into a reaction and the surroundings get cooler. They have a POSITIVE H
• H products > Hreactants
27kJ + NH4NO3(s) NH4(aq)+1+NO3(aq)
-1 OR
NH4NO3(s) NH4(aq)
+1 + NO3(aq)
-1H = + 27 kJ
Reaction Co-ordinates
What is the difference between Temperature & Heat?
Temperature•Instrument: thermometer•Units: Celsius, Fahrenheit, Kelvin•Definition: A measure of the average kinetic energy of the molecules in a substanceA measure of the motions of the moleculesA measure of how hot or cold something is
What is the difference between Temperature & Heat?
Heat•Instrument: calorimeter•Units: calories, joules•Definition: The total amount of energy in a substance.A form of energy that is transferred between objects because one is warner than the other.Heat transfer is always from hot to coldDepends on 3 things:1. amount of substance (mass)2. Temperature change 3. type of material (specific heat)
Units of Heat Energy
• A calorie is defined as the amount of heat needed to raise the temperature of 1 g of water by 1 C
1 cal= 4.184 J• Food “Calories” are kilocalories.
1kcal = 1000 calories.
Temperature ≠ Heat
Greater Thermal Energy
Specific Heat• Amount of heat required to raise the
temperature of 1 g of a substance by 1 C• Different substances have different specific
heats.Water has a specific heat of 4.184 J/gC. Iron(Fe) has a specific heat of .449 J/gC. Gold (Au) has a specific heat of .129 J/gC.
• The higher the specific heat the more energy it takes to change its temperature.
Calculating Heat
c= specific heatq = heat in joules or galoriesm= mass T = change in temperature = Tf – Ti
c= q_ mT
Example
• A 155 g sample of an unknown substance was heated from 25.0 C to 40.0 C. The substance absorbed 5696 J of energy. What is the specific heat?
Example
• How much heat is needed to change the temperature of 12.0 g of silver with a specific heat of 0.057 cal/g°C from 25.0°C to 83.0 °C?
Measuring Heat in a Calorimeter
• A coffee cup calorimeter measures heat at constant pressure; works on the premise that the amount of heat released in a reaction(-q) or physical change is equal to the amount of heat absorbed by the water(+q) - q = +q
• Rearrange the specific heat equation:
q = m x c x T
Example• A piece of unknown metal with mass 17.19 g is heated to an
initial temperature of 92.50 °C and dropped into 25.00 g of water (with an initial temperature of 24.50 °C) in a calorimeter. The final temperature of the system is 30.05°C. What is the specific heat of the metal? Specific heat of water = 4.184 J/g°C
Example• A 32.07 gram sample of vanadium was heated to 75.00 °C (its
initial temperature). It was then dumped into a calorimeter. The initial temperature of the calorimeter’s water was 22.50 °C. After the metal was allowed to release all its heat to the calorimeter’s water, 26.30 °C was the final temperature. What mass of distilled water was in the calorimeter?
• Specific heat of vanadium = .4886 J/gC Specific heat of water = 4.184 J/g°C
Measuring Heat during Phases Changes
Heat of Fusion/Solidification• Heat of fusion (Hfus ) is the heat energy required
to melt one gram of a solid at its melting pointFor water, Hfus = 334 J/g
q = Hfus x mass
• Heat of solidification (Hsolid ) is the heat energy lost when one gram of a liquid freezes to a solid at its freezing point
For water, Hsolid = -334 J/g
q = Hsolid x mass
Heat of Vaporization/Condensation• Heat of vaporization (Hvap) is the heat to vaporize
one gram of a liquid at its normal boiling pointFor water, Hvap= 2260 J/g
q = Hvap x mass • Heat of condensation (Hcond ) is the heat energy
released when one gram of a liquid forms from its vapor
For water, Hcond = -2260 J/g
q = Hcond x mass
Example
• How much heat is needed to melt 500.0g of ice at 0 C?
Example
• How much heat is evolved when 1255 g of water condenses to a liquid at 100°C?
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