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?