Chapter 8: Energy
Coffee cup calorimetry
A technique for measuring heat changes that uses an insulated container (such as a polystyrene coffee cup) to measure heat changes
Bomb Calorimetry
A technique for measuring heat changes using a sealed container; commonly used to measure high-energy reactions.
Calorimetry
An experimental technique used to measure heat changes
Guide to solving temperature changes: enthalpy vs specific heat
Specific heat of a substance is used when a gas, liquid, or solid changes temperatures without a phase change. Enthalpy of fusion or vaporization is used during the phase change.
The acid-base neutralization reaction shown here is an exothermic process. How many grams of NaOH are neutralized if 218.5kJ of heat are released in the process? HBr (aq) + NaOH (aq) → NaBR (aq) + H₂O (l) ∆H(rxn) = -55.8 kJ
((x) g NaOH) * (55.8kJ/1mol NaOH) * (1mol NaOH/40g) = 218.5 kJ solve equation x = 156.6308244 3 significant figures 157 g NaOH
Aluminum has a specific heat of 0.91 J/g *°C. If a 4,000 g sheet of aluminum absorbs 28,000 jokes of energy, how much will the temperature increase?
(28000J/4000g) * (g * °C/0.91 J) = 7.7°C
Reaction enthalpy
(∆H rxn) The amount of heat energy that is absorbed or released in a chemical reaction at constant pressure.
A student places a block of hot metal into a coffee cup calorimeter containing 100.0g of water. The water temperature rises from 23.1 °C to 31.5 °C. How much heat (in calories) did the water absorb: How much heat did the metal lose?
1300 calories absorbed and lost.
Sodium hydroxide dissolves in water in an exothermic process as shown here. How much heat is released if 139.8g of NaOH dissolve in water? NaOH (s) + NaOH (aq) ∆H(rxn) = -44.0 kJ
139.8 g NaOH * (-44.0 kJ/1 mol NaOH) * (1 mol NaOH/40.0g) = -153.78 kJ 3 significant figures -154 kJ
Methane is the main component of natural gas. Using the reaction enthalpy given here, calculate the heat energy produced by the combustion of one kilogram of methane. CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) ∆H (rxn) = -861.3 kJ
1kg CH₄ * (-861.3kJ/1mol CH⁴) * (1000g/1kg) * (1mol CH₄/16.05g) = -53663.5514 kJ 4 significant figures -5370 -5.370 * 10⁴ kJ
Write a balanced equation for Magnesium and oxygen combining to form magnesium oxide (exothermic).
2Mg(s) + O₂(g) → 2MgO(s) + heat
An engineer tests the thermal properties of a metal alloy. Using a 50.0 g sample, she finds that adding 485 J of heat energy to the alloy causes a temperature change of 4.10 °C. What is the specific heat of this alloy?
485J/(50.0g * 4.10 °C) = 2.37 J/g * °C
Heat energy
A form of kinetic energy involving the kinetic energy of the particles within a substance; heat energy specifically refers to the total kinetic energy transferred from one substance to another.
Temperature
A measure of the average kinetic energy of the molecules in a substance.
Sublimation
A phase change in which a solid passes directly into the gas phase sublimation - solid → gas (endothermic) deposition - solid ← gas (exothermic)
Endothermic change
A physical or chemical change that absorbs energy from the surroundings. A process that absorbs energy Enthalpy of products is higher than reactants, results in a positive ∆H
Exothermic change
A physical or chemical change that releases energy to the surroundings. A process that gives off energy Enthalpy of products is lower than reactants, results in a negative ∆H value
Phase Change
A physical process in which a subs. Both phases are present at the same time. Both phases oaf matter exist simultaneously and the heat energy added or removed only works to change the phase (temperature remains constant during phase change) There is a characteristic quantity of heat needed to perform the phase change per gram (or per mole) of a material.
How may the potential energy of a tightly pulled bowstring be released?
As the tension is released, energy is released. If an arrow is seated on the string, the energy is then transferred to the arrow.
Zinc has a specific heat of 0.39 J/g*°C while iron has a specific heat of 0.45 J/g*°C. If a 100g sample of each metal is cooled from 100 °C to room temperature (25 °C), which one releases more heat?
Because iron has the greater specific heat, it releases more heat energy.
Write a balanced equation for calcium hydroxide forming calcium oxide plus water (endothermic).
Ca(OH)₂ (s) + heat → CaO(s) +H₂O(l)
Coffee cup calorimetry vs bomb calorimetry
Coffee cup calorimetry takes place at a constant pressure Bomb calorimetry takes place at a constant volume.
Is this an endothermic or exothermic reaction? heat + NH₄Cl(s) → NH₄Cl(aq)
Endothermic (heat is absorbed to change the solid to an aqueous solution)
Is this an endothermic or exothermic reaction? heat + H₂O(l) → H₂O(g)
Endothermic (heat is entering the system and converting a liquid to a gas)
How may the potential energy of an anvil positioned at the top of a cliff be released?
Energy is released when it falls off the cliff.
Potential energy
Energy that is stored
Consider the following chemical reaction. Is this change endothermic or exothermic? Which has the higher potential energy, the starting materials or the products.
Exothermic (heat is released into surroundings with the products). Products have less potential energy than reactants (some of the potential energy of the reactants was converted to heat energy in the reaction).
Is this an endothermic or exothermic reaction? H₂O(g) →H₂O(l) + heat
Exothermic (heat is released when the gas converts to liquid)
Is this an endothermic or exothermic reaction? K₂CO₃ (s) → K₂CO₃ (aq) + heat
Exothermic (heat is released when the solid converts to an aqueous solution)
How to determine how much heat energy it takes to complete a phase change:
Heat = n ∆H fus (where n is the number of moles) heat = n ∆H vap (where n is the number of moles) Units of enthalpy (H) Enthalpy always discussed as a change (∆H) ∆H units are kJ/mol
Write a balanced equation for liquid water freezing, forming into ice cubes (exothermic).
H₂O(l) → H₂O(s) + heat
Law of conservation of energy
In a chemical or physical change, the total energy of the universe remains constant.
Surroundings
In thermodynamics, everything that exists around the system being studied.
System
In thermodynamics, the part of the universe being studied.
Energy
The ability to do work
Fuel value
The amount of heat energy that can be released by a combustion reaction of a certain substance.
Heat of vaporization (∆H vap)
The amount of heat per gram or per mole required for a phase change that occurs at the boiling point.
Heat of fusion (∆H fus)
The amount of heat per gram or per mole required for a phase change that occurs at the melting point.
Heat capacity
The amount of heat required to raise the temperature of a given object.
Specific heat
The amount of heat required to raise the temperature of one gram of a substance by one degree Celsius; some times called the specific heat capacity. (c). a proportionality constant that relates heat to a temperature change heat=mc∆T
How may the potential energy of a plate of spaghetti be released?
The chemical energy is released when it is digested.
How may the potential energy of a stick of dynamite be released
The chemical energy stored in it is released when it detonates.
Kinetic energy
The energy of motion; the faster an object is moving, the greater the kinetic energy it has.
Joule (J)
The standard unit of energy; 1J = 1kg*m²/s²
Isothermal
The temperature T of a system remains constant: ΔT = 0.
Boiling Point
The temperature at which a substance goes from a liquid to a gas (or from a gas to a liquid) boiling - liquid → gas (endothermic) condensation - liquid ← gas (exothermic)
Melting point
The temperature at which a substance goes from a solid to a liquid (or from a liquid to a solid). melting - solid → liquid (endothermic) freezing - solid ← liquid (exothermic)
Work
The transfer of energy from one form to another.
A synthethic fuel blend is tested by bomb calorimetry. It is found that an 80.10-g sample releases 28,096kJ of heat. What is the fuel value of this blend in kJ/g?
Use Fuel Value kJ/g 28,096kJ/80.10g = 350.7615481 three significant figures 350.9kJ/g
When many ionic compounds dissolve, they release heat. For example, when calcium sulfate mixes with water, the temperature of the water may rise by 10 ⁰C. If the calcium sulfate is the system, what makes up the surroundings? Is this reaction endothermic or exothermic?
Water is the surroundings, and the reaction is exothermic.
Is the wax in the candle burning (system: candle) endothermic or exothermic?
Within the candle system, exothermic.
Is warming your hands by a fire (system: your hands) endothermic or exothermic
Within the hand system, endothermic.
Is the water in a lake releasing heat to the air, forming ice (system: water) endothermic or exothermic?
Within the water system, exothermic.
Thermodynamics
the scientific field that deals with energy and temperature changes