Chapter 7: Chemical Energy
What is thermal energy?
Random kinetic energy
What bonds are stronger in an endothermic reaction?
Reactants Energy that flows into the system as heat is used to increase the potential energy of the system.
What is an exothermic reaction?
Results in the evolution of heat
What is the way that energy transfer is divided between work and heat?
Specific conditions - the pathway
Enthalpy is a ____ function because _____
State function Energy, pressure, and volume are all state functions
What is another term for state function?
State property
What is the law of conservation of energy?
States that energy can be converted from one form to the other but can neither be crated or destroyed
What is thermodynamics?
Study of energy and its interconversions
What is initial energy (E) of a system?
Sum of the kinetic and potential energies of all the "particles" in the system
What is heat capacity?
The amount of energy required to raise the temperature of an object by one degree Celsius
What does energy required to heat a compound or element depend on?
The amount of substance present
What is energy defined as?
The capacity to do work or produce heat.
At constant pressure, what does the change in enthalpy equal?
The energy flow as heat
If the change in enthalpy is negative, _______ and the reaction is _______
The enthalpy of the products is less than the reactants Exothermic - generation of heat
If the change in enthalpy is positive, _______ and the reaction is ____
The products of the reaction will have a greater enthalpy than the reactions Endothermic - heat is absorbed by the system
What is enthalpy?
The property of the system equal to E + PV, where E is the internal energy of the system, P is the pressure of the system, and V is the volume of tech system.
What is calorimetry?
The science of measuring heat
What do we always take the point of view of?
The system
What are the two parts of the universe when it comes to the combustion of methane?
The system and the surroundings
What is the effect of the ball on the hill?
The temperature of the hill increases very slightly as ball rolls down.
What are the two ways to transfer energy?
Through work and through heat
Regardless of the condition f the hill's surface, the ______ ______ transferred will be constant
Total energy
Describe an analogy for difference between a state function and a non state function
Traveling from Chicago to Denver, the change in elevation is always 4606 feet. The distance travels depends on how you make the trip. Elevation is a function that does not depend on the route, but distance is pathway dependent. Elevation is a state function and distance is not
What is important in understanding energy changes in chemical reactions?
Understanding of the nature of these chemical bonds between products and chemical bond between reactants
What must change for pressure-volume work to be performed?
Volume
What is the work equation?
W = FD
What is an example of potential energy?
Water behind a dam has potential energy that can be converted to work when water flows through turbines and creates electricity
What is a common type of work associated with chemical processes? Give an example
Work done by a gas (though expansion) or work done to a gas (compression) Ex: heat from combustion in an engine expands the gases in the cylinder to push on the piston
What is the equation for enthalpy change in a chemical reaction?
change in enthalpy = enthalpy of products - enthalpy of reactants
Heat capacity equation
heat absorbed / increase in temperature
What is the most common unit for pressure in chemistry?
the atmosphere
What is the equation for internal energy
ΔE = q + w ΔE = change in systems internal energy q = heat w = work
What is the formula for energy change under constant volume?
ΔE = qv
What is the change in enthalpy?
ΔH = Q Change in enthalpy = heat
What is the atmosphere equivalent to?
1 standard atmosphere = 1 atm = 760 mm Hg
What is a very important function of the state function:
A change in this function in going from one state to another is independent of the particular pathway taken between two states
What do thermodynamic quantities always consist of?
A number giving the magnitude of the change A sign indicating direction of flow
What is an example of constant-volume calorimetry?
A photographic lightbulb flashes and becomes hot; reaction is inside the bulb where volume does not change.
What is state function?
A property that is independent of the pathway, refers to a property of the system that depends only on the present state rather than past or future
What may differ depending on the pathway?
Amounts of heat and work
What is constant-pressure calorimetry
Atmospheric pressure remains constant during the process
What bonds are stronger in an exothermic process?
Bonds are stronger on the product side More energy is being released by forming new bonds in the products than is consumed to break the bonds in the reactants
What is the device used experimentally to determine the heat associated with a chemical reaction?
Calorimeter
Explain a little bit about cellular respiration and energy
Cellular respiration extracts the energy stored in sugars and other nutrients to drive the various tasks of the cell
What is the living cell powered by?
Chemical reactions
What is a simple calorimeter?
Coffee cup calorimeter Two nested styrofoam cups with a cover, stirrer and thermometer are inserted
What is a coffee cup calorimeter an example of?
Constant-pressure calorimetry
What are the problems with fossil fuels?
Depleting more than can be extracted and refined Waste products from burning fuel effect environment (CO2) may warm the earth and cause changes in climate Impurities react with air to produce pollution
What is constant-pressure calorimetry used in?
Determining changes in enthalpy (heat reactions) for reactions occurring in solution
Where does energy, released as heat, come from in an exothermic reaction?
Difference in potential energies between reactants and products Energy gained by the surroundings must be equal to the energy lost by the system
What is potential energy?
Energy due to position or composition
What is kinetic energy?
Energy due to the motion of the object and depends on the mass of the object m and the velocity
What are the signs for energy?
Energy flows into system (endothermic) then the sign will be positive because of energy increase Energy flows out of a system (exothermic) the sign will be negative because of energy decrease
What are the signs for work?
Energy flows into system (endothermic), surroundings dow work on system, w is positive Energy flows out of system (exothermic), system does work on surroundings, w is negative
What happens to energy in an exothermic reaction?
Energy flows out of the system
Are energy and heat/work state functions?
Energy is a state function Heat and work are not
What are the surroundings?
Everything else in the universe Furnace and room in methane combustion
What does the P in PΔV stand for?
External pressure - the pressure that causes a compression or that resists and expansion
What is the law of conservation of energy in relation to thermodynamics?
First law of thermodynamics The energy in the universe is constant
What can change the internal energy of a system?
Flow of work, heat, or both
What is work?
Force acting over a distance
What is pressure?
Force per unit area
What is the transfer of energy that causes the ball to stop rolling?
Frictional heating
Explain the different ways heat and work can be balanced in the ball scenario
Ground is so rough all energy is expended in frictional heating and no work is done, moves so slowly B cannot move
What is the enthalpy equation?
H = E + PV
What is specific heat capacity?
Heat capacity given per gram of substance
What is molar heat capacity?
Heat capacity given per mole of substance
What is an endothermic reaction?
Heat flows into a system
In this chapter, what will we concentrate on in terms of energy?
Heat transfer that accompanies chemical processes.
Energy is ______ on the pathway, but work and heat are both ______ on the pathway
Independent Dependent
Where does the mercury measurement come from for pressure?
Instruments for measuring pressure are based on the height of a mercury column in mm that the gas pressure can support
Define heat
Involves the transfer of energy between two objects due to temperature differences
What is one of the most important characteristics of energy?
It is conserved
What are the units for specific heat capacity?
J/°C·g J/K·g
What is molar heat capacity's units?
J/°C·mol J/K·mol
Is heat a substance contained by an object?
No, it transfers
What is calorimetry based on?
Observing the temperature change when a body absorbs or discharges energy as heat
What can energy be converted from?
One form to another
How do substances respond differently to being heated?
One substance might require a great deal of heat to raise its temperature by one degree, while others can do so with relatively little heat
What is energy the essence of?
Our very existence as individuals and a society
What is the system?
Part of the universe on which we wish to focus attention Reactants and products of methane combustion
What is the difference between the energy stored in chemical bonds of products and the chemical bonds of reactants?
Potential energy
Describe energy of a ball rolling down a hill
Potential energy is changed to kinetic energy as ball rolls downhill, if the ball hits another ball some energy is transferred, and most importantly kinetic energy is transferred to surface of hill as heat
What is converted in any exothermic reaction?
Potential energy is converted to thermal energy via heat
Define temperature
Property that reflects the random motions of particles in a particular substance