Chem 1211 Unit 3

What is concentration?

Concentration: amount of substance present per given volume

SI unit: molarity (M)

Molarity (M):

moles of solute per liters of solvent

Notation

•Concentration of species A is denoted as [A] (e.g. [NaCl] = 3.0 M)

•3.0 M = 3.0 mol/L

How do you calculate molarity of ions in a solution?

Recall dissociation/hydration reactions

You will need to write a balanced hydration reaction to calculate the concentration of ions

Concentration of ion will be higher than the compound itself

What is dilution?

Dilution:

the process of adding a solvent to lower the concentration of solute in solution

•mol of solute after dilution = mol solute before dilution

Which ions are leftover in precipitation reactions?

• No ions from the LR that went into the solid will be leftover

• There will be leftover ions from the XS reactant and the LR that went into the aqueous product

What is energy?

Energy

• The capacity to do work

• SI units: Joule (J)

• Work: Force acting over distance

Kinetic vs potential energy

KInetic energy:

• Thermal — random m0lecular movement

• Electrical — movement of electrons

Potential energy

• Chemical — energy stored in bonds in molecules

• Electrostatic — seperation of electrical charges

System vs surroundings

System — Part of the universe chosen to study

• Beaker

• Solid object

• Solution

Surroundings — everything else around the system

What are the types of systems?

Open system

• Matter and energy can be exchanged between system and surroundings

  • Open beaker

Closed system

• Only energy can be exchanged

  • Flask with a stopper

Isolated system

• Neither matter or energy can be exchanged

  • Insulated thermos

What is the first law of thermodynamics?

Energy cannot be created or destroyed, but can be converted from one form to another

What is internal energy?

Internal energy

• The sum of all kinetic and potential energy in a system

• Systems can exchange internal energy through heat (q) or work (w)

|\E = q + w

|\E system = -|\E surroundings

What are exothermic vs endothermic reactions?

Exothermic

• Energy is flowing out of the system

• |\E is negative

• SURROUNDINGS WILL “FEEL” HOT

  • Heat will be introduced into surroundings

Endothermic

• Energy is flowing into the system

• |\E is positive

• Suroundings will “feel” cold

What is heat?

Heat (q)

• The transfer of thermal energy between a system and surroundings due to a temperature difference

• SI unit: Joule (J)

• 1 calorie (cal): Quantity of heat needed to change the temperature of one gram of water by one degree celsius

  • 1 cal =4.184 J

What happens when two objects at different temperatures are brought into contact?

Eventually the two objects will reach the same temperature (thermal equilibrium)

What is pressure-volume (PV) work?

Work involved in the expansion or compression of a gas

• Work = -external pressure x |\V

• Work is negative if volume is expanding (energy leaving system)

• Work is positive if volume is compressing

How do you know PV work is happening?

There will be a change in moles of gas from the reactant to product side of a reaction

State vs path function

State function — properties of a system that don’t depend on how the system reached that states

• Temperature

• Internal energy

Path function — Properties that depend on the path taken to reach a particular system state

• Work

• Heat

What is heat capacity? Specific heat capacity?

Heat capacity — the quantity of heat required to change the temperature of a system by one degree

• Unites : J/ ‘C or J/K

Specific/molar heat capacity: the amount of heat energy required to raise the temp of one gram/mole of a substance 1 ‘C at constant pressure

• Units J/ (g x ‘C)

• q = m x Cs x |\T

How does the law of conservation of energy relate to heat equations?

q system + q surroundings = 0

q system = -q surroundings

q sub2 = -q sub1

mCT = -mCT

What is enthalpy?

Enthalpy (H) — The change in energy of a system, at constant pressure

• IT HAS NOTHING TO DO WITH WORK, ONLY HEAT

• It is the sum of the change in internal energy and the PV product

Change in enthalpy (|\H) — Heat exchanged at constant pressure

• Positive enthalpy means endothermic reaction

• Negative enthalpy means exothermic reaction

• Enthalpy is often called “the heat of”

What is standard enthalpy of formation?

Standard enthalpy of formation (|\Hvf) — the change in enthalpy that accompanies the formation of 1 mole of a compounds from its elements with all substances in their standard state

• For a pure element, standard enthalpy of formation is 0

• For H+ ions it is also 0

What does standard state mean? What are the standard states?

Standard state of an element or a compounds is defined as the most stable form of the substance in the physical state that exists at 1 bar of pressure

• Solid/ liquid: Pure substance at 1 bar

• Gas: Ideal gas at 1 bar

• Aqueous: At the concentration of 1M and at 1 bar

• Temperature is not part of standard state

• Notated by a degree mark next to enthalpy

List of standard enthalpies of formation

Calculating reaction enthalpy equation

ALWAYS PAY ATTENTION TO WHETHER ITS JOULES OR KILOJOULES IN A PROBLEM

C (graphite) is a standard state of carbon

What is enthalpy of reaction?

Enthalpy change per “mol of rxn”

• units: kj/mol

What is calorimetry and calorimeters?

Calorimetry - experiments that measure the amount of heat transfer from or to a substance

Calorimeters - a simple calorimeter is a coffe cup that doesn’t allow any outside heat in or out

• Experiments must be at constant pressure

• q surroundings = - q system

In calorimetry experiments, what variables apply to the surroundings and what apply to the system?

Surroundings (calorimeter)

• Volume of water/solution

  • mL or L

• Density of water

• Initial temp of water, Ti

• Final temp of solution, Tf

• Specific heat capacity

System (rxn)

• Mass of compound

• Molar mass of compounds

• Heat of solution of compounds

  • change in H = KJ/mol

• Final temp of solution, Tf

What is EM radiation?

Energy transmitted when electric and magnetic fields are propagated as waves through space

What are some components of a wave?

Wavelength (weird teepee symbol) - distance between peaks or troughs

• Units: meters (m)

Frequency (v) - # of cycles per second

• Units: hertz (Hz) = inverse seconds

C = ^ x V

• C = Speed of light = 2.998 × 10^8 m/s

Constructive vs destructive interference

• Constructive interference - when two waves are in sync and amplify when added together

• Destructive interference - when two wave don’t sync up and and cancel out when added together

Order of the EM spectrum (from low frequency to high)

• Long radio waves

  • AM radio waves

  • FM radio waves

• Microwaves

• Infrared waves

• Visible light

• UV rays

• X-rays

• Gamma rays

Rich Men In Vegas Use Xpensive Girls

Visible light order from low to high frequency

• Red

• orange

• yellow

• blue

• indigo

• violet

Roy-g-biv

What did Max planck do?

• He studied the radiation emitted from heated solids (black body radiation)

• Disproved that radiation intensity would increase indefinitely as wavelength decreases

What is planck’s constant in energy equation?

E = nhv

n = positive integer, having to do with electron levels

Planck’s constant (h) = 6.626 × 10^-34 J x s

v = frequency in inverse seconds

what is planck’s energy change equation?

• If an electron changes from a higher energy level to a lower one, energy is emitted as EM radiation

change in E = E (higher n) - E (lower n) = change in nhv

If change in n is 1, E = hv

What is the relationship between energy, wavelength, and frequency?

E = hv

c = ^ x V

V = C/^

So, E = hc/

What is the photoelectric effect?

• Einstein discovered

• It occurs when electrons are ejected when light hits a surface of light-sensitive material (metal), but only if the frequency is at or higher than the threshold

• Light behaves as both a wave and a particle (wave-particle duality)

What is the energy of a photon? What is a photon?

A photon is a pack of energy

E photon = hv

What is a line emission spectra?

It is what happens when excited particles are seperated by a prism into visible wavelengths (wavelengths are presented on the figure, longest wavelength on right)

What does the Bohr model of the atom suggest?

• Suggests electrons can only occupy certain energy levels

Bohr Model

• Electrons move around the nucleus in one fixed set of circular orbits

• No energy is emitted as long as the electron remains in a given orbit

• An atom emits energy as a photon when the lectern falls into a smaller orbit with lower energy

• THis only works for one-electron systems like hydrogen

What are the different energy states for a hydrogen atom? What is the formula for change in energy as hydrogen releases a photon?

what is the relationship between mass and wavelength?

de broglie states that larger mass and higher speed equal shorter wavelength

photons are absorbed by metal surface is frequency is above threshold, but electrons are ejected