unit 5: limiting reactants, percent yield, states of matter

limiting reactant

the reactant that runs out first in a chemical reaction and therefore produces LESS of the product.

• it “limits” the amount of product you can make

excess reactant

the reactant you have more of, “extra”

steps to find determine the limiting and excess reactant

1. convert both givens to the moles of one of the products (doesn’t matter which one)

2. the limiting reactant is the reactant that give you the smaller amount of the product (that you calculated)

steps to find the excess reactant that remains after the reaction is complete

1. find the Limiting Reactant (convert both givens to moles of the same product and the smaller number is the LR)

2. find the used mass of excess reactant by converting the given mass of the LR to the mass of the excess reactant

3. for the excess reactant do given mass - used mass to find left over mass

what is percent yield (definition and formula)

it’s the measure of a reaction’s efficiency

(actual yield/theoretical yield) x 100

actual yield

the amount of product ACTUALLY produced in an experiment

key words in problems: IS made, IS produced, IS formed, IS obtained, IS recovered

theoretical yield

the amount of product that you CALCULATE that you could have made if the experiment went perfectly

key words in problems: SHOULD have been made or it’s what you calculate using stoich

yield means

product

what can never be the actual or theoretical yield

any reactants (substances to the left of the arrow) because yield means product

how to find percent yield (steps)

1. determine the actual the theoretical yields from the problem

2. convert the actual and theoretical to the SAME UNITS

3. plug into the formula (don’t forget to multiply by 100)

steps to do % yield backwards (finding the actual or theoretical yield)

1. convert given grams of whatever is given (actual or theoretical) to MOLES of what you are finding (for example i could be given grams of WO3 and i need to find moles of tungsten will be used so i would covert the mass of WO3 to moles of tungsten)

2. plug whatever you got into the correct spot of % yield formula (for example i would put the moles of tungsten i just calculated into the “actual yield” part of the formula because in the problem it said “how many moles of tungsten will be produced)

at high temperatures (atoms moving fast) and low pressures, gases are considered _____ meaning that __________________________ (no IMFs so the gas particles are free to move around their container)

ideal, they don’t interact with each other

when gas is ideal

gas particles don’t interact and spread out as much as possible

what happens if a gas is compressed (due to high pressure or low temps bc of slow moving particles)

the particles begin to feel the attractive forces (IMFs) between them, and will condense into a liquid so no longer ideal

states of matter with IMFs from strongest to weakest when at the same temperature and pressure

solids, liquids, gases

strongest to weakest IMF

H-Bonding, Dipole-dipole, London Dispersion Forces

H-bonding

lots of energy required to become gases (high boiling points)

type of molecule: polar, H-N, O, F

Dipole-dipole

in between H-bonding and LDFs strength wise and are polar

type of molecule: polar

London Dispersion Forces

liquids become gases easily (low boiling points)

type of molecule: nonpolar

liquids in terms of volume and shape

definite volume but no definition shape

properties of liquids

• much more dense than gases (liquid is 1250x more dense than water vapor

• has stronger IMFs than gases

• particles don’t have fixed positions (because no definite shape) but strong IMF’s limit their range of motion

• incompressible

• can flow + diffuse (less-so than gases, so they’re less fluid)

solids in terms of volume and shape

definite volume and definite shape

how does a liquid become a solid (in terms of energy)

decrease the average energy of a liquid which will freeze it and make it a solid

properties of solids

• there must be stronger IMFs than a liquid at a given temperature acting between the particles of a solid

• typically more dense than liquids

gases in terms of volume and shape

no definite shape or volume

properties of gases

• very different from solids and liquids

• particles are far apart

• less dense than solids and liquids

• will completely fill any container

Kinetic Molecular Theory

explains properties of solids, liquids, and gases in terms of energy and IMFs

1. gases consist of small particles that move

2. attractive forces between gas particles are very small

3. the actual volume occupied by gas molecules is very small compared to the volume the gas occupies

4. the average kinetic energy (speed) of gas molecules depends on Kelvin Temperature

5. gas particles are constantly moving in straight paths until they collide with each other or with their container

how to convert from Celsius to Kelvin

Tk = Tc + 273

the relationship between speed of particles and temperature

they go up together and go down together

elastic collisions

particles bouncing off each other, TOTAL kinetic energy remains constant

properties resulting from KMT (kinetic molecular theory)

• gases being compressible

• diffusion

• effusion

• pressure

• atmospheric pressure

gases being compressible

you can eliminate the empty space in a sample of a gas

diffusion

ability to mix particles of different substances dur to their constant, random motion

effusion

ability of a gas to move through a small opening

graham’s law of effusion

the rate of effusion or diffusion of a gas is INVERSELY proportional to the square root of its molar mass

rate of effusion and diffusion = 1/√molar mass

pressure

force over area —> force is due to collisions between gas particles and the walls of their container

units of pressure

atmospheres (atm)

pascals (Pa)

Torr

millimeters mercury (mmHg)

inches mercury pound per square inch (psi)

the conversions between most of the pressure units

1 atm = 760 mmHg = 760 torr = 101.325 kPa

atmospheric pressure

pressure due to air

when internal pressure = external pressure: you feel nothing wrong

when internal pressure is nothing (0 mmHg or no air) with an external pressure of 760 mmHg: the external pushes on the internal and the person in the internal conditions, feels a tight squeeze inside

melting

when a substance goes through a phase change from solid to liquid

vaporization

when a substance goes through a phase change from liquid to gas

sublimation

when a substance goes through a phase change from solid to gas

evaporation

when particles at the surface of a substance with the greatest KE overcome the attractive forces of the liquid and break free into the atmosphere which becomes vapor

volatility

how readily a liquid can evaporate

boiling

this differs from evaporation because this is when vapor pressure = atmospheric pressure and the vaporization if throughout the whole substance not just at the surface

vapor pressure

when particles escape from liquid into the atmosphere and then those particles start colliding with thing, it causes pressure which is called vapor pressure

when VP = AP, liquid boils

desposition

when a substance goes through a phase change from gas to solid

condensation (condensing)

when a substance goes through a phase change from gas to liquid

freezing

when a substance goes through a phase change from liquid to solid

phase diagrams

a graph demonstrating the comparison of pressure vs temp that shows what phase a substance is in under different conditions of pressure and temp

triple point

when 3 phases co-exist on the graph

phase changes

the lines on the phase diagram