Chemistry Unit 7


measure of the average speed of molecules Celsius = Kelvin-273 Kelvin = Celsius+273

*Celsius and Kelvin are not the same values, but they have the same increments

Factors Which Affect Phase Change


2.Pressure increase in temperature causes the molecules to move faster. Solid to liquid to gas Decrease in temperature causes the molecules to slow down. Gas to liquid to solid

2.The more pressure a liquid is under, the harder it is for that substance to become a gas. i.e.)slightly increasing the pressure by putting a lid on top of a pot of boiling water. The liquid condenses on the top before falling back as a liquid

Collision Theory

For a reaction to occur, particles must collide with an effective collision(proper angle – must have a positive and negative to create a bond, and sufficient energy)

Reaction Rate Influences:

1.Nature of reactants -how reactive are the substances? (will it be a fast or slow reaction? i.e.copper and water reacts slowly during oxidation) -Are the reactants even able to react? (i.e.Gold doesn’t react with water)

2.Concentration – the amount of substance -high concentration of reactants = higher reaction(rxn) rate -low concentration of reactants = lower rxn rate

3.Temperature -increase in temp. = higher rxn rate (due to faster moving molecules and thus more collisions) -decrease in temp. = lower rxn rate 4.Presence of a Catalyst – substance that speeds up a chemical reaction without changing the outcome of the reaction

Activation Energy Diagram

chart which shows progression of energy during a reaction.

Activation energy is the amount of energy required to start a chemical reaction

Heat of reaction(?Hr) is the heat gained or lost during chemical reaction

1.Endothermic Reaction

2.Exothermic Reaction

1.heat is absorbed – it is gained. Energy of reaction(?Hr) is positive

2.heat is released – it is lost. Energy of reaction(?Hr)is negative.


Reversible Reaction

Can produce “product” or may reverse to produce “reactant”

Catalyzed Reaction

Lowers the activation energy needed to produce products


force per unit area(caused by atoms or molecules hitting walls of container)

Factors affecting pressure are: -number of molecules or mass(# of collisions)

-volume of the container(# of collisions)

-temperature (speed with which atoms hit container walls)

Gas Laws

the “Gas Laws” describe the behavior of gases under various conditions of temperature, pressure and volume

Boyle’s Law

P1V1 = P2V2

The pressure exerted by a gas varies inversely(opposite:increase of volume, decrease in temp., and vice versa) with the volume(if the number of atoms and the temperature remain constant)

Gay Lussac’s Law

V1/T1 = V2/T2

The pressure of a gas varies directly(same:temp. increases, pressure also increases) with the temperature(in Kelvin)(if the number of molecules and the volume remain constant)

Charles’ Law

V1/T1 = V2/T2

The volume of a gas varies directly with the temperature(in Kelvin)(if the number of molecules and the pressure remain constant)

Combined Gas Law

P1V1/T1 = P2V2/T2

incorporates Boyle’s, Charles’ and Gay Lussac’s Laws into one mathematical formula

Gas Laws Units

Pressure: P (atm, mmHg, torr, or KPa)

Volume: V (L or mL)

Temperature: T (Kelvin)

Ideal Gas Law (use Sig Figs)

PV = nRT

Describes the behavoir of an “ideal gas”(no attraction between molecules)

Ideal gas- an imaginary gas that perfectly fits all assumptions of the Kinetic-molecular theroy

P= pressure (any unit)

V= volume (liters)

n = number of moles (moles)

R= gas “constant”

T= temperature (K)

Example 1)What pressure is exerted by 4.50 moles of gas in a 198 liter container at 8.0°C?

PV = nRT…

crpss out like units…P(198L) = 4.50mol(.0821(Latm/molK)(281K)…

198P = 103.815atm…divide by 198 on both sides

Answer: P= .52atm

Ex.2) What is the volume occupied by 9.45 grams if C2H2 at STP?

…convert the mass of C2H2 into moles: 9.45gC2H2 x (1mol/26gC2H2)= .363moles

PV = nRT

(1atm)V = .363mol(.0821Latm/molK)(273K)

V= 8.13atm

Dalton’s Law of Partial Pressure

In a mixture of gases, the total pressure of the mixture is equal to the sum of the pressures that each gas would exert by itself in the same volume

Ptotal = P1+P2…+Pn

Example)What is the atomospheric pressure if the partial pressure of N2, O2, and Ar are 604.5mmHg, 162.8mmHg, and 0.5mmHg respectively?

604.5 + 162.8 + 0.5 = 767.8mmHg!

Gas Laws Examples (use Sig Figs)

Ex.1) A gas occupies 25.0mL at 17.0?C. If the volume is decreased to a 12.0ml, find the new temperature in degrees Celsius. Assume constant pressure.

V1/T1 = V2/T2

17.0 + 273 = 290K (convert ?C to Kelvin)

…25mL/290K = 12.0ml/T2…cross multiply

…T2(25mL) = (12.0ml)(290K)…divide each side by 25mL to get T2 by itself

T2= (12.0)(290K)/25 = 139.2K…convert back to Celsius

139.2 – 273 = -133.2…use sig figs

Answer = 130?C


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