Thermochemistry review

thermochemistry definition

The branch of chemistry that studies the energy changes in chemical reactions.

Energy is?

the ability to do work (to make things move).

What is the SI unit for energy?

joule (J) or kilojoule (kJ)

What is kinetic energy?

is the energy that an object has because of its motion.

What is potential energy?

is the energy that an object has because of its position relative to other objects and the level of its attraction to other objects

What is the law of conservation of energy called?

Energy can not be created or destroyed, but energy can be transferred from one object to another and energy can change form.

What are the three ways particles can move?

Vibration, rotation, and translation

Vibrational movement is the:

The movement of the atoms along the bonds in a molecule.

Rotational movement is:

When an atom or molecule "spins" on its axis.

Translational movement is when:

A molecule moves from one place to another.

Temperature is:

A measure of the average kinetic energy of the particles in a substance.

Thermal energy is:

is the total kinetic energy of the particles (either atoms or molecules)

What does thermal energy depend on?

1. how many particles there are

2. The particles temperature

What is heat?

Thermal energy in transit, energy transfer

Where is chemical potential energy stored?

In the bonds between atoms

What does the amount of chemical potential energy depend on?

1. The number of bonds (more bonds, more potential energy)

2. The type and lenght of the bonds (shorter bonds have smaller chemical potential energy)

Enthalpy is

the total internal energy of a substance at a constant

pressure

Enthalpy is meassured in

kJ/mol

What is a system?

the reactants and products that we are studying

And exothermic reaction is

The products have a lower enthalpy than the reactants, energy is released and absorbed by the system, energy "exits", it is a product, released

An endothermic reactions is

The products have a higher enthalpy than the reactants, energy must be absorbed by the system, "energy enters", it is a reactant, required

ΔH value for exothermic reactions is

Negative

ΔH value for endothermic reaction is

positive

A thermochemical equation contains...

the amount of energy lost or gained during a chemical reaction

Using the conversion factor method how do you find ΔH

mass x mol/molar mass x enthalpy value/ coeffcient for molecule

What is a calorimeter used for?

Is a well insulated styrofoam cup in which a chemical reaction can be carried out in, the purpose of this is to ensure the only heat lost is by the chemical reaction and not by the outside forces

What is the surroundings?

Whatever is around the reactants and products

If the reactants have a higher enthalpy (longer bonds) that the products:

1. The enthalpy of the system will go down, and energy will be released

2. ΔH is negative

3. The surroundings will absorb the energy and the temperature will go up

4. Q will be positive

If the reactants have a lower enthalpy (shorter bonds) that the products:

1. The enthalpy of the system will increase, and energy will be absorbed

2. ΔH is positive

3. The surroundings will provide the energy and the temperature will go down

4. Q will be negative

What is specific heat capacity?

The amount of energy required to raise the temperature of 1.00g of a substance to 1.00 C

For ΔH\=-Q

Constant pressure must be achieved in the enviroment, no gases used or produced

how to calulate ΔH/mol

Q\=mcΔt

How to calculate %error

(accepted value-experimental value/ accepted value)x100

Assumptions in calorimetry lab:

1) No heat is lost to, or gained from, the calorimeter or anything outside it.

2) That all of the energy released by the reaction is converted to thermal kinetic energy and transferred to the solution as heat. No energy is lost or produced in any other form such as sound, light or electricity.

3) Density and specific heat capacity (c) of a dilute solution is the same as the density and specific heat capacity of pure water.

4) The reaction is carried out at constant pressure, so Q \= - ΔH.

5) No energy lost from the system if gaseous products are formed or water evaporates from surroundings.

6) assume that there is no transfer of mechanical energy to the water as heat when the solution is stirred.

Calorimetry calculations

1. Q\=mcΔt

2. ΔH\=-Q

3. Find mol

4. ΔH/mol

Hess's law of heat summation

When a reaction can be expressed as the algebraic sum of two or more reactions, then ∆H for the overall reaction is equal to the algebraic sum of the ∆H values of the individual steps.

Rules for Hess's Law

If the reaction is multiplied (or divided) by some factor, ΔH must also be multiplied (or divided) by that same factor.

If the reaction is reversed (flipped), the sign of ΔH must be changed from a negative to positive, and vice versa.

What is metals standard state?

solid

What is noble gases standard state?

Gases

What is HOBrFINCl standard state?

H2(g), O2(g) , Br2(l) F2(g), I2(s) N2(g) Cl2(g)

Standard enthalpy of formation equation Σ

ΔHo\= ΣΔHf(products)-ΣΔHf(reactants)

What is bond energy?

The amount of energy that is required to break a chemical bond

Estimating enthalpy equation:

∆H \= ∑ bond energy of reactants - ∑ bond energy of products

Reaction rate is

A measure of how quickly or slowly reactants are consumed (used up), or products are formed, in a chemical reaction.

if a reaction is fast, the value for the rate will be

larger

if a reaction is slow, the value for the rate will be

smaller

To determine reaction rate

Reaction rate\= change in quantity of substance/change in time

Initial rate:

The rate at the very beginning, when the concentration of reactants and products are very close

Instaneous rate:

The rate of the reaction at a single moment of time, calculated using tangeant

Average rate

the rate of the reaction at a specific moment of time

The Kinetic Molecular Theory of Matter

states that matter is made up of particles and that these particles are in constant motion

Collision Theory:

States that in order for a chemical reaction to occur, the reactant particles must collide with each other

Activation energy

The minimum energy required for an effective collision

Activated Complex

A theoretical high-energy particle that exists for a fraction of a second at the instant the particles collide

if a collision is effective, the activated complex breaks down into the

products

if a collision is ineffective, the activated complex breaks down into the

reactants

5 things that affect reaction rates:

Temperature, Concentration, Surface Area, The Nature of the Reactants , The Presence of a Catalyst

Catalysts do/do not affect....

do affect activation energy, do not affect enthalpy change

elementary steps

most reactions proceed as a series of steps,

Reaction mechanism

The series of elementary steps that take place as a reaction proceeds from reactants to products

Reaction intermediate

some molecules are formed in an early elementary step and used in a later step, RI do NOT appear in the overall chemical equation.

for drawing reaction mechanism use.... to move up and..... to move down

Ea and ∆H

The RDS is the \______ step

Slowest

the reaction rate units should aways be

mol/L//////s

The units for k are

L(m-1)/mol(m-1) x s

A proposed reaction mechanism is plausible (possible) if:

1. the sum of the steps of the reaction mechanism adds up to give the overall reaction equation &

2. the molar coefficients of the reactants in the RDS agree with the exponents for these reactants in

the Rate Law.

Catalyst

A substance that changes the rate of a chemical reaction, but is not consumed during the reaction.

A homogeneous catalyst

is in the same phase as the reaction

a heterogeneous catalyst

is in a different phase as the reaction

A positive reaction

speeds the reaction up by lowering the Ea of the rate determining step

A negative catalyst

slows a reaction down by raising the Ea of the rate

determining step

the standard state of carbon at SATP is

graphite

the specific heat capacity of water is

4.184 J/g C

Enthalpy

The total chemical potential energy of a substance

Effective collision

when the replusion between the valence electrons will break the existing bonds and allow new bonds to form

Activation energy determines the\_________∆H determines\_______

Ea determines rate, ∆H determines which direction is favoured

first order unit for k

1/s

zero order for k

mol/l/s