Thermochem (Thx Charlotte!)

What is energy (physics def.)?

The ability to do work.

Why is energy difficult to define(2 points)?

It occurs in many different forms.

It doesn't exist on its own(it's a property of a system).

What are the two types of energy that can be associated with matter?

KINETIC and POTENTIAL energy.

What is kinetic energy?

Energy of movement.

What components are included in kinetic energy (3 types)?

Vibrational movement, rotational movement and transitional movement.

What is potential energy?

Stored energy/Energy of position.

What components are included in potential energy (3 types)?

Intramolecular energy, intermolecular energy and nuclear forces.

What are the units of energy?

Joules (J) \= kg x m^2/s^2

How much energy is a joule?

Approx. the amount required to lift an apple (100 g) 1 metre.

What is THERMAL ENERGY?

TOTAL potential and kinetic energy of a substance.

What happens when thermal energy is absorbed?

The absorbing object gets warmer.

What happens when thermal energy is released?

The releasing object gets cooler.

What is HEAT?

The TRANSFER of thermal energy from a warm object to a cool object and vice versa.

What is TEMPERATURE?

The AVERAGE kinetic energy of molecules.

What unites measure temperature?

Celsius, farenheit and kelvin.

What is kelvin?

The international system for metric temperature scale.

What is the difference between CELSIUS and KELVIN?

They're starting point: (the change in 1 K \= the change in 1 C) & (T in K \= T in C + 273.15)

Is there a limit to how hot and cold we can get?

No limit to hot, but there is a limit to cold (b/c there are no negative kelvins).

What degree celsius is equal to 1 kelvin?

-273.15 Celsius (Absolute zero) \= 1 Kelvin

What is absolute zero?

The point at which all thermal energy is lost, thus the molecules stop moving (0 Kelvin or -273.15 Celsius).

What is a chemical system?

A system composed of reactants and products under study.

What are the surroundings of a system?

All the matter that is NOT part of the system.

What are the three type of systems?

Closed, open and isolated.

What happens during an interaction between systems and surroundings?

An exchange of energy and matter.

What is an OPEN system?

Where energy and matter are exchanged between a system and its surrounding. (pot without lid)

What is a CLOSED system?

Where energy is exchanged with the surrounding, but matter is not. (pot with lid on; heat is released)

What is an ISOLATED system?

Where neither energy nor matter is exchanged with the surrounding. (very difficult to achieve, therefore very few examples)

What is an ENDOTHERMIC reaction?

A chemical reaction where thermal energy is absorbed from the surroundings to break bonds.

What is an EXOTHERMIC reaction?

A chemical reaction where thermal energy is released into the surroundings from the forming of bonds.

What happens when bonds are formed?

Thermal energy is released (exothermic).

What happens when bonds are broken?

Thermal energy is absorbed (endothermic).

What is specific heat capacity?

The quantity of thermal energy required to raise the temperature (average kinetic energy) of 1g of a substance by 1 degree celcius.

What unit measures specific heat capacity?

J/g x C

What has a higher heat capacity: metal slide or plastic slide? Why?

The plastic slide has a higher heat capacity because the quantity of thermal energy required to raise 1g of the plastic by 1 degree celsius is greater (it takes more time and effort to heat).

What does SATP stand for?

Standard ambient temperature and pressure.

What are the standard ambient temperature and pressure?

Temperature: room temp. (25 C)

Pressure: 101 kPa

What is calorimetry?

The process of measuring energy charges during a physical and chemical charge.

What is used to measure energy changes?

Calorimeters

What are the energy changes calorimeters measure?

Thermal energy released by a system and absorbed by the surrounding.

What calculation is used in calorimetry?

q\=mc(change in T)

How is the change in temperature calculated?

Final temp. - Initial temp. \= Change in temp.

Where can we usually find the specific heat capacity ('c') of a substance?

From a table.

What does 'q' give us?

Heat \= the total amount of energy (kinetic+potential) absorbed or released by a chemical system.

In q\=mc(delta T), what unit of measurement is mass ('m')?

Grams

In q\=mc(delta T), can 'q' be negative? Why?

Yes. It means that the reaction is exothermic (more heat is released than absorbed during the reaction).

What does is mean when 'q' is positive in q\=mc(delta T)?

The reaction is endorthermic (more energy is absorbed than released during the reaction).

What is the first law of thermodynamics?

Energy cannot be created or destroyed/"Energy is conserved". The energy of the universe is constant.

What equation represents the FIRST law of thermodynamics?

q of system +q of surroundings = 0

OR

q of system = -q of surroundings; -q of system = q of surroundings.

What is the initial heat capacity of water?

4.19 K/g x C

When using q\=mc(delta T) with a solution, what 'c' is used?

Water's specific heat capactiy of 4.19.

What is the SECOND law of thermodynamics?

When two objects are in thermal contact, heat is ALWAYS transfered from the object at a higher heat content to the object at a lower heat content until the two objects are at the SAME heat content.

What is enthalpy?

The TOTAL thermal energy (heat content) of the system, plus the pressure and times the volume.

What is the equation that represents enthaply?

H \= E + PV

Is it possible to measure the total thermal energy (enthalpy of a system)?

No because of the number of breaking and forming bonds that take place during a reaction at an intramolecular level.

What kind of enthalpy CAN be measured?

The change in enthalpy of a system.

What equation is used to measure the change in enthalpy of a system?

change in H \= change in E + change in (PV)

What magnitude is the change in pressure and volume assumed to be in a lab?

Zero (0)

Is the statement change in: H\=change in E\=q correct?

For this class, yes.

What does change in H \= - or + |q| mean? Why is q - or +?

It assumes change in enthalpy and heat are the same thing. 'q' is - or + depending on whether or not the equation is endothermic (+) or exothermic (-).

What is: change in H\=H of products - H of reactants representing?

The enthalpy change formula.

What type of chemical reaction is occuring when the change in enthalpy is positive?

Endothermic b/c of absorption/increase of energy.

What type of chemical reaction is occuring when the change in enthalpy is negative?

Exothermic b/c release/decrease of energy.

How are enthalpy changes represented in equations(3 ways)?

1. Within the equation: + change in H on the reactant side = endothermic & + change in H on the product side = exothermic.
2. Outside of the equation: change of H degree of reaction = - ________ kJ or change of H degree of reaction = ________ kJ.
3. Potential Energy Diagrams:

EXOTHERMIC - reactant energy is above the product energy & distance between is change in H.

ENDOTHERMIC - product energy is above the reactant energy & distance between is change in H.

What unit of measurement represents heat versus what represent change in enthaply?

Heat \= J; Enthalpy \= kJ

How can we calculate enthalpy(3 ways)?

1. Measure in a lab (using calorimetry).
2. Hess' Law (for chemical changes made in several steps).
3. Enthalpies of formation

How is molar enthalpy represented?

Change in H subscript 'r'

What do thermochemical equations show?

The thermal energy change that accompanies a chemical reaction.

What is the molar enthalpy of a substance?

The energy charge per ONE mol of a substance.

Using thermochemical equations, how do we find the molar enthalpy of a substance?

Divide all coefficients and the enthalpy represented in the equation by the value that decreases the coefficient by the desired substance to 1.

What is the representation of a molar enthalpy of combustion and where are they found? What process is used to find them using a thermochemical equation?

Change in H subscript comb; found in reference table and through the same process of finding molar enthalpy in a thermochemical equation.

What affects the amount of energy involved in a energy change?

The quantity of matter undergoing the change.

What equation is used to find the change of enthalpy with moles?

Change in enthalpy = n x change in molar enthalpy.

n = number of moles.

Molar enthalpy = kJ/mol

What is the Law of Additivity of Reaction Enthalpies?

The total enthalpy change of a reaction is independent of the route taken. Therefore there may be more than one possible route to get from reactants to products, but all that matters are the beginnings reactants and end products themselves, not the number of steps.

What is Hess's Law?

By adding or subtracting chemical reactions with the same products or reactants as the one you're interested in, you can figure out the overall heat of the reaction that you're interested in.

Why might Hess's Law be useful?

It can help when studying reactions that can't be analyzed through calorimetry because they occur too fast or too slow or are too dangerous.

What are the rules of applications fro Hess' Law (two of them)?

1. If you reverse a chemical equation (change the products to the reactions side and the reactions to the product side), you must also change the sign of enthalpy (delta H) by multiplying it by -1.
2. The magnitude of enthalpy change is directly proportional to the number of moles of reactants and products in a reaction. If coefficients in a balanced equation are multiplied by a factor, the value of enthalpy change must be multiplied by that same factor.

How can you identify a Hess's Law question?

There will be one main thermochemical equation and many others available for your use.

How is standard enthalpy of formation represented?

deltaH to the degree and subscript f

What is: standard enthalpy of formation?

The enthalpy change when 1 mol of a compound is formed from its elements.

Are all substances in their standard state in standard enthalpy of formation? What is "standard state"?

Yes. When a substance is in its most stable form at SATP.

What are the standard STATES for the elements?

Most = solids

Noble gases and diatomic elements = gases

Bromine and mercury = liquids

Substances in solutions = concentration of 1 mol/L

What is the standard enthalpy of formation for elements in their natural states? Why is it that?

Zero (0). Because they're in their most fundamental form.

What equation is used to calculate the standard enthalpies of formation?

How can you estimate the enthalpy change?

With the quantity of energy of the chemical bonds of the reactants and products.

What is bond dissociation energy and what is the sign of the values?

The energy required to break a bond. Values are positive.

Why are bond dissassociation energies reported as averages?

B/c the depend on the types of atomes and types of bonds in the same molecule.

When is an exothermic reaction the result (bond energies)?

When the bonds in the products are stronger than the bonds in the reactants.

When is an endothermic reaction the result (bond energies)?

When the bonds in the reactants are stronger than the bonds in the products.

How do you estimate the change in enthalpy using bond energies (equation)?

Change in enthalpy \= sum of bonds broken (reactants) - sum of bonds formed (products)

What bonds have larger bond energies: multiple or single? Why?

Multiple bonds have larger bond energies and are generally stronger. The more the number of bonds between atoms, the shorter the bond length.

What sign represents "sum of" in equations.

Sigma (weird looking 'E')

What is chemical kinetics?

The study of reaction rates and reaction mechanisms.

What are reaction mechanisms?

Events at a molecular level that control the speed and outcome of a reaction.

What is a reaction rate?

The speed at which a reaction is performed.

Does thermodynamics give any indication as to how fast a reaction will take place?

NO

How are rates of reaction usually obtained?

By measuring the rate at which a product is formed or a reactant is consumed over a series of time intervals.

What is the formula for finding the rate of a reaction?

rate\=the absolute value of change in quantity of change in time

Is there only one property that can be divided by time to find the reaction rate?

No. You can divide the change in mass, change in absorbance, change in frequency, etc. to find a rate. It depends on the type of reaction.

What must you remember to include when finding the rate of consumption? (rate\=change in reaction over change in time)

A negative sign beside the change in reactant b/c the quantity of the reactant decreases over time.

When finding the rate of production, will the answer be positive?

Yes, b/c the concentration of product will increase over time.