Chem 1A Midterm 2- Energy

Haven't done lesson 21 yet but will get there

Law of conservation of mass

Mass (atoms/matter) can’t be created or destroyed

In a balanced chemical equation, there should be (same/different) number of each element on either side

Same

When balancing a chemical equation, what part do you change to balance?

Stoichiometric coefficient

What are tips for balancing a chemical equation?

1. Start with balancing whatever is in the fewest formulas

2. Balance polyatomic ions as a single element

What is the limiting reactant?

The reactant that runs out first, stopping the reaction and limiting how much product is made

What is the opposite of the limiting reactant?

Excess

T/F: You cannot have fractional stoichiometric coefficients

False- you can, since the coefficients represent ratios and not the whole amount of each element

How do you find the limiting reactant?

Use unit conversion to find how much product can be made with either reactant, whichever makes the LEAST amount of product is the limiting reactant

What is heat capacity?

(C) how much energy is needed to raise temperature

What unit is temperature in, in heat capacity?

J/mol

What is specific heat?

(c) heat capacity per g or mol

What unit is specific heat in?

J/ g C

or

J/ mol C

(Heat capacity/specific heat) is impacted by volume

Heat capacity is impacted by volume

Specific heat is not

What is a factor in specific heat, in the case of liquids and solids?

Bonding strength/ IMFs

What are degrees of freedom?

Ways for a molecule to move when influenced by heat energy

What are the ways something can move, according to degrees of freedom?

Rotations

Vibrations

Translations

Which kind of movement increases temperature?

Translations ONLY

If something rotates or vibrates, it has a higher ____

Specific heat

More degrees of freedom means a (higher/lower) final temp

More degrees of freedom means a lower final temp

If something moves more, it will be (more/less) hot

If something moves more, it will be less hot

If something moves less, it will be (more/less) hot

If something moves less, it will be more hot

Since most of its energy can go into translational movement, which would increase the T

(More/less) bonds means more vibrations

More bonds means more vibrations

What kind of bonds limit rotations?

Linear bonds

(Weaker/stronger) IMFs make a molecule easier to move, resulting in (low/high) heat capacity and (low/high) temperature

Weaker IMFs make a molecule easier to move, resulting in low heat capacity and high temperature

What are the equations for heat input/output?

q = mcΔT or q = CΔT

What do the variables in q = mcΔT and q = CΔT stand for?

q - heat input or output

m - mass in mol

c - specific heat

C - heat capacity

ΔT - change in temperature (final - initial)

In calorimetry, what two values are always the same between the surroundings and object?

Final temperature

In calorimetry, how is q (heat input/output) related to either object?

The q of one object is the inverse of the other; or in other words

q (surroundings) = - (q object)

Because the amount of heat one object loses is the same amount as what the other gains

What is the q of an exothermic reaction?

q < 0

Which way does energy flow in an exothermic reaction?

Energy exits the system

How does an exothermic reaction feel?

Hot!

Which way do KE and PE flow in an exothermic reaction?

High → low

What is the q of an endothermic reaction?

q > 0

What is the flow of energy in an endothermic reaction?

Energy ENTERS system

How does an endothermic reaction feel?

Cold!

What is the flow of KE and PE in an endothermic reaction?

Low → high

Which reaction has an energy input?

Endothermic

Which reaction has an energy output?

Exothermic

What is sublimation?

Phase change from solid to gas

What does potential energy (at least in this class) refer to?

How close particles are to one another

What sign is attraction?

Negative

What sign is repelling?

Positive

A small radius/close together particles indicate a (weak/strong) attraction and (very/less) negative PE

A small radius/close together particles indicate a strong attraction and very negative PE

A large radius/far apart particles indicate a (weak/strong) attraction and (very/less) negative PE

A large radius/far apart particles indicate a weak attraction and less negative PE

On a phase change graph, what does an angled slope indicate?

KE change, temperature change

On a phase change graph, what does a constant slope indicate?

PE change, phase change

Explain why PE change does not change temperature

Temperature will increase until a phase change, when another particle movement overtakes translation and causes temp to be constant during phase changes

What is enthalpy?

Heat, ΔH

T/F: ΔH of opposite phase changes are the same integer

F: ΔH of opposite phase changes are opposite integers

ex) ΔH(vaporization) = -ΔH(condensation)

When looking for which system has lower PE, what can you look for?

• Phase

  • From highest to lowest PE: gas, liquid, solid

• Bond #

  • More bonds = more attractions = negative PE

Describe what is happening in this image, and how it relates to attraction/repulsion

Read the graph from right to left

1) The distance (radius) decreases, and not much attraction is “felt” because particles are too far apart

2) The potential energy drops suddenly (indicating strong attraction!) when the particles are at optimal distance

3) As the radius decreases further, repulsion force dominates attraction force, and potential energy increases (strong repulsion)

4) Repulsion force will cause radius to increase to an optimal distance

Where on the graph do particles “want” to be?

Particles want to be at the lowest PE possible, at a distance that is not too close or far

What is optimal bond length?

Just the right length of bond so that repulsion does not over take attraction

What is optimal bond energy?

The amount of energy input, in kJ/mol, required to break bond

What makes a strong bond?

• Higher order (more bonds)

• More polar (a more polar bond indicates very partial charges that are attracted to one another)

• Shorter bond length (look at the atom size)

When comparing two bonds’ attraction/repulsion graph, what can you look for?

(example pictured)

• Radius: a shorter bond length will be closest to the axis (smaller r = stronger attraction)

• Depth of curve, which represents bond strength (deeper curve = more PE, stronger attraction)

T/F: Energy is stored in bonds, and breaking those bonds releases energy

False!! Common misconception, though :D

Bond breaking is (endothermic/exothermic)

Bond breaking is endothermic

It requires energy input to break the bonds

How can bond breakage release energy?

Forming an even stronger bond

Which characteristics does low PE correspond with?

• (Negative/positive)

• (Strong/weak) attraction

Low PE = negative = strong attraction

Which characteristics does high PE correspond with?

• (Negative/positive)

• (Strong/weak) attraction

High PE = positive = weak attraction

What sign is an exothermic ΔH(rxn)?

ΔH(rxn) < 0

or

ΔH(rxn) is negative

What sign is an endothermic ΔH(rxn)?

ΔH(rxn) > 0

or

ΔH(rxn) is positive

In an exothermic reaction, products have (higher/lower) PE and (higher/lower) bond energies

In an exothermic reaction, products have lower PE and higher bond energies

In an endothermic reaction, products have (higher/lower) PE and (higher/lower) bond energies

In an endothermic reaction, products have higher PE and lower bond energies

What does Hess’s Law say?

If a reaction can be written as a sum of several steps, the ΔH is equal to the sum of Δ of each step

What is a pure element?

The most stable version of an element @ 1 atm and 298 K

What state of matter are most elements in at standard conditions? What about noble gases?

Most elements are solids at standard conditions

Noble gases are gases

Which elements want to be “doubled” at standard conditions?

H O N Cl Br F

What is enthalpy of formation? aka ΔH_f^o

Heat energy released or consumed when 1 mol of substance is formed from pure elements in the most stable form @ standard state

What unit is enthalpy of formation in?

kJ/mol^-1

What is important to note about pure elements in ΔH_f^o equations?

Pure elements ΔH_f^o = 0 since you don’t form anything with pure elements (they’re already pure, as the name suggests)

What is the ΔH_rxn^o equation?

ΔH_rxn^o = ∑nΔH^o_f(product) - ∑nΔH^o_f(reactants)

aka

[number of mols (enthalpy of formation of products)] - [number of mols (enthalpy of formation of reactants)]

Given the three ways below to model PE or KE changes, which model corresponds to which energy change?

q = mcΔT

ΔH_fus/con/vap

ΔH_rxn

KE: q = mcΔT

PE: ΔH_fus/con/vap and ΔH_rxn

Is enthalpy alone enough to predict the spontaneity of changes?

No!

What is spontaneity?

The “natural” direction of change

Is spontaneity always +ΔH?

No- can be +ΔH, -ΔH, or ΔH = 0

What is entropy (S)?

Dispersion/spread of matter and energy

How is entropy measured?

The number of ways you can arrange a system

Many ways to arrange a system = (higher/lower) entropy

Higher

Fewer ways to arrange a system = (higher/lower) entropy

Lower

What is a perfect crystal, and what are its conditions?

Perfect crystals occur at T = 0 K, there is no entropy

What is the ΔS of everything other than a perfect crystal?

S > 0

When comparing entropy, what can you look at?

• Phase of matter

  • # of particles

  • Which is hotter

• # of molecules

• Spread

Arrange phases of matter by entropy, high to low

Gas > liquid > solid

If both objects are gases, what can you look at to determine higher entropy?

Which has more particles

More particles = higher entropy

If both objects are solids, what can you look at to determine higher entropy?

Which is hotter

Do more or less molecules signal higher entropy?

More

Does less or more spread out particles signal higher entropy?

More spread

What is the equation for ΔS^o_rxn?

ΔS^o_rxn = ∑nS^o_products - ∑nS^o_reactants

What does a negative ΔS indicate?

Nonspontaneous

What does a positive ΔS indicate?

Spontaneity

What is the relationship between ΔS_surr and ΔH_sys?

ΔS_surr and ΔH_sys are always the opposite sign of each other

Are ΔS_surr and ΔS_sys always opposite signs from one another?

No! They can be, but it’s not a rule

What is the equation for ΔS using ΔH and T?

ΔS = ΔH/T

When should you use ΔS = ΔH/T?

Constant T, like during a phase change

What is the equation for ΔS_total?

ΔS_surr + ΔS_sys = ΔS_total

What sign is ΔS_total?

Should always be ΔS_total > 0

What is the second law of thermodynamics?

Changes occur so universal entropy increases, aka so that ΔS_total is positive

If ΔS_total is positive, it is (nonspontaneous/spontaneous)

If ΔS_total is positive, it is spontaneous