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Energy

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Valerie Henderson

on 22 April 2016

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Transcript of Energy

Energy
What is energy?
Energy is the capacity to do work or produce heat.
molar heat of combustion
copy and paste as needed and take advantage of an infinite canvas!
Aluminum duckie
Al-duckie) problem
Calorimetry Mini-Lab Report
Peer review of Mini-Lab Report
Students previously learned about one type of energy (kinetic) and will now learn about chemical potential energy. We will learn about forms of energy, sources of energy, how it is stored and energy transfer.
molar heat of combustion part II
Types
of
Energy
Transfer
Sources
Forms of Energy
kinetic
potential
The energy of position
Chemical Potential Energy
stored in chemical bonds
3 Types of Potential Energy
Gravitational Potential Energy
Elastic Potential Energy
Chemical Potential Energy
released during chemical reactions
Can you name things that contain chemical potential energy and describe the chemical reaction that releases that energy?
Chemical Bonds - Review
Individual atoms have relatively low stability (noble gases are the only exception)
Individual atoms become more stable when they are part of a compound.
What holds compounds together?
chemical bonds
When 2 Hydrogen atoms bond, what element do they both have the same lewis structure as?
Helium
= low potential energy and high stability
bond length = distance between 2 bonded atoms @ their minimum potential energy
What happens to potential energy as the attractive electric force pulls them closer together?
aka: energy is released as the attractive force pulls them closer together
as potential energy goes down, the system gives off energy
Bond energy
energy required to break a bond between 2 atoms
What kind of bond will form?
What are the bond types we've learned about?
we're going to focus on types of covalent bonds
In a covalent bond, are electrons taken or shared?
They aren't always shared equally.
What does electronegativity mean?
ability to attract electrons
Electronegativity determines which element has a stronger attraction for electrons.
In an H molecule, electronegativity is the same (because it's 2 of the same type of atom).
2
Nonpolar covalent:
Valence electrons are the electrons that participate in bonding.
Which are those?
Outermost (highest energy level electrons)
valence electrons are shared equally
What do you think happens when the electronegativities are not the same?
Which has a higher electronegativity?
carbon and
oxygen atom
bonding
oxygen
Which one wants the electrons more?
polar covalent bond
valence electrons are shared unequally
For Carbon and Oxygen because Oxygen has a greater electronegativity, the electrons will be pulled closer to the oxygen and it will have a partial negative charge.
Why partial - charge?
electrons pulled towards it
polar molecules
What does the word polar mean?
A polar bond has ends that have partial charges. The less electronegative element is partially positive and the more electronegative element is partially negative.
This is called a dipole.
polarity is related to bond strength
Would a greater electronegativity difference be associated with a stronger or weaker bond?
bond
energy
570
(KJ/mol)
432
366
298
stronger bond
bringing back bonding:
Draw the Lewis dot structure for oxygen and hydrogen
How many Hydrogen atoms will bond with 1 oxygen?
2
What do you think the arrows show?
a dipole (which atom has the partial negative charge because
it attracts the electrons more)
A water molecule has what we call a bent shape
because it has a dipole moment.
Carbon dioxide vs Water
Because carbon dioxide is linear (doesn't have a dipole), the attractive force between carbon dioxide molecules is small.
The attractive force between water molecules is strong because of the dipole (partial positive and partial negative charges),
attractive force between water molecules is strong
Water molecules want to stick together (cohesion) so more energy is needed to separate water molecules from each other.
lone pairs: electrons which don't participate in bonds
A straight line shows a bond formed between 2
electrons.
A single bond is chemical bond formed by 2 electrons.
double bond:
sharing of 2 pairs (4 total) electrons
triple bond:
sharing of 3 pairs (6 total) electrons
For covalent substances, forces that act between molecules
intermolecular forces
dipole-dipole or london dispersion forces or van der waals forces
Intermolecular Forces
short-range and decrease rapidly as molecules get farther apart
These forces are only effective when molecules get close to each other.
Do you think they have an effect on gases?
*Remember KMT
*
Not much
Because the forces are weak, they act only on neighboring molecules
dipole-dipole forces:
the positive end of one molecule attracts the negative end of a neighboring molecule
When polar molecules get close and attract each other, the force is significant if the degree of polarity is fairly high (high polarity = greater difference in electronegativity)
When molecules are very polar, the dipole-dipole force is stronger.
What do you think the boiling point of a substance might tell you about the strength of the bonds of the substance?
When you boil something, what do you add?
heat
higher boiling point = more energy added
(more energy is needed to be added to break stronger bonds)
energy
stronger
London Dispersion Forces
named after German chemist Fritz W. London
experienced by nonpolar molecules
The strength of london dispersion forces between nonpolar particles increases as the molar mass of the particles increases because, generally as mass increases, so does number of electrons.
Breaking Down Bonding
Essential Questions:
1. What is a polar covalent bond and what is a nonpolar covalent bond?
2. Why does water have a high surface tension (the water molecules stick together)?
3. What is the difference between a single, double and triple bond?
4. What are the two types of intermolecular forces and what is the difference?
Enthalpy:








the sum of the internal energy of a system plus the product of the system's volume multiplied by the pressure that the system exerts on its surroundings
energy added during melting or removed during freezing is called the enthalpy of fusion (fusion means melting)
Dipole: has partial negative and
positive poles ( =ends)
Van der Waals Forces:
sometimes used as a synonym for all intermolecular forces
How can geckos climb walls?
On their feet, they have tiny hair-like projections (setae)
These create van der waals forces between them
and the thing they're climbing.
What is a van der waals force?
How do babies get fingers and toes?
week 8: hands and feet look like paddles
week 9: Toes can be seen
How did the feet that looked like paddles develop toes?
Exothermic vs Endothermic
One absorbs heat and one gives off heat
What's the difference?
Etymology -

Exothermic
exo
therme
outside of
heat
What about endothermic?
inside
heat
greek
outside, out of
within
So what happens to heat transfer for endothermic system?
system gains heat from its surroundings
Etymology -

Exothermic
exo
therme
outside of
heat
What about endothermic?
inside
heat
The root exo- means:
The root endo- means:
Labeling:
System:
Surroundings:
The thing you're looking at
everything else
Enthalpy of fusion:
(aka: latent heat of fusion)
aka: heat of fusion
Hydrogen bond: electrostatic attraction between polar molecules that occurs when Hydrogen is bound to a highly electronegative element.
Which elements have the highest electronegativity?
F = most electronegative
N and O also have high electronegativity
The Hydrogen experiences attraction another nearby highly electronegative element.
Heat vs Temperature
Thermochemistry
study of changes that occur during chemical reactions and changes of state.
Why do some substances increase temperature quicker than others?
What is temperature?
measure of average kinetic energy
Heat is the total energy of molecular motion in a substance while temperature is a measure of the average energy of molecular motion in a substance
What's the difference?
Heat energy depends on the speed of the particles, the number of particles (the size or mass), and the type of particles in an object.
Temperature does not depend on the size or type of object.
For example, the temperature of a small cup of water might be the same as the temperature of a large tub of water, but the tub of water has more heat because it has more water and thus more total thermal energy.
Need an example?
Calorimetry
precise measurement of exact amount of heat flow into or out of a system for chemical and physical processes
First law of thermodynamics
energy can't be created or destroyed
heat released by the system is equal to the heat absorbed by its surroundings
or
Heat absorbed by the system is equal to the heat released by its surroundings
It can be changed to different forms
Temperature is a measure of molecular motion and is measured in C or K

Thermal (heat) energy is transferred spontaneously from an object at a higher temperature to another object at a lower temperature.

Thermal equilibrium is reached when the two objects are at the same temperature.


Heat is measured in Joules (J) or calories (cal).

A sum-up of Calorimetry
How do we increase the temperature?
add energy
What kind of energy do we add to increase the temperature?
heat energy
o
Temperature Changes:
What happens to the temperature of an object when it absorbs heat?
What happens to the temperature of an object when it releases heat?
The temperature change can be used to determine the amount of heat that was absorbed or released if the relationship between these two quantities is known.
Heat energy is measured in?
Joules (J)
or
Calories (cal)
One J is defined as the amount of energy expended by a force of 1 newton moving an object 1 meter in
the same direction as the force.
The unit is named after the English physicist James Prescott Joule, whose work led to the theory of conservation of energy, and the first law of thermodynamics, in the 19th century. Through his experiments, he was able to determine that energy changes from one form to another when work is done, but cannot be created from nothing or destroyed.
He was John Dalton's student.
1 calorie = amount of energy needed to raise 1 g of water 1 degree celsius
1 calorie = 4.184 J
calories on food packages = kilocalories (kcal)
The specific heat capacity (C) of any substance is defined as the amount of energy needed to increase the temperature of 1 g of the substance by 1 ºC. What is the specific heat capacity of water? What are the units?

4.184 J
Water has a very high specific heat capacity.
This is what accounts for moderate climates in coastal areas.
specific heat capacity
calorimetry p.o.g.i.l.
Objective:
To determine based on data analysis from experiments an equation to determine heat absorbed or released by a system in calorimetry problems.
enthalpy of vaporization
aka: latent heat of vaporization
vaporization means evaporation
as a liquid evaporates, a lot of energy is needed to separate the particles
Calorimetry Problem
Help Center
heat released when 1 mole of the substance undergoes complete combustion.
always exothermic (releases heat)
to show this, the sign of the ( H) heat of combustion is negative (-)
energy (q) released by the surroundings = energy (q) absorbed by system
surroundings =
system =
bath water
Al-duckie
heat flows from hot to cold
(bath water will release heat energy which will be absorbed by the Al-duckie)
q
water
=
q
Al-duckie
- sign means the system is releasing energy
-
q = m c T
m c T
water
water
water
=
m c T
Al-duckie
Al-duckie
Al-duckie
molar heat of combustion is given in units of KJ/mole
H
=
q
q
material
=
(1560 J)
1560 J is equivalent to mc
=
4,992 J
x
1000 J
1 KJ
=
5.0 KJ
step 1: find q
H = -q
= -5.0 KJ
Step 2: Find moles (because molar heat of combustion is in KJ/mol
What letter do we used for moles?
n
n = 0.1964 g
How do you convert from g mole?
multiply by the g/mole
x
1 mole
108.1 g
=
0.00182 mol
step 3: Use the KJ you figured out and the moles to express in KJ/mol
-5.0 KJ
0.00182 mol
=
2.7 x 10
3
-
KJ
mol
Enthalpy
= total energy of a system
The only way to measure energy is through a change in energy.
(this means we can't find H but we can find a change in H)
H
The enthalpy change for one mole of a pure substance is called:
molar enthalpy change
H
H
=
mc
T
Using molar heat of combustion to find temperature:
step 1: calculate the change in molar enthalpy
How you ask?
H =
KJ
mol
You have the molar heat of combustion of CH O
4
but wait, that's only for 1 mole!
So, you need to figure out the actual change
convert grams to moles
multiply that by the molar heat of combustion
Now, you have the change in molar enthalpy in KJ
3.2 C
o
q = mc
T
(which is change H)
Now, you have the energy (q)
(because
H = q)
q = 35, 100 J
Why did I convert the q in KJ to J?
-
-
1 mL of water has a mass of 1 gram.
1 mL = I gram
Question:

Claim (Hypothesis):

Methods:

Evidence:

Analysis of Evidence

Accept or Reject the claim:
must be testable
What you think
(data)
use a data table!
use our equation...(hint: think solve-crumple-toss)
What did you find out?
You must get your Mini-Lab report peer reviewed by 2 peers (2 other students) before you may submit it.
Please use the peer review evaluation form.
(procedure)
It is 2-sided. 1 reviewer on the front, one on the back.
What does specific heat capacity mean, really?
What is specific heat capacity?
the energy required to raise the temperature of 1 gram of substance by 1 degree celsius
What is the specific heat of liquid water?
4.184
4.184 what?
monk seals?
cute but no.
J
g
o
C
The grams and degree Celsius are on the bottom because of the word "by" in the definition
In a sentence:
This means it takes 4.184 Joules to raise the temperature of each gram of water by 1 degree Celsius.
Heating and Cooling Curves
of phase changes
What is a phase change?
(what is a phase?)
solid, liquid, gas
What might a phase change mean?
Energy Unit Assessment Help
heat flows from the hotter thing to the colder thing until both their temperatures are the same.
Identify the system and the surroundings. Draw a basic calorimeter and show the direction of heat flow.
exothermic reactions release (give off) heat and the sign of H is negative.
Endothermic reactions absorb heat and the sign of H is positive
Level 1:
Level 3:
Be able to solve the Al-Duckie Problem (solve-crumple-toss #14)
Understand the differences and similarities between temperature and heat (Energy transfer notes)
Level 4:
Be able to solve problem #16 from solve-crumple-toss
AND the energy and changes of matter ChemQuest
*Hint for #16 -- use the distributive property of addition
Level 2:
Calorimetry Reference Info
c= specific heat capacity
c is different for:
different susbstances
substances in different states of matter
different for solid
different for liquid
different for gas

4.814 J
g
C
C
H O
2
=
C
ice
=
2.1 J
g
C
C
steam
=
1.7
J
g
C
Use these specific heat values depending on the temperature of water. They only work for water.
If the temperature (in Celsius) is...
Below O
Between 0 and 100
above 100
The state of matter of water is...
solid
liquid
gaseous (water vapor)
What do people call it?
ice
water
steam
Two main categories of energy sources:
Renewable
Nonrenewable
What do you think the difference is between the two categories?
Full transcript