Help Session on Standard MNRG.5.b

This standard is a pretty straightforward one to master, once you know what to look for. 

There are some groups or families of elements that have special names, probably because of their importance in the early days of chemical understanding.  For quick reference, and to make it easier to communicate with each other, this standard states the expectation that you will be able to identify elements that belong in the groups shown on the drawing below:

image of periodic table with important group names

This periodic table shows the location of the alkali metals, alkaline earth metals transition metals, halogens and noble gases.

After seeing where the locations are, look at a standard periodic table, like this one and try to identify each of the following:

1 calcium (Ca)

2 lithium (Li)

3 xenon (Xe)

4 gold (Au)

5 chlorine (Cl)

6 argon (Ar)

7 barium (Ba)

8 sodium (Na)

9 iron (Fe)

10 bromine (Br)

Check your answers here.

 

Help Session on Standard MNRG.5.a (GA Chemistry)

This standard is really easy to master, if you know the basic layout of the periodic table.  There is a stairstep-looking boundary that divides the periodic table into its two main parts.  It starts below boron (B) and jogs down underneath silicon (Si), arsenic (As) and tellurium (Te).  Every element that touches this line, except aluminum (Al) is a metalloid (also called a semimetal).  To the right of the boundary are the nonmetals.  Hydrogen looks like it’s on the left, but it is actually also a nonmetal.  To the left of the boundary are the metals, including the two rows on the bottom of the table that start with Ce and Th:

image of periodic table with metal-nonmetal boundary

The blue boundary divides the periodic table into metals (purple zone), nonmetals (green zone) and metalloids (touching the boundary).

 

 

Help Session on Standard MNRG.2.b

This standard states:
I am able to state and apply the First Law of Thermodynamics.

We started out with a Statement of the First Law of Thermodynamics. I followed it up with an alternative version, shown in blue:

first law whiteboard

The 1st Law of Thermodynamics, stated 2 different ways.

Then I showed a way of representing the 1st Law of Thermodynamics symbolically.  Whatever energy goes into any device, system or process is the maximum you can get out after the device or process has transformed the energy.

first_law_graphic

A symbolic way of stating the 1st Law of Thermodynamics

Then I showed an example of what this means, using a scheme many non-scientists over the years have tried to accomplish–using the output from a generator to power the motor that drives the generator:

perpetual motion machine image

This perpetual motion machine couldn’t power anything.

You can never accomplish anything with a scheme like this, because the very maximum amount of energy the generator could produce is the amount needed to power itself.  (Actually, there is a 2nd Law of Thermodynamics that says you can’t even power the motor with this generator, but that’s another topic for another time.)

Next, we looked at several examples of energy sources we use here on earth to heat ourselves and do work.  It turns out that all the energy (even the “free” stuff we get from renewable methods like solar, wind and biomass) is not created, but originally came from the sun, our big power source for this planet.

energy source image

All the energy sources we use in Earth come from the sun either directly or indirectly.

 

 

 

Help Session on Standard MNRG.4

This standard states:

I am able to categorize a sample of matter as one of the following:

  • Heterogeneous mixture
  • Homogeneous mixture
  • Compound
  • Element

To show the difference, I started out with some quick visual definitions:

element

Elements have their own spot on the periodic table.

An element is the purest form of matter.  Anything that has it’s own spot on the periodic table of the elements is an element.

If two or more elements combine in a chemical reaction, they will form a new type of pure substance that has a distinct ratio of amounts one element to another.

Just because two elements are mixed together doesn’t mean that a compound has formed.  There must be a chemical reaction that produces something completely new.

compound image

Compounds form in chemical reactions between elements.

When two or more substances are placed together and don’t react chemically, the result is what we call a mixture.  Sometimes, it’s obvious that what we have is a mixture, because we can see it by eye.  Mixtures like this are called heterogeneous mixtures (“hetero-” = different):

hetero_mixture_image

Heterogeneous mixtures appear to be mixtures by eye.

Sometimes, the particles in the mixture are so small that we can’t really tell they are mixtures, unless we know for sure.  These mixtures are called solutions or homogeneous mixtures (“homo-” = same).  Homogeneous mixtures are easy to confuse with pure substances, just by looking at them:

homo_mixture_image

Homogeneous mixtures look like pure substances by eye.

Next, we looked at real life examples of these classes of matter.

copper image

Copper is an example of an element because it is on the periodic table.

water_image

Pure (distilled) water is an example of a compound because H2O is a definite ratio of elements.

bean_salad_image

This bean salad is a heterogeneous mixture because you can easily see with your eye that it contains more than one type of substance.

KoolAid_ image

This Kool-Aid is a homogeneous mixture because it is made of more than one substance, but I can’t tell that just by looking at it.

Help Session on Standard NMHT.4.c

Alex, Alexis, Jordan and Whitney were in attendance.  We started out by reviewing the precision reporting rule for adding and subtracting:

add/subtract rules

add/subtract rules

 

The people who were there wanted an example right away, so I used 12.5 + 3.26, which adds to 15.76.  Since 12.5 is only known to the precision of the nearest 0.01, we have to round our answer to the nearest 0.01, as well.  This means we should actually report an answer of 15.8.

adding examples

adding examples

Next, I gave examples of adding and subtracting:

subtracting examples

subtracting examples

In the first adding example, we round to the nearest 0.01, because that is the precision of both the measurements being added.  In the second example, we round to the nearest 10, because that is the precision of the least precise of the two measurements, 30, which is written without a decimal point.  The only adequate way to report this precision is with scientific notation, so it is written as 3.0 x 10².

In the first subtraction example, the answer is rounded to the nearest 0.1, because the least precise measurement, 1.2, is only known to the nearest 0.1.  In the second subtraction example, the answer is rounded to the nearest 1, because that is the precision of 2, which is the least precise of the measurements.

Next, we reviewed the precision reporting rule for multiplication and division:

multiply/divide rules

multiply/divide rules

Right after that, I showed examples for multiplication and division:

multiplication examples

multiplication examples

In the first multiplication example, the answer is rounded to 2 sig. figs. because the least precise measurement, 3.0, shows only 2 sig. figs. In the second multiplication example, the answer is rounded to 1 sig. fig. because the least precise measurement, 4, shows only 1 sig. fig.

division examples

division examples

In the first division example, the answer is rounded to 2 sig. figs. because the least precise of the two measurements, 4.0, is shown to only 2 sig. figs. In the second division example, the answer is rounded to 3 sig. figs. because both measurements have 3 sig. figs.

Practice Problems

Practice Problems

Last, I posted 4 practice problems:

Try them yourself.  See if you get the right answer by checking here.

Help Session on Standard NMHT.4.a

Today, during B Lunch, we had a help session on Standard NMHT.4.a, which is, “I am able to differentiate between accuracy and precision in data sets.”  Whitney, Alexis and Alex showed up for the session.

I started out by showing two targets.  The first target had shots from two accurate rifles:

accurate rifles

accurate rifles

Accuracy means “centered on the target” for a rifle, so both these rifles are accurate, even though 4 of the 5 circled bullet holes are barely on the target.  The second had shots from two precise rifles:

precise rifles

precise rifles

 

 

The rifle that is off to the upper right is precise, because precise for a rifle means “close shot grouping.”

Then we talked about what it means for measurements to be precise and accurate.  Precise measurements agree closely with each other.  In other words, the range of values in a group of measurements is small.  Accurate measurements agree with a true or accepted value.  In other words, the average of a group of measurements that is accurate agrees with a true or accepted value for the quantity that is being measured.

Next, we looked at 4 sets of seven measurements and worked out a description for each, in terms of accuracy and precision:

accurate and precise

accurate and precise

 

These measurements are accurate, because the average of their values is the same as the true value of 10.  They are also precise, because the range, 0.2 is small, compared to the average (about 2%).

 

 

accurate, not precise

accurate, not precise

These measurements are accurate, because their average agrees with the true value, but they are not precise, because the range, 8, is large, compared to the average (about 80%).

 

 

 

precise, not accurate

precise, not accurate

These measurements are not accurate, because the average does not agree with the true value, but they are precise, because the range is small, compared with the average.

 

 

 

not accurate, not precise

not accurate, not precise

These measurements are neither accurate nor precise, because the range is large and the average does not agree with the true value.

After this, the students were invited to take a short assessment to demonstrate proficiency on Standard NMHT.4.a