We last left off in the mind-boggling Unit 5. Let’s spend some time in a little more straight forward unit: chemical reactions. Don’t worry, I didn’t forget about the mole! It is coming up in Unit 7.
Here is what we added to the model so far in Unit 5:
- All atoms contain mobile, negatively charged particles called electrons whose charge is balanced by the positive (pudding) core of the atom. (Thomson’s Plum Pudding Model)
- In metals, the positive core has a weaker attraction to the electrons so electrons can move more freely than in non-metals, allowing metals to conduct electricity.
- Metals tend to lose electrons and become positively charged cations and non-metals tend to gain electrons and form negatively charged anions.
- Ions are charged all over and attract ions of opposite charge from all directions. When ions of opposite charges are attracted to each other, they form ionic bonds. Ionic substances are bonded throughout and have high melting/boiling points.
- When the electrons of two non-metal atoms are attracted to the other’s positive core, a covalent bond is formed. Molecular compounds are bonded within molecules but the molecules are only attracted to each other through intermolecular attractions. Molecular substances have lower melting/boiling points compared to ionic substances.
- Molecules can be attracted to each other through induced dipole-dipole attractions and permanent dipole-dipole attractions.
- Ionic compounds are named by writing the metal first and then dropping the ending of the non-metal and adding the suffix -ide.
- Molecular compounds are named by using the prefixes -mono, -di, -tri, -tetra, etc. to denote how many atoms of each element are present in the compound. The first element only gets a prefix if there is more than 1. For the second element, you must drop the ending and add the suffix -ide.
Yes, that was a LARGE unit!
I kick off Unit 6 by blowing stuff up (because that’s what chemistry is, right?)
Chemical Reaction Demos
I think a unit on chemical reactions should start with some chemical reactions. Insert your favorite demos here. I like to use smashing thermite, the blue bottle, mossy zinc and hydrochloric acid and of course igniting hydrogen balloons from gas produced from the previous reaction.
I need to set up this demo in the future!
I have students observe the reactions and tell me how they know a chemical reaction occurred. By the end of the class we have a good list of macroscopic observations that tell us a chemical reaction has happened.
I then use the Zn and HCl reaction to introduce what is happening at the particle level. I have students draw out the particle models of the skeleton equation and they can see that it does not follow the law of conservation of mass. That is a big chemistry faux pas! The only way to fix this is to add more HCl to the reaction. This tells us that 1 zinc atom reacts with 2 hydrochloric acid molecules to form one molecule of hydrogen gas and one compound of zinc chloride. That sets us up nicely for balancing equations.
For balancing equations, it all comes down to practice. I start my students with balancing skeleton equations and then I have them move on to constructing their own skeletons from word problems. I have every student start by drawing the particle models to balance equations. Some students graduate from this quickly while others are always stuck to it. I just encourage students to do whatever works from them and I always leave individual whiteboards (sheet protectors with a white piece of paper) out on my desks during this unit for students who need them.
I try to break up the monotony of balancing equation worksheets with some games. Sometimes I do speed competitions (by volunteer only so students who aren’t super fast balancers don’t feel any extra pressure) or group games like board hockey.
Once students are comfortable with balancing equations, we can move on to classifying reactions.
Classifying Chemical Reactions
I start this new topic with a pretty standard chemical reactions types lab. Students complete a series of mini experiments that are representative of the different reaction types. I like to have 2 reactions for every reaction type. I give students the reaction type and skeleton equation for each reaction. Students must record their observations, balance the equations and draw the corresponding particle models for each reaction. In that aspect, there is some confirmation built into this lab but the goal is not predict products but to find patterns.
To whiteboard this lab, I have each group whiteboard a different reaction. We then talk through each reaction and find patterns in the similar reaction types. The key questions in whiteboard meeting are: “what is similar between the two reactions you saw of this type?” and “why do you think it is called insert reaction type here.“That helps us come up with a set of rules. The rules are far more meaningful to students when they come up with them themselves versus being given the patterns through notes.
After the lab, I have students classify the reactions on a worksheet that they already balanced the equations for and we whiteboard it the next day. After classifying chemical reactions, we move on to the last topic of the unit!
Energy and Chemical Reactions
This topic brings back an old favorite, the LOL chart! Before I introduce the new and improved LOLOL chart, I show students one of my favorite demos.
Don’t worry, I use a test tube and a Swedish Fish but you get the idea. This is a very exothermic reaction so it gets the conversation about heat and reactions started. I have students balance and classify the equation and then I draw an LOLOL chart on the board. This is where I introduce Echemical, which was foreshadowed in Unit 3. I ask students where they think chemical energy comes from and they can easily tell you,”from chemical bonds.” This is where you need to address the big misconception that energy is stored in bonds. It takes energy to break bonds and energy is released when bonds are formed. Collegeboard has a quick explanation of this misconception with some nice real-life examples like, “why is hydrogen such a good fuel source if it’s not storing lots of energy in its bonds?” You can also mention activation energy here and how some reactions need a bit of energy to get started but do not require a constant energy input to proceed (I like to use the example of burning magnesium ribbon).
After that discussion, I take the students observations about the gummy bear reaction and fill in the LOLOL chart accordingly.
The Swedish Fish (sugar) starts off at room temperature. After the reaction, the products are very hot. That heat had to come from somewhere and it wasn’t from the surroundings. That means it must have come from within the system; enter Echemical. After a while, the products cool down but the reaction is over so the chemical energy stays the same. That heat leaves the system so the reaction is exothermic. The LOLOL chart tells us that more energy was released forming new product bonds than what was used to break the original reactant bonds.
This is a good time to show an endothermic demo as well. I like ammonium nitrate and water because I use ammonium chloride and barium hydroxide later on for the practicum.
I have students try to whiteboard the LOLOL chart for this reaction and then we have a quick board meeting. All that is left then is some practice… and a practicum!
Unit 6 Practicum
For the Unit 6 practicum, I try to bring in as many learning targets as possible. I give students 2 reactions to observe: magnesium ribbon and hydrochloric acid and ammonium chloride and barium hydroxide. Students must give the signs that a chemical reaction has occurred, write the balanced equation, draw the particle models, classify the reaction and draw the LOLOL chart representing the observed temperature change.
That is it for Unit 6! It is small but mighty! Let’s take a look at the model so far…
- Chemical reactions can be identified by a change in color, temperature or odor or the formation of a precipitate or a gas
- Particles can rearrange during a chemical reaction but mass must be conserved (total number of particles does not change)
- Chemical reactions occur in predictable patterns
- It takes energy to break bonds and energy is released when bonds are formed
- Exothermic reactions release heat when the chemical energy of the system is decreased. Endothermic reactions absorb heat when the chemical energy of the system is increased.
Next up… THE MOLE!