What IS a chemical reaction? Our episode starts with differentiating between physical (1:08) and chemical changes (1:47).
What IS a chemical reaction? Our episode starts with differentiating between physical (1:08) and chemical changes (1:47). To represent chemical reactions, chemists use three different types of chemical equations: Molecular equations that show neutral compounds (4:50), complete ionic equations that show soluble ionic compounds as free floating ions (5:30) and net ionic equations (NIE) (6:30). NIEs remove the spectator ions and show only the ions that are reacting. Tip: Don’t forget your coefficients! (7:10).
Question:(8:17) What type of covalently bonded molecules are often represented in ionic form when dissolved in water?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap types of chemical equations.
Lets Zoom Out:
Unit 4 - Chemical Reactions
Topics 4.1 and 4.2
Big idea - Transformations
Unit 4 is titled “Chemical Reaction” - but what is a chemical reaction? When I boil a pot of water for my pasta, I can observe bubbles (- who else has to think of Bubbles from Finding Nemo?). I can also see “bubbles” when I mix baking soda and vinegar. Boiling water, however, is NOT a chemical reaction. What is the difference and how are we representing and communicating a chemical reaction? Today’s episode recaps chemical reactions and the different types of chemical equations.
Let’s zoom in:
We are starting with a rather dry definition: Physical changes are characterized by a change in property, but not a change in composition. What does that mean? When a substance undergoes a physical change it doesn’t change its identity - like with our example - its still water, even though its changed from liquid to gas form. It has undergone a change in physical state, but kept its identity: still water molecules, still one oxygen atom covalently bonded with two hydrogen atoms. So, where do the bubbles come from? The water molecules on the bottom of your pot enter the gaseous phase first and then rise to the surface of the water.
A chemical change, on the other hand, is characterized by a change in identity - forming a new substance, with different composition and properties. Our baking soda - NaHCO3, sodium bicarbonate - reacts with vinegar - aka acetic acid - to form CO2, which is released as a gas (bubbles!, bubbles!, bubbles!), water and sodium acetate. And, these three products have a different identity! Chemical bonds have been broken and new ones formed, the composition, the chemical makeup has changed. We can identify chemical changes by looking for the production of heat or light, formation of a gas or precipitate or a color change. These all indicate that the new substance has different properties!
Describing our observations on the macroscopic scale is a great first step. But in chemistry, we want to visualize the chemical reaction, the change in composition and identity. We accomplish this using chemical equations.
Chemical equations use the chemical formulas of reactants and products. If you have multiple reactants and/or products, they are connected with a plus sign. To indicate the chemical reaction from reactants to products, we use a “yield” - in the form of an arrow. The arrow is REALLY important. And it might be a pet peeve of our author Sarah, but we do NOT use an equation sign. Why not? Because the reactants and products are not THE SAME. That’s the whole point of it - it is a chemical reaction. They are NOT equal in terms of properties.
Yes, there are aspects about it that are equal, Sarah admits that: The law of conservation of mass needs to be taken into account. Mass can neither be created nor destroyed. That means, we have to have equal numbers of each element before and after the reaction. Or: What goes in, has to come out. Just in a different arrangement. We do that by using coefficients to balance the chemical equation.
There are different types of chemical equations and their use depends on the context: molecular equations, complete ionic equations and net ionic equations. Let’s take a closer look at them.
Molecular equations are the type of equations you’ve probably seen the most. If it just says “write the equation” it usually means the molecular equation. In a molecular equation, all components are shown with their chemical formulas, as neutral compounds and, if you have the information, with their state in parenthesis. For example: In a double displacement reaction, an aqueous solution of potassium iodide reacts with an aqueous solution of silver(I) nitrate to form an aqueous solution of potassium nitrate and solid silver(I) iodide. The reactants, as well as the products, are ionic compounds. You wouldn’t be able to tell with a molecular equation, though.
But what if I want to communicate that these compounds are ionic and three of the four compounds dissolve in water? Then I can use a complete ionic equation: In this representation, I am taking into account the formation of free floating ions in the aqueous solution. I would therefore write these free floating ions by themselves: Potassium cation plus iodide anion plus silver(I) cation plus nitrate anion react to form potassium cation, nitrate anion and solid silver(I) iodide. As you’ve noticed, I didn’t write the silver(I) iodide as ions: Because they are forming an insoluble precipitate. Therefore, there are no longer free floating ions and this complete ionic equation reflects that. Generally, all solids, liquids and gases have to be written as a molecule, even in a complete ionic equation.
But you’ve also noticed, that the potassium cation and the nitrate anion are both free floating ions as reactants and as products. They are just bystanders and do not participate in the chemical reaction. In chemistry, we call them spectator ions. To represent “what is actually happening”, we can reduce the complete ionic equation to the net ionic equation. The net ionic equation, or NIE, shows only the species that are participating in the chemical reactions. It therefore leaves out the spectator ions. In our example, the NIE would be: the aqueous silver(I) cations react with the aqueous iodide anions to form solid silver(I) iodide.
Our example was pretty straight forward. One word of caution for different examples: Do not forget the coefficients. For example, if you have: sulfuric acid + 2 sodium hydroxide yields 2 water plus sodium sulfate, then the coefficients are also used in the complete ionic and net ionic equations. The NIE would be: 2 H+ react with 2 OH- to form 2 H2O.
To recap:
During chemical changes a substance undergoes a transformation into a different substance with new properties. A chemical reaction can be represented using balanced chemical equations. Three types of chemical equations are used: molecular chemical equations, showing all compounds as molecules, complete ionic equations showing the dissolution of ionic compounds in aqueous solutions and net ionic equations (NIE). NIEs only show the ions that are participating in the chemical reaction.
Coming up next on the APsolute RecAP Chemistry Edition: Dive Deeper: Stoichiometry
Today’s Question of the day is about types of species.
Question: What type of covalently bonded molecules are often represented in ionic form when dissolved in water?