Balancing Chemical Equations Instructions to teacher: Write in your name and the date of the synchronous session in the slide above.

In this session we will, Review the meaning of chemical equations Discuss the need for balancing chemical equations Practice balancing equations

Chemical Equations Mathematical Equations Chemical Equations Example: x + 2x = 3x purely mathematical Example: C3H8 + 5O2 → 3CO2 + 4 H2O describe experimental observations define the quantities and kinds of substances involved in chemical reactions Instructions to teacher: Explain that we usually think of equations as being purely mathematical, but that chemical equations are used to describe experimental observations. They express two important facts about a chemical reaction: the quantities of substances involved and the kinds of substances involved. Then go to the next page to begin examples.

A Chemical Equation C3H8 + 5O2 → 3CO2 + 4H2O reactants products letters: element symbols large numbers: coefficients small numbers: subscripts Instructions to teacher: Point out the parts of a chemical equation, naming and pointing out reactants, products, element symbols, coefficients, and subscripts.

What do the numbers mean? C3H8 + 5O2 → 3CO2 + 4H2O C3H8 means you have 3 atoms of C and 8 atoms of H 5O2 means you have 5 molecules of O2, and O2 means you have 2 atoms of O, so you have 5 x 2 = 10 atoms of O in 5O2 O2 Instructions to teacher: Explain that the numbers tell you how many atoms of each element you have. Subscripts apply to the single element that they follow. Coefficients apply to the whole molecule that they precede. Explain that if there is no number written, it is understood to be “1.” Lead students through the examples given on the screen, explaining how coefficients are multiplied by subscripts to find the number of atoms when necessary. Then, ask students to tell you how many atoms of each element are in the products to make sure that they understand how these numbers are derived. they should see that there are 3 atoms of C and 6 atoms of O in 3CO2; there are 8 atoms of H and 4 atoms of O in 4H2O. 5O2

Conservation of Mass C3H8 + O2 → CO2 + H2O Law of conservation of mass: numbers of each element must be the same in both products and reactants. Element # of atoms in reactants # of atoms in products C H O Instructions to teacher: Explain how the law of conservation of mass says that matter can be neither created nor destroyed; therefore, there must be the same number of atoms of each elements in the products and reactants. Explain that making sure that this law is obeyed is called balancing the equation. Ask students to look at the equation on the screen and discuss whether or not it is balanced. Have them fill in the table to check if the equation is balanced. Guide them through filling in the table to make sure they understand how it should be filled in. They should see that there are 3 atoms of C, 8 atoms of H and 2 atoms of O in the reactants; there is 1 atom of C, 2 atoms of O, and 2 atoms of H in the products. This equation is not balanced.

Balancing an Equation C3H8 + O2 → CO2 + H2O Balance C atoms Balance H atoms C3H8 + O2 → 3CO2 + 4H2O Balance O atoms C3H8 + 5O2 → 3CO2 + 4H2O Instructions to teacher: Explain that you can see that this equation is unbalanced. To satisfy the law of conservation of mass, you must rewrite the equation in a way that satisfies the law. Make it clear that you never alter subscripts and you never alter the elements; you can only add coefficients, and you should do so in a way that uses the smallest possible coefficient. Point out that you often have to balance an equation by trail and error. With practice, it gets easier. A good way to start is to pick an element that only appears once in both the products and reactants. Here you could start with C. In the reactants you have 3 atoms of C, and you have 1 atom of C in the products. Ask the students what kind of coefficient they could add to balance just the C atoms? They could add a 3 in front of CO2. They could then balance the H atoms and then the O atoms. Guide students through this example to make sure they understand where each coefficient comes from. Point out that you don’t necessarily have to balance the equation by always moving from left to right in order and that sometimes you will have to work by trial and error.

Practice Instructions to teacher: Link students to http://funbasedlearning.com/chemistry/chembalancer2/default.htm and practice balancing some equations. (or some other website of your choice.) Note that at this site, you have to write in “1” coefficients. There are 10 examples here for students to work through. You may add additional examples if you want. Work through the examples until you are confident that the students understand how to balance the equations.