Chapter 6 section 1 Chemical equation

1. Chapter 6 section 1 Chemical equation Writing Chemical EquationsGr.10

2. Writing Chemical Equations • reactant : is a substance that undergoes a reaction. • When reactants undergo a chemical change, each new substance formed is called a product. • For example, a familiar chemical reaction involves the reaction between iron and oxygen (the reactants) that produces rust, which is iron (III)oxide (the product). • The simplest reactions involve a single reactant or a single product, but some reactions involve many reactants and many products.

3. CHEMICAL REACTIONS SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")! Reactants: Zn + I2 Product: Zn I2

4. Chemical Equations 4Al (s) + 3O2 (g) ---> 2Al2O3 (s) The numbers in the front are called Stoichiometric _Coefficients The letters (s), (g), and (l) are the physical states of compounds.

5. Introduction • Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken • Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. • Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

6. Parts of a Reaction Equation • Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). • A + sign separates molecules on the same side • The arrow is read as “yields” • Example C + O2 CO2 • This reads “carbon plus oxygen react to yield carbon dioxide”

7. Chemical Change • Atoms in the reactants are rearranged to form one or more different substances • Old bonds are broken; new bonds form Examples: Fe and O2 form rust (Fe2O3) Ag and S form tarnish (Ag2S) LecturePLUS Timberlake

8. Learning Check E1 Classify each of the following as a 1) physical change or 2) chemical change A. ____ a burning candle B. ____ melting ice C. ____ toasting a marshmallow D. ____ cutting a pizza E. ____ polishing silver LecturePLUS Timberlake

9. Solution E1 Classify each of the following as a 1) physical change or 2) chemical change A. __2__ a burning candle B. __1_ melting ice C. __2__ toasting a marshmallow D. __1__ cutting a pizza E. __2__ polishing silver LecturePLUS Timberlake

10. Chemical Reaction A process in which at least one new substance is produced as a result of chemical change. LecturePLUS Timberlake

11. A Chemical Reaction Reactants Products LecturePLUS Timberlake

12. Word Equations • The simplest way to represent a reaction is by using words to describe all the reactants and products, with an arrow placed between them to represent change. • Reactants are placed to the left of the arrow, and products are placed to the right. • Plus signs are used to separate reactants and also to separate products.

13. Word Equations • The compound in vinegar that is involved in the reaction is acetic acid, and baking soda is sodium hydrogen carbonate. • The equation for the reaction of vinegar and baking soda can be written using the chemical formulas of the reactants and products.

14. Chemical Equations • Symbols in the parentheses are put after formulas to indicate the state of the substance. • Solids, liquids, gases, and water (aqueous) solutions are indicated by the symbols (s), (l), (g), and (aq). • Now the equation tells us that mixing an aqueous solution of acetic acid (vinegar) with solid sodium hydrogen carbonate (baking soda) results in the formation of an aqueous solution of sodium acetate, liquid water, and carbon dioxide gas.

15. Energy and Chemical Equations • Noticeable amounts of energy are often released or absorbed during a chemical reaction. • Some reactions absorb energy. If energy is absorbed, the reaction is known as an Endothermic reaction. • For a reaction that absorbs energy, the word energyis sometimes written along with the reactants in the chemical equation. • For example, the equation for the reaction in which water breaks down into hydrogen and oxygen shows that energy must be added to the reaction.

16. Exothermic reactions. • Reactions that release heat energy are called exothermic reactions. • When writing a chemical equation for a reaction that produces energy, the word energy is sometimes written along with the products. • Some of this energy is in the form of light. • You may have also noticed that the word energy is not always written in the equation. • It is used only if it is important to know whether energy is released or absorbed.

17. Balancing Chemical Equations • The mass of the products is always the same as the mass of the reactants that react to form them. • The law of conservation of mass: Matter is neither created nor destroyed during a chemical reaction. • The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the products. • For example, consider the equation that represents breaking down carbonic acid into water and carbon dioxide.

18. Lavoisier, 1788 Chemical Equations Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.

19. Balancing Equations • When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. • Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

20. Subscripts vs. Coefficients • The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

21. Chemical Equations 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) This equation means 4 Al atoms + 3 O2 molecules ---produces---> 2 molecules of Al2O3

22. Steps to Balancing Equations There are four basic steps to balancing a chemical equation. • Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! • Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. • Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. • Check your answer to see if: • The numbers of atoms on both sides of the equation are now balanced. • The coefficients are in the lowest possible whole number ratios. (reduced)

23. Some Suggestions to Help You • Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. • IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element) • (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

25. Balancing Equations 2 2 ___ H2(g) + ___ O2(g) ---> ___ H2O(l) What Happened to the Other Oxygen Atom????? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

26. Balancing Equations ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s) 2 3

27. Balancing Equations ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g) ____B4H10(g) + _____ O2(g) ----> ___ B2O3(g) + _____ H2O(g)

28. Balancing Equations Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na3PO4 + Fe2O3 ----> Na2O + FePO4

29. Good luck