Writing and Balancing Chemical Equations Chemical Equations Chemical
Writing and Balancing Chemical Equations
Chemical Equations • Chemical reactions result in chemical changes. – Chemical changes occur when new substances are created. – The original substance(s), called reactants, change into new substance(s) called products.
Chemical Equations • Chemical reactions can be written in different ways. – A word equation: • Nitrogen monoxide + oxygen nitrogen dioxide – A symbolic equation: State of matter • 2 NO(g) + O 2(g) 2 NO 2(g) - Letters indicate the state of each compound. (aq) = aqueous/dissolved in water Coefficients (s) = solid - Indicate the ratio of compounds in the reaction. ( ) = liquid - Here, there is twice as much NO and NO 2 than as is O 2. (g) = gas See pages 202 - 203 (c) Mc. Graw Hill Ryerson 2007
Conservation of Mass in Chemical Change • Chemical change means new compounds are created. – No new matter is created or destroyed; atoms are just rearranged. – All of the matter in the reactants = all of the matter in the products. – John Dalton, 200 years ago, realized that atoms simply rearrange themselves during chemical reactions. – Number of each atom in reactants = number of each atom in products.
Conservation of Mass in Chemical Change • The Law of Conservation of Mass: – In chemical reactions, atoms are neither created nor destroyed. – This law was developed by Antoine and Marie-Anne Lavoisier in the 1700 s. – Mass of reactants = mass of products If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O 2) = mass of products (exhaust).
Writing and Balancing Chemical Equations • The simplest form of chemical equation is a word equation. – Potassium metal + oxygen gas potassium oxide • A skeleton equation shows the formulas of the elements/compounds. – A skeleton equation shows atoms, but NOT quantities of atoms. Is this equation balanced? ? • K + O 2 K 2 O See page 206 (c) Mc. Graw Hill Ryerson 2007
Writing and Balancing Chemical Equations • A balanced chemical equation shows all atoms and their quantities – Balancing ensures that the number of each atom is the same on both sides of the reaction arrow. – Always use the smallest whole-number ratio. Balance K + O 2 K 2 O • 4 K + O 2 = 2 K 2 O See page 206 (c) Mc. Graw Hill Ryerson 2007
Word Equations Skeleton Equations
Counting atoms: 2 methods, same meaning These two diagrams/equations look different, but both represent the same thing
How to count atoms
Counting Atoms to Balance an Equation • Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations. – Word equation: methane + oxygen water + carbon dioxide – Skeleton equation: CH 4 + O 2 H 2 O + CO 2 • To balance the compounds, look at how many atoms of each element occur on each side of the reaction arrow. The same number of atoms must be on each side. – Skeleton equation: CH 4 + O 2 H 2 O + CO 2 1 carbon, 4 hydrogen, 2 oxygen 1 carbon, 2 hydrogen, 3 oxygen – To balance, find coefficients that equate atoms on both sides – Balanced equation: CH 4 + 2 O 2 2 H 2 O + CO 2 1 carbon, 4 hydrogen, (2× 2) oxygen 1 carbon, (2× 2) hydrogen, (2× 1)+2 oxygen
Practice: Balance the following chemical equation: Balancing Step 1: * A skeleton equation is simply a representation of the chemical equation before any balancing is done
Balancing equations: Step 2 Balancing Step 2: • Now the equation is perfectly balanced
Strategies for Balancing Equations • Balance chemical equations by following these steps: – Trial and error will work but can be very inefficient. – Balance compounds first and elements last. – Balance one compound at a time. – Only add coefficients; NEVER change subscripts. – If H and O appear in more than one place, attempt to balance them LAST. – Polyatomic ions (such as SO 42–) can often be balanced as a whole group. – Always double-check after you think you are finished.
• Balance the following: – Fe + Br 2 Fe. Br 3 2 Fe + 3 Br 2 2 Fe. Br 3 – Sn(NO 2)4 + K 3 PO 4 KNO 2 + Sn 3 (PO 4)4 3 Sn(NO 2)4 + 4 K 3 PO 4 12 KNO 2 + Sn 3 (PO 4)4 – C 2 H 6 + O 2 CO 2 + H 2 O 2 C 2 H 6 + 7 O 2 4 CO 2 + 6 H 2 O
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