Unit 10: Energy in Chemical Reactions Cypress Creek High School Chemistry 1L Chapter 10

States of Matter Some chemical equations indicate the state of matter of the substances – Solid (s) - baking soda – Liquid (l) - water – Gas (g) - carbon dioxide – Aqueous (aq) - vinegar and sodium acetate Aqueous means the substance is dissolved in water. For example, NaCl (aq) means table salt dissolved in water. 

Chemical Equations –The simplest way to represent this reaction is by using words –Chemical equations replace chemical names with chemical formulas Reactants (LEFT) substances that undergo a reaction Products (RIGHT) new substances formed in a reaction Yield sign (MIDDLE) represents the change/reaction

Chemical Equations Practice What is the chemical equation for the following reactions? What are the reactants and products? 1) Water and sulfur trioxide combine to form sulfuric acid 2) Potassium chlorite decomposes to form potassium chloride and oxygen H 2 O + SO 3 → H 2 SO 4 reactants product KClO 2 → KCl + O 2 reactant products

Review of Chemical Equations Label the following chemical equation: SnO 2 (s) + 2H 2 (g)  Sn(s) + 2H 2 O(g) productsreactants state of matter subscript coefficient yield sign

Law of Conservation of Mass The LCM says matter is neither created nor destroyed – The mass of the reactants is always the same as the mass of the products. – The number of atoms of each element is always the same in the reactants and the products.

Law of Conservation of Mass Examples Hydrogen gas and oxygen gas combine to create water. 2H 2 + O 2  2H 2 O The large numbers, known as coefficients, indicate how many moles of each compound there are. This helps to satisfy the LCM! They apply to every element in the compound (2H 2 O means 4 H’s & 2 O’s) They are the coefficients should be in the lowest whole number ratio Particles are rearranged, not created or destroyed!

Law of Conservation of Mass Examples This should be a review - just count moles of atoms! How many carbon atoms are conserved? Hydrogen atoms? Oxygen atoms?

LCM Examples Calcium chloride and sodium sulfate combine to form calcium sulfate and sodium chloride. CaCl 2 + Na 2 SO 4  CaSO 4 + 2NaCl Before and After: Does the mass change? Do the number of atoms change? # atoms in the reactants # atoms in the products Ca11 Cl22 Na22 S11 O44 Total10

Balancing Chemical Equations Because the Law of Conservation of Mass (LCM) states that matter is not created or destroyed, the number of atoms of each element must be identical in the reactants and products. This means the chemical equation is balanced. Example 1: # atoms in the reactants # atoms in the products H22 C11 O33 Total66 This equation is balanced because it satisfies the LCM Writing coefficients is not necessary because each compound has a coefficient of 1

Balancing Chemical Equations Example 2: # atoms in the reactants # atoms in the products Na12 O34 H12 C11 Total69 This equation is not balanced because it does not satisfy the LCM Writing coefficients is necessary So we have to balance the equation…

Balancing Chemical Equations # atoms in the reactants # atoms in the products Na22 O44 H22 C11 Total99 Example 2 continued: The process of balancing chemical equations requires trial and error. You may not alter the subscripts because it changes the substance! You may only place coefficients in front of the compounds. These act as multipliers to balance the number of atoms. By placing a 2 in front of NaOH, the equation now satisfies the LCM and is balanced ***Tip: balance hydrogen and especially oxygen last - they often fall into place !

Balancing Chemical Equations Practice Write a balanced chemical equation for the following reactions. Place coefficients in the blanks. Chemical equations never actually write the coefficient “1”, but for this activity write a number in every blank. A) ___ K(s) + ___ H 2 O(l)  ___ H 2 (g) + ___ KOH(aq) B) ___ CaCl 2 (aq) + ___ Na 2 CO 3 (aq)  ___ CaCO 3 (s) + ___ NaCl(aq) C) ___ N 2 (g) + ___ H 2 (g)  ___ NH 3 (g)