1 Chapter 2 Chemical Change

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3 Chemical Reactions A chemical reaction is a process in which reactants change to produce new products with different properties. The best way to describe a chemical reaction is to write a chemical equation. reactants products (the arrow indicates the direction of the reaction)

4 Evidence of Chemical Reactions 1. Color change - the final products have a different color than starting materials 2. Odor change - - the final products have a different odor than starting materials

5 Evidence of Chemical Reactions 3. State change - the final materials may include a substance in a state that differs from a starting material. - commonly either a gas or a solid precipitate forming. 4. Energy change - when a chemical reaction occurs energy in the form of heat light sound or electricity is absorbed or released.

balancing chemical rxn eqn A balanced chemical equation is one in which the total number of each kind of atom or ion in the reactants is equal to the total number of the same kind of atom or ion in the products. To balance an equation we use a coefficient in front of the chemical formula.

7 Rules for Writing Balanced Chemical Equations 1. Write the chemical formula for each of the reactants and products, including the state of matter for each. 2. Try balancing the atom or ion in the greatest amount. Use coefficients in front of the formula to balance an atom or ion - note: leave hydrogen and oxygen to the end.

8 Rules for Writing Balanced Chemical Equations 3. Ensure that coefficients cannot be reduced by a common factor. 4. Check the final reaction equation to ensure that all atoms and ions are balanced.

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Chemical Amount Calculations for molar mass 1. Write the chemical formula for each substance. 2. Determine the amount in moles of each element in the formula.

11 3. Use the molar masses from the periodic table and the chemical amounts from step 2 to determine the mass of one mole of the chemical. (n=m/M or m = nM) 4. Communicate the amount as grams per mole to two decimal places.

12 Example 1: Calculate the molecular mass of Hydrogen Chloride HCl. Solution: The molecular mass of HCl = atomic mass of H + atomic mass of Cl = = 36.5g

13 Example 2: Calcium phosphate has the formula Ca 3 (PO 4 ) 2. Calculate the mass of one mole of calcium phosphate. 3 mole of Ca 2 mole of P 8 mole of O. 1 mol Ca is 40.08g X 3 mole = g 1 mol P is 30.97g X 2 mole = 61.94g 1 mole O is 16g X 8 mole = 128g 1 mole of Ca 3 (PO 4 ) 2 is g

14 Example 4: 100g of Ca 3 (PO 4 ) 2 is how many mole? Solution:

Classifying Chemical Reactions 1. Formation (F) or Simple Composition Element + element > compound Eg2Na (s) + Br 2(l) ----> 2NaBr (s) 2Mg (s) + O 2(g) ----> 2MgO (s) 2. Simple decomposition(SD) Compound ---> elements Eg2HgO (s) ----> 2Hg (l) + O 2(g) 2H 2 O (l) ----> 2H 2(g) + O 2(g)

16 Classifying Chemical Reactions 3. Single Replacement element + compound -> element + compound Eg - metal replaces metal ion to produce a new metal in the following reaction Cu (s) + 2AgNO 3(aq) -----> 2Ag (s) + Cu(NO 3 ) 2(aq) non-metal replaces non-metal ion to produce a new non-metal in the following reaction Cl 2(g) + 2NaI (aq) > I 2(s) + 2NaCl (aq)

17 Classifying Chemical Reactions 4. Double Replacement - occurs between ionic compounds in solution. - the ions change partners EgCaCl 2(aq) + 2NaNO 3(aq) -----> Ca(NO 3 ) 2(s) + 2NaCl (aq) Note: some products have low solubility and will form a precipitate (s)

18 Classifying Chemical Reactions 4. Double Replacement Another example of a double replacement reaction is the reaction between an acid and a base producing water and an ionic compound. This is a neutralization reaction. HCl (aq) + KOH (aq) ----> H 2 O (l) + KCl (aq) Acid base water ionic compound

19 Classifying Chemical Reactions 5. Combustion Reactions - burning a substance in oxygen - burning a carbon compound produces CO 2(g) and H 2 O (g) EgC 3 H 8(g) + 5O 2(g) --> 3CO 2(g) + 4H 2 O (g) propaneoxygencarbon dioxide water - burning hydrogen produces H 2 O (g) - burning sulfur produces SO 2(g)

20 Predicting Chemical Reactions 1. Classify the reaction type based on the reaction generalizations. 2. Predict the products of the reaction based on the reaction type. 3. Write the chemical formulas for each of the reactants and products. 4. Balance the equation without changing the chemical formulas.

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22 Types of Energy Change 1. Endothermic - energy is absorbed by the system from its surroundings. Eg - cold packs - photosynthesis Reactants + Energy > Products

23 Types of Energy Change 2. Exothermic - energy is released by the system to its surroundings. Eg- hand warmers - hot packs - cellular respiration Reactants > products + energy

24 Types of Energy Change 2. Exothermic - energy is released by the system to its surroundings. Eg- hand warmers - hot packs - cellular respiration Reactants > products + energy