1 chapter 2 chemical change. 2 3 chemical reactions a chemical reaction is a process in which...

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Chapter 2Chapter 2

Chemical Change

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Chemical ReactionsChemical 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)

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Evidence of Chemical ReactionsEvidence of Chemical Reactions1. 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

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Evidence of Chemical ReactionsEvidence of Chemical Reactions3. 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.

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2.3- balancing chemical rxn eqn2.3- balancing chemical rxn eqnA 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.

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Rules for Writing Rules for Writing Balanced Chemical EquationsBalanced 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.

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Rules for Writing Rules for Writing Balanced Chemical EquationsBalanced 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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2.4- Chemical Amount2.4- Chemical AmountCalculations for molar mass

1. Write the chemical formula for each substance.

2. Determine the amount in moles of each element in the formula.

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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.

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• Example 1:

• Calculate the molecular mass of Hydrogen Chloride HCl.

• Solution:

• The molecular mass of HCl = atomic mass of H + atomic mass of Cl

• = 1.0 + 35.5

• = 36.5g

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• Example 2: • Calcium phosphate has the formula

Ca3(PO4)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 = 120.24 g • 1 mol P is 30.97g X 2 mole = 61.94g• 1 mole O is 16g X 8 mole = 128g• 1 mole of Ca3(PO4)2 is 310.18 g

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• Example 4:

• 100g of Ca3(PO4)2 is how many mole?

• Solution:

•

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2.5-Classifying Chemical Reactions2.5-Classifying Chemical Reactions

1. Formation (F) or Simple Composition

Element + element ------> compound

Eg 2Na(s) + Br2(l) ----> 2NaBr(s)

2Mg(s) + O2(g) ----> 2MgO(s)

2. Simple decomposition(SD)

Compound ---> elements

Eg 2HgO(s) ----> 2Hg(l) + O2(g)

2H2O(l) ----> 2H2(g) + O2(g)

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Classifying Chemical ReactionsClassifying Chemical Reactions

3. Single Replacementelement + compound -> element + compound

Eg - metal replaces metal ion to produce a new metal in the following reaction

Cu(s) + 2AgNO3(aq) -----> 2Ag(s) + Cu(NO3)2(aq)

non-metal replaces non-metal ion to produce a new non-metal in the following reaction

Cl2(g) + 2NaI(aq) ------> I2(s) + 2NaCl(aq)

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4. Double Replacement

- occurs between ionic compounds in solution.

- the ions change partners

Eg CaCl2(aq) + 2NaNO3(aq) ----->

Ca(NO3)2(s) + 2NaCl(aq)

Note: some products have low solubility and will form a precipitate (s)

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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) ----> H2O(l) + KCl(aq)

Acid base water ionic compound

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5. Combustion Reactions

- burning a substance in oxygen

- burning a carbon compound produces CO2(g) and H2O(g)

Eg C3H8(g) + 5O2(g) --> 3CO2(g) + 4H2O(g)

propane oxygencarbon dioxide water

- burning hydrogen produces H2O(g)

- burning sulfur produces SO2(g)

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Predicting Chemical ReactionsPredicting 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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Types of Energy ChangeTypes of Energy Change1. Endothermic - energy is absorbed by

the system from its surroundings.

Eg - cold packs

- photosynthesis

Reactants + Energy ------> Products

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Types of Energy ChangeTypes of Energy Change2. Exothermic - energy is released by the

system to its surroundings.

Eg - hand warmers

- hot packs

- cellular respiration

Reactants ------> products + energy

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Types of Energy ChangeTypes of Energy Change2. Exothermic - energy is released by the

system to its surroundings.

Eg - hand warmers

- hot packs

- cellular respiration

Reactants ------> products + energy

1 chapter 2 chemical change. 2 3 chemical reactions a chemical reaction is a process in which...

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