Physical and Chemical Change CHAPTER 4 Physical and Chemical Change 4.2 Chemical Reactions

Electrolysis ? ? Experimental setup: Observations: An electric current transfers a large amount of energy to water. ? Observations: The mass of water decreases over time. The volume of gas increases.

Electrolysis ? ? Experimental setup: Observations: Hypothesis: An electric current transfers a large amount of energy to water. ? Observations: The mass of water decreases over time. The volume of gas increases. Hypothesis: The gas produced is water vapor.

Electrolysis ? ? Hypothesis: Observations on the gases: The gas produced is water vapor. ? Observations on the gases: 1. When cooled, the gas does not condense into liquid water. 2. One of the gases burns. 3. One of the gases causes a flame to get brighter.

Electrolysis ? ? Hypothesis: Observations on the gases: Conclusion: The gas produced is water vapor. ? Observations on the gases: 1. When cooled, the gas does not condense into liquid water. 2. One of the gases burns. 3. One of the gases causes a flame to get brighter. Conclusion: The gas produced is not water vapor.

Electrolysis ? ? Clues: One gas burns. One gas causes a flame to get brighter. The gases come from water. ? ?

Electrolysis Clues: One gas burns. One gas causes a flame to get brighter. The gases come from water. +

The energy from the electric current was high enough to cause a chemical change through a chemical reaction. + chemical reaction: a process that rearranges the atoms in any substance(s) to produce one or more different substances. chemical change: a result of chemical reaction.

+ A chemical equation

The chemical equation oxygen oxygen hydrogen hydrogen How many oxygen atoms are present on each side? What about hydrogen atoms? hydrogen hydrogen

How can we make both sides the same number of each atom? The chemical equation ! 1 2 oxygen oxygen This equation is not balanced. 2 2 hydrogen hydrogen How can we make both sides the same number of each atom?

The chemical equation coefficients oxygen oxygen hydrogen hydrogen Count again the number of oxygen and hydrogen atoms on each side, considering the coefficients. hydrogen hydrogen

The chemical equation 2 2 coefficients oxygen oxygen 4 4 hydrogen

The chemical equation 2H2O(l) 2H2(g) + O2(g) Reaction arrow Reactants Products 2H2O(l) 2H2(g) + O2(g) Coefficients tell you how many of each molecule participate in the reaction. (No coefficient appears when it equals 1)

The chemical equation Rules of balancing a chemical equation: 1. Determine if the equation is balanced. Count the number of each kind of atom on either side of the arrow. 2. If it is not balanced, use coefficients. Ex: H2O becomes 2H2O 3. Do not change the subscripts inside the molecular formulas. Ex: H2O cannot become H3O 4. Check that the equation is balanced. Count the number of each kind of atom on either side of the arrow with the new coefficients.

Is it balanced? Reactants Products Iron (Fe) Oxygen (O)

Is it balanced? Reactants Products Iron (Fe) 1 2 Oxygen (O) 3

Not balanced Reactants Products Iron (Fe) 1 2 Oxygen (O) 3

The chemical equation Rules of balancing a chemical equation: 1. Determine if the equation is balanced. Count the number of each kind of atom on either side of the arrow. 2. If it is not balanced, use coefficients. Ex: H2O becomes 2H2O 3. Do not change the subscripts inside the molecular formulas. Ex: H2O cannot become H3O 4. Check that the equation is balanced. Count the number of each kind of atom on either side of the arrow with the new coefficients.

Add coefficients ? ? ? Reactants Products Iron (Fe) Oxygen (O)

4 3 2 Reactants Products Iron (Fe) 4 Oxygen (O) 6 Balanced!

a sugar (glucose) gases a gas Photosynthesis is the basis of most life on Earth.

Verify that the equation is balanced Reactants Products Carbon (C) Oxygen (O) Hydrogen (H) Verify that the equation is balanced

Number of atoms or moles Verify that the equation is balanced Reactants Products Carbon (C) 6 Oxygen (O) 18 Hydrogen (H) 12 Number of atoms or moles

Use the formula mass to convert from moles to grams.

108 g + 264 g 372 g 180 g + 192 g 372 g Total mass of reactants Total mass of products 108 g + 264 g 372 g 180 g + 192 g 372 g

Conservation of mass 108 g + 264 g 372 g 180 g + 192 g 372 g Total mass of reactants Total mass of products 108 g + 264 g 372 g 180 g + 192 g 372 g Conservation of mass

Total mass of reactants = Total mass of products conservation of mass: law that states that, in any chemical reaction, the total mass remains the same. Total mass of reactants = Total mass of products

All chemical reactions involve three key components: Reactants Products Energy Reactants Products Energy (in or out)

Energy “in” Energy as a reactant. reactants products An endothermic reaction requires an input of energy.

Energy “out” Energy as a product. reactants products An exothermic reaction releases energy.

Energy is absorbed Energy is released

First law of thermodynamics: Energy can neither be created nor destroyed. Energy is absorbed Are these violations of the first law? Energy is released

First law of thermodynamics: Energy can neither be created nor destroyed.

What is missing? Photosynthesis Respiration Mixing water and CO2 doesn’t make sugar and O2 because energy input is needed. Respiration Energy input is not needed but just mixing sugar and O2 doesn’t make water and CO2.

What is missing? A candle burning gives off heat. It is an exothermic reaction. However, the candle does not spontaneously light itself up.

What is missing? A candle burning gives off heat. It is an exothermic reaction. However, the candle does not spontaneously light itself up.

Activation energy C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J

Activation energy C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J

Activation energy C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J

Activation energy C6H12O6 + 6O2 6H2O + 6CO2 + 2,800,000J an energy barrier

Chemical equations must be balanced so that mass is conserved conservation of mass: law that states that, in any chemical reaction, the total mass remains the same. In an exothermic reaction energy is released. In an endothermic reaction energy is absorbed.