UNIT: Chemical Reactions TOPIC: Properties of Reactions Objectives: Lesson 1 of 3 To learn how we recognize a chemical reaction has took place To learn what a chemical reactions is To learn how we balance chemical equations To learn the symbols for each physical state in a chemical reaction

Quickwrite In 1-2 sentences answer one of the questions below: What sort of clues do think indicate that a chemical reaction is happening in your car engine???? What sort of clues indicate a chemical reaction when you drop a piece of alka selzter in water???? What are some by products produced when your body breaks down sugar, C6H12O6????? What are some clues that this reaction is occuring in your body????? Hint: think of temperature!!!!

Evidence for a Chemical Reaction How do you know a chemical reaction has occurred? Chemists look for visual clues such a color or heat change, a solid forms, bubbles are produced, a flame occurs However, reactions are not always visible Sometimes the only signal that a reaction is occurring is a change in temperature as heat is produced

What are some clues that a chemical reaction has occurred? A color change A solid forms Bubbles form A heat change A flame is produced

Chemical Reactions CH4 O2 CO2 H2O C H + O O + Chemists have learned that a chemical change always involves a rearrangement of the ways in which atoms are grouped and combine with one another For example, consider the reaction that occurs with your laboratory bunsen burners In this reaction, methane (CH4) reacts with oxygen (O2) in the air As you know, a flame is produce and heat is released As the atoms rearrange themselves, two new chemicals are produced carbon dioxide (CO2), and water (H2O) CH4 O2 CO2 H2O C H + O O +

Chemical Reactions CH4 O2 CO2 H2O + O Such a change is called a chemical reaction Notice, the atoms were rearranged, Oxygen combined with Carbon and Hydrogen to produce 2 new chemicals: Water & Carbon Dioxide Chemical reactions obey the Law of Conservation of Matter which states: Atoms cannot be destroyed, they can only be rearranged CH4 O2 CO2 H2O + O Methane (CH4) Water(H2O) Carbon Dioxide (CO2) Oxygen(O2)

What is a Chemical Reaction? A chemical reaction is the change that occurs when atoms rearrange themselves and form new compounds Chemical reactions obey the first LAW of conservation of MASS or MATTER which states: Atoms and matter cannot be created or destroyed, they can only be rearranged

Chemical Equations We represent chemical reactions by writing a chemical equation in which the chemicals present before the reaction are on the left of the arrow, and the chemicals formed by the reaction are on the right of the arrow The arrow indicates the direction of the change CH4 + O2 CO2 + H2O

Chemical Equations In the reaction of methane with oxygen, we have: CH4 + O2 CO2 + H2O The chemicals present on the left side of the equation are known as the reactants The chemicals present on the right side of the equation are known as the products Reactants Products

What is a Chemical Equation? An equation that uses the formulas of chemicals compounds and elements to show the reactants and products in a chemical change Chemicals present before the reaction are on the left of the arrow (reactants), and the chemicals formed by the reaction are on the right of the arrow (products) Ex: CH4 + O2 CO2 + H2O

Balancing Chemical Reactions It is important to realize that in a chemical reaction, atoms are neither created nor destroyed In other words, their must be the same number of each type of atom on the product side as on the reactant side of the arrow

Balancing Chemical Reactions Remember, atoms cannot be created or destroyed in chemical reactions, they can only be rearranged! The total number of atoms must be the same on both sides of the arrow Balancing Chemical Reactions Notice, there are 2 Oxygen atoms on left side and 3 Oxygen atoms on the right side; also there are 4 Hydrogen on the left side and 2 Hydrogen on the right side We can fix this by putting in 1 more Oxygen molecule on the left (reactant) side and 1 more Water molecule on the right (product) side The table now has equal number of atoms on both sides of the equation In other words, the reaction and table below show that 1 Oxygen atom is created and 2 Hydrogen atoms are destroyed Therefore, the reaction cannot happen this way! By counting the number of atoms, you can see that the chemical equation is balanced and shows the actual numbers of molecules involved in this reaction We group molecules together by putting coefficients in front of them: The reaction below is not balanced We can see it is not balanced by taking the reactants & products apart: CH4 CO2 H2O O2 + O2 + + + 2 H2O + + 2 H H C H O O O O O C O H O H H O H Left Reactant Side Right Product Side C = O = H = 7 6 9 1 1 4 2 3 4 4 4 2 Total Number of Atoms

Balancing Chemical Reactions Notice, there is 1 Oxygen atoms on left side and 2 Oxygen atoms on the right side; therefore the chemical equation is NOT balanced We can fix this by putting in 1 more Water molecule on the left(reactant) side and 1 more Hydrogen molecule on the right(product) side The table now has equal number of atoms on both sides of the equation By counting the number of atoms, you can see that the chemical equation is balanced and shows the actual numbers of atoms and molecules involved in this reaction In other words, the reaction and table below show that 1 Oxygen atom is created Therefore, the reaction cannot happen this way! We group molecules together by putting coefficients in front of them: Remember, atoms cannot be created or destroyed in chemical reactions, they can only be rearranged! The total number of atoms must be the same on both sides of the arrow The reaction below is not balanced We can see it is NOT balanced by taking the reactants & products apart: H2O H2O O2 H2 H2 + 2 2 + + H O H H H H H O O H O H 3 Left Reactant Side Right Product Side O = H = 6 1 2 4 2 4 2 2 4 Total Number of Atoms

Subscripts and Coefficients When balancing chemical equations, it is important to note that the subscripts can never change For example, in the equation below, the subscript of 4 in methane cannot be changed, neither can the subscripts 2 in Oxygen, Carbon dioxide and Water CH4 + O2 CO2 + H2O Only coefficients can be added by putting whole numbers in front of the compounds when balancing chemical reactions 2 2

How Do we Balance Chemical Equations? By placing coefficients in front of each molecule or compound Subscripts cannot be changed! Ex: CH4 + 2O2  CO2 + 2H2O

Balancing Chemical Equations Remember: When balancing chemical equations, it is important to note that the subscripts can never change!!!! ___O2 ___O3 ___ Mg + ___ O2 ___ MgO 3 2 2 2

Determine how many atoms of each element are in each compound below: Practice: Determine how many atoms of each element are in each compound below: ___ Na + ___ Cl2 ___ NaCl ___ Fe + ___ O2 ___ Fe2O3 ___ Al + ___ O2 ___ Al2O3 ___ Na + ___ F2 ___ NaF 2 2 4 3 2 4 3 2 2 2

Physical States In a reaction, we often use symbols to represent the physical state of the reactants and products Look at the reaction below when sodium metal reacts with water: Na(s) + H2O(l) H2(g) + NaOH(aq) Symbol State The symbol (s) tells us that Sodium (Na) is in the solid state The symbol (l) tells us the water (H2O) is in the liquid state The symbol (g) tells us the hydrogen (H2) is in the gaseous state Finally, the symbol (aq) tells us that sodium hydroxide (NaOH) is aqueous or dissolved in water (s) solid (l) liquid (g) gas (aq) aqueous

What do the symbols represent in a chemical equation? The symbols represent the physical state of the reactants and products Na(s) + H2O(l) H2(g) + NaOH(aq) Symbol State or dissolved in water (s) solid (l) liquid (g) gas (aq) aqueous

Summary (you can always write your own summary) Explain the Law of Conservation of Mass in your own words and apply it to chemical reactions: What is the difference between the reactants and products in a chemical reaction? Describe the process of balancing chemical reactions, be sure to explain how you count atoms on both sides of the equation What can you not change when balancing chemical reactions? Is the reaction below balanced? Explain why or why not: K + Cl2  KCl