CHAPTER 8 CHEMICAL EQUATIONS AND REACTIONS CHEMICAL EQUATIONS
- Slides: 35
CHAPTER 8 CHEMICAL EQUATIONS AND REACTIONS
CHEMICAL EQUATIONS AND REACTIONS A chemical reaction is the process by which one or more substances are changed into different substances.
• Chemical reactions are described by CHEMICAL EQUATIONS, which represent the identities and relative amounts of the reactants and products in the reaction. – Ex: (NH 4)2 Cr 2 O 7(s) N 2 (g)+ Cr 2 O 3 (s)+ 4 H 2 O (g) – ammonium dichromate yields nitrogen, chromium(III) oxide and water – (s), (g), (l) , (aq)= solid, gaseous, liquid state, or aqueous solution
Indications that a chemical reaction has occurred. • The formation of a precipitate. A precipitate is a solid formed in a chemical reaction that is different from either of the reactants.
• The formation of a gas is another sign that a reaction may have occurred. – The formation of bubbles when two liquids are mixed usually indicates that a gas has formed. – A gas can also be formed when a solid is added to a solution.
• Evolution of heat and light indicates that a chemical reaction is occurring.
• A color change may also indicate that a chemical reaction has occurred. – A reaction has occurred if two solutions are mixed and there is a color change that is not simply the result of a dilution of one of the reactant solutions. A color change may also occur when a solid and liquid are mixed.
Characteristics of Chemical Equations 1. The equation must represent known facts. 2. The equation must contain the correct formulas for the reactants and products. 3. The law of conservation of mass must be satisfied.
Word Equations • WORD EQUATION - describes a chemical change using the names of the reactants and products. • REACTANTS - the substances that exist before a chemical change (or reaction) takes place. • PRODUCTS - the new substance(s) that are formed during the chemical changes.
Try this: What is the word equation for the following reaction: CH 4(g) + O 2(g) CO 2(g) + H 2 O(g) • The arrow is read as reacts to yield or yields • Identify the reactants and products. • The formula equation is shown and represents the reactants and products of a chemical reaction by their symbols or formulas.
• COEFFICIENTS: the numbers in front of the formulas of substances in chemical equations which tell the relative number of moles, molecules, or formula units taking part in a chemical reaction. (We use moles most of the time. ) • Sample Problem 2: What does the following say? 2 Na. I + Cl 2 2 Na. Cl + I 2
Balancing Chemical Equations • BALANCED EQUATION - the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product. – Count the number of atoms of each element on each side of the equation. These must be equal • The fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation relates to what law?
Try these: • Determine whether the following equations are balanced. 2 Na + H 2 O 2 Na. OH + H 2 CH 4 + O 2 CO 2 + 2 H 2 O • 2 Na 2 Na 2 H 4 H • 1 C • 4 H • 2 O 1 C 4 H 4 O
• There are many symbols used in chemical equations. See table 8 -2 page 246 for the complete list. You need to know these.
Steps for balancing chemical equations • Step 1: Write a word equation for the reaction. • Step 2: Write the correct formulas for all reactants and products. – be on the lookout for the diatomic elements like H 2, N 2 etc. • Step 3: Determine coefficients that make the equation balance. • Try this : Barium and chlorine react to yield barium chloride. – Ba + Cl 2 Ba. Cl 2
• Begin balancing with the most complicated-looking group. A polyatomic ion that appears unchanged on both sides of the equation can be counted as a single unit. • Save the elemental (single elements) reactant and products for last, especially if it is hydrogen or oxygen. Keep your eye out for diatomic molecules such as oxygen, hydrogen, and the halogens. • If you get stuck, double the most complicated-looking group and try again. • Finally, make sure that all coefficients are in the lowestpossible ratio. • Know when to quit! None of the reactions you will encounter will be that difficult. If the coefficients are getting wild, double-check what you’ve done since you may have a simple mistake.
Write a balanced equation for the reaction between the following: • chlorine and sodium bromide to produce bromine and sodium chloride. • aluminum sulfate and calcium chloride to produce aluminum chloride and calcium sulfate. Cl 2 + 2 Na. Br 2 Na. Cl + Br 2 Al 2(SO 4)3 + 3 Ca. Cl 2 2 Al. Cl 3 + 3 Ca. S O 4
WHEW! That was fun • Home fun- Read the section and do the practice problems. • Pg 254 q 1 -5 • Pg 269 q 1 • Pg 270 q 18 – 23 • These are due tomorrow.
Ch 8 sec. 2 TYPES OF CHEMICAL REACTIONS • Reactions are classified according to similarities and regularities. • There are five different types of reactions that we will discuss. (Note that there are other types, but these are fairly common and what we will be using. )
1. SYNTHESIS REACTION • AKA composition reactions • In a synthesis reaction two or more substances combine to form a more complex substance. • Two or more reactants yielding one product is another way to identify a synthesis reaction. • For example, hydrogen gas combined with oxygen gas can produce a more complex substance-----water. The chemical equation for this synthesis reaction looks like this: H 2 + O 2 H 2 O How does this look to you? • This is represented by A + X AX
2. DECOMPOSITION REACTION • In a decomposition reaction a more complex substance breaks down into its more simple parts. • One reactant yields two or more products. Basically, synthesis and decomposition reactions are opposites. For example, water can be broken down into hydrogen gas and oxygen gas. The chemical equation for this decomposition reaction looks like: reactant -------> product + product 2 H 2 O 2 H 2 + O 2 • This is represented by AX A + X
3. SINGLE REPLACEMENT REACTION • In a single replacement reaction a single uncombined element replaces another element in a compound. • Two reactants yield two products. • For example when zinc (single uncombined) combines with hydrochloric acid, the zinc replaces the hydrogen. • The chemical equation for this single replacement reaction looks like this: Zn + 2 HCl Zn. Cl 2 + H 2 • This is represented by : A + BX AX + B or • Y + BX BY + X
4. DOUBLE REPLACEMENT REACTION • In a double replacement reaction parts of two compounds switch places to form two new compounds. • Two reactants yield two products. • For example when silver nitrate combines with sodium chloride, two new compounds--silver chloride and sodium nitrate are formed because the sodium and silver switched places. • The chemical equation for this double replacement reaction looks like: • Ag. NO 3 + Na. Cl Ag. Cl + Na. NO 3 • This is represented by: AX + BY AY + BX
5. COMBUSTION REACTION • A substance combines with oxygen, releasing a large amount of energy in the form of heat and light. • What combustion reaction was witnessed during the lab?
Synthesis Reaction Trends • Elements react with oxygen to form oxides. • Nearly all metals react with oxygen to form oxides. Group I and II metals react easily. – What forms when oxygen reacts with barium? with potassium? • Some metals can form more than one oxide. – Ex: Fe. O and Fe. O 2 (Two different oxidation states for iron) • Non-metals also form oxides. Ex carbon, sulfur, hydrogen.
Synthesis Reaction Trends • Metals react with halogens to form ionic compounds, but you already knew that. • Oxides of metals react with water to form hydroxides. Ex: Ca. O + H 2 O Ca(OH)2 • Many oxides of nonmetals (in the upper right corner) react with water to form oxyacids. – Ex: SO 2 + H 2 O H 2 SO 3
Decomposition Reaction Trends • Decomposition by electric current is called electrolysis. This is used to separate water into its component parts. • Oxides of less-active metals (usually in the center of the table) decompose into their elements when heated. • Metal carbonates decompose to produce metal oxides and carbon dioxide. • Metal hydroxides (except group I metals) decompose to form metal oxides and water. • Metal chlorates decompose to form metal chlorides and oxygen. • Some acids decompose into nonmetal oxides and water – Ex: Carbonic acid yields carbon dioxide and water.
SINGLE REPLACEMENT REACTION TRENDS • More active metals can replace less active metals in solutions. Ex- aluminum can replace lead in lead nitrate solution. (This will be more easily understood when we talk about the activity series next week). • Hydrogen in water is replaced by Group I metals. Brainiac: Alkali Metals • Hydrogen in an acid is replaced by a metal forming a salt and hydrogen gas. • More active halogens replace less active halogens in a compound.
Balance the following equations (make sure they are correct) and identify the type of reaction • (NH)2 S + Zn. Cl 2 NH 4 Cl + Zn. S • Ba + H 2 O Ba(OH)2 +H 2 • double replacement • single replacement • H 2 O H 2 + O 2 • decomposition • K + Cl 2 KCl • synthesis • Ba + Cl 2 Ba. Cl 2 • synthesis • Al+ Pb(NO 3)2 Al(NO 3)3 + Pb
HOME FUN • PG 264 q 1 -4 • pg 269 q 8 • pg 271 q 23, 26 -30
Activity Series • The ability of an element to react is referred to as the elements ACTIVITY. • A list of elements organized according to their activity is called an ACTIVITY SERIES. • The most active elements are placed above less active elements and can replace less active elements in reactions.
• For metals greater activity means a greater ability to lose electrons. • For nonmetals greater activity means a greater ability to gain electrons. • Just because we can write equations for reactions doesn’t mean the reactions will actually occur. • The activity series lets us predict whether a reaction will occur or not.
You have an activity series on your “magic sheet” woohoo! • Try these: Will the following reactions occur? For reactions that will occur, write the products and balance the equations. • Mg. Cl(aq) + Zn(s) • Al(s) + H 2 O(g) • Cd(s) + O 2(g) • I 2(s) + KF(g) • no • yes. 2 Al(s) + 3 H 2 O(g) Al 2 O 3 (s) + 3 H 2 (g) • yes 2 Cd(s) + O 2(g) 2 Cd. O (s) • no
Home fun • pg 267 q 1 -3 • pg 271 -272 q 35 -37
MAIN POINTS • Chemical equations summarize chemical reactions, using chemical formulas to represent the reactants and products, rather than full names. • All the atoms that appear on one side of a balanced equation must also appear on the other side. Chemical reactions adhere to the Law of Conservation of Mass (matter). • Real chemical reactions are more complex than the neat summaries depicted in equations. Some reactants usually remain unreacted, some side reactions generate additional minor products, and most reactions proceed through a variety of chemical intermediates which are usually not included in the equations. • The main factors influencing reaction rates are temperature, concentration, and the presence of catalysts. • Chemical equations can be used to calculate the specific quantities of reactants and products involved in the reactions, either as masses or as moles. The general term for the process of performing such calculations is stoichiometry.
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