Chemical Changes

Chemical Changes Glenn V. Lo Department of Physical Sciences Nicholls State University

Physical vs. Chemical Change • CHEMICAL CHANGE: Transformation of one or more substances to other substance(s); “change in composition” • If a change does not involve a change in composition, it is called a PHYSICAL CHANGE. • A chemical change involves: • A decrease in the amount of one or more substances; “reactants” • An increase in the amount of other substance(s); “products”

Example • Suppose a drop of water added to a white solid results in the formation of a blue solid. Identify the reactants and products.

Example • During a chemical change, the amount of substance X increases from 5.0 g to 6.0 g. Is X a reactant or a product?

Atomic Interpretation of Chemical Change • A chemical change involves a rearrangement of atoms; bonds between atoms are broken and/or new bonds are formed. • Example: Is the change illustrated below a chemical or physical change?

Example • Consider the following pictorial representation of a chemical change. What are the reactants? Products? Which molecules are not involved in the reaction?

Chemical equation • A symbolic representation for a chemical change. Example: reaction of chalk and hydrochloric acid CaCO3(s) + 2 HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g) • Formulas of reactants on the left • Formulas of products on the right • Coefficients: numbers in front of formulas: 1, 2, 1, 1, 1 • Physical states: (l)=liquid, (g)=gas, (s)=solid, (aq)=“aqueous” or dissolved in water

Balanced Equation • There should be equal numbers of each kind of atom on both sides of the arrow (before and after the change has occurred). If this is done, the chemical equation is said to be “balanced.” • Counting # of atoms: subscript x coefficient

Example • Consider the balanced equation: CaCO3(s) + 2 HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g) How many Cl atoms are shown on either side of the arrow? Verify that the equation is balanced.

Balanced equation • The coefficients needed to balance an equation is not unique. • Example: Consider the balanced equation: CaCO3(s) + 2 HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g) What coefficients are needed to balance the equation if the coefficient of HCl is 6?

Smallest whole number coefficients • It is customary, but not necessary, to use the smallest set of whole numbers as coefficients of a balanced equation. • How to tell if we have the smallest set of whole number coefficients: • Smallest coefficient is 1, or • If we divide all coefficients by smallest coefficient and we end up with non-whole numbers, we already had it.

Example • Which of the following sets of coefficients can be reduced to a set of smaller whole numbers? A. 1, 3, 3, 2 B. 5, 2, 3, 4C. 3, 6, 6

Fractional Coefficients • It is OK to use fractional coefficients. • Interpret fractional coefficents as fractions of a group count. • Example: ½ CO2 is interpreted not as ½ of a molecule, but ½ of a “mole” of molecules • 1 mole = 602 billion trillion

Fractional Coefficients • To change fractional coefficients to whole number coefficients, multiply by the least common multiple of the denominators. • Example: change the coefficients to whole numbers: 1/2 N2 + 3/2 H2  NH3

Balancing tips • Adjust coefficients only; do not alter formulas • First balance elements that show up just once on both sides of the arrow. • Adjust coefficients upward. Use smallest possible whole numbers. • Always recheck after adjusting a coefficient.

Balancing by inspection • Balance: CH4 + O2 CO2 + H2O

Balancing using algebra • Use letters to represent coefficients. Example:w CH4 + x O2 y CO2 + z H2O • Set up equations to balance each element: • To balance C: w = y • To balance H: 4w = 2z • To balance O: 2x = 2y + z

Chemical vs. Physical Change • Chemical change involves change in composition: monitor substances present in the mixture • If change can be explained by unchanged formula units just moving apart or closer together: most likely physical change (example: melting, vaporization, sublimation)

Signs of Chemical Change • Some signs that a chemical change has occurred: • Unusual color change • Change in odor • Burning (production of heat and light) • Precipitation • Gas formation (odor or bubbles) • NOTE: Absence of a visible change does not necessarily mean that no change has occurred!

Examples • Chemical changes • Food rotting • Gasoline burning • Stain fading when treated with bleach • Physical changes • Water boiling • Ice melting • A chalk split (by teacher’s hand) in two

Test Yourself A mixture contains 25.0 g of substance X and 10.0 g of substance Y. A few minutes later, due to a chemical change, it is found that the mixture now contains 30.0 g of substance X and 5.0 g of substance Y. For the reaction described here... A. X is the reactant and Y is the product B. Y is the reactant and X is the product C. Both X and Y are reactants D. Both X and Y are products

Test Yourself Consider the pictorial representation shown below for a process at the atomic/molecular level. Which process involves a chemical change? A. 1 only B. 2 only C. both D. neither 1 2

Test Yourself Consider the pictorial representation shown below for a process at the atomic/molecular level. Which process does not involve a chemical change?

Test Yourself Which pictorial representation best accounts for the conversion of water from liquid to steam?

Test Yourself Consider the chemical equation for the hydrogenation of acetylene: C2H2 + 2 H2 C2H6 Which of these is the formula of a reactant in this reaction? A. C2H2, B. 2 H2, C. C2H6

Test Yourself Which of the following is not a balanced chemical equation? A. 2 H2 + O2 2 H2O B. 4 H2 + 2 O2 4 H2O C. Both A and B D. None of the above

Test Yourself Consider the following chemical equation where coefficients are represented by the letters w, x, y, and z: w Al2O3 + x H2SO4  y Al2(SO4)3 + z H2O If w=1, then which of these is true? A. y=2, B. z=3, C. x=6

Test Yourself Consider the combustion of propane: C3H8 + O2  CO2 + H2O (unbalanced) Using the smallest set of whole numbers, the sum of the coefficients in the balanced chemical equation is: A. 7, B. 9, C. 12, D. 13

Test Yourself What is the chemical equation for the combustion of acetylene as represented below?

Test Yourself Using the smallest set of whole numbers, the coefficient of NH3 in the balanced chemical equation for the reaction illustrated below is... A. 2, B. 3, C. 4, D. 6

Simple Types of Reactions • Combination (synthesis, composition) • Decomposition • Displacement (single replacement) • Metathesis (double replacement, double decomposition) • Combustion

Combination • Also known as composition, synthesis • Two or more reactants  one product • Common examples: • two elements  compound • ammonia (NH3) + acid  ammonium salt • metal oxide + water  metal hydroxide • nonmetal oxide + water  acid • metal hydroxide + CO2 metal bicarbonate • salt + water  salt hydrate

Combination examples • NH3(g) + HCl(g)  • Al(s) + Br2(l)  • Mg(s) + O2(g)  • MgO(s) + H2O(l)  • LiOH(s) + CO2(g)  • SO2(g) + H2O(l)  • CuSO4(s) + H2O(l) 

Decomposition • One reactant  two or more products • Usually requires heat or electricity •  above or below arrow indicates “heat” • Common examples: • metal carbonate  metal oxide and CO2(g) • metal bicarbonates metal hydroxide and CO2(g). • metal hydroxides metal oxide and H2O(g). • acids  nonmetal oxide and H2O(g). • metal chlorates  metal chloride and O2(g). • peroxides  oxide and O2(g). • binary compounds the elements

Decomposition examples • CaCO3(s)  CaO(s) + ? • Limestone is CaCO3. Quicklime, used to make cement, is CaO.

Decomposition examples • NaN3(s)  Na (s) + ? • Used in airbags

Displacement • Element + compound  element + compound • Common examples in aqueous solution: • Metal displacing hydrogen from water or acid • Metal displacing another metal • Halogen displacing another halogen

Activity Series • More “active” metal tends to displace a less active one: • K>Ca>Na>Mg>Al>Mn>Zn>Cr>Fe>Ni>Sn>Pb>H >Cu>Bi>Sb>Hg>Ag>Pt>Au • Memory aid: Peter Carly Simon Made A Mangy Zebra Carry Iron Nails To Liverpool. He Caught Billy, A Mexican Silver Plated Goat.

Displacement Example • Metal displacing another metal • Example: Mg(s) + CuCl2(aq)  ?

Displacement example • Thermite reaction: (used for welding railroad tracks) Al + Fe2O3

Test Yourself Which of the following is more likely to occur? A. Cu(s) + MgCl2(aq)  Mg(s) + CuCl2(aq) B. Cu(s) + AgNO3(aq)  Cu(NO3)2(aq) + Ag(s)

Displacement of hydrogen from H2O • Products are H2(g) and metal hydroxide • Example: Mg(s) + H2O(l)  ?

Displacement of hydrogen from acid • Products are H2(g) and salt • Example: Mg(s) + HCl (aq)  ?

Most Active metals • Most active metals: K, Ca, Na • very reactive; readily displace hydrogen from cold H2O (regardless of what else is in the solution) • Example: Na(s) + H2O(l)  ?

Example • What is the chemical equation for the reaction that occurs when K(s) is mixed with NaCl(aq). A. K(s) + NaCl(aq)  KCl(aq) + Na(s) B. 2 K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)

Second Most Active Metals • Mg, Al, Mn, Zn, Cr, Fe • readily displace H2 from hot steam and acids • Reaction is slow with cold water

Third most active metals • Third most active metals: Ni, Sn, Pb • readily displace H2 from acids, but not from water

Least Active Metals • Other metals will not displace H from water or acids • H> Cu>Bi>Sb>Hg>Ag>Pt>Au

Activity Series • Halogens: F > Cl > Br > I • Example: • Cl2(aq) + NaBr(aq) http://www.youtube.com/watch?v=EqPxOq9NeCE

Metathesis • Involves two reactants the yield ions in aqueous solution • Reaction can be thought of as an exchanging of partners • Typical examples: • Precipitation • Acid-base neutralization • Gas formation

Chemical Changes