HEAT Video 6.1

Table I  Exothermic reactions release heat and have negative values.  Example: When Carbon and Oxygen react they release 393.5kJ of heat per mole reacted.  Endothermic reactions absorb heat and have positive values.  Example: When Nitrogen and Oxygen react they absorb 182.6kJ of heat per mole.

Table I examples  When C 2 H 4 is formed, is heat released or absorbed?  Table I: 2C + 2H 2  C 2 H 4 Δ H=52.4kJ  When 2 moles of CH 4 burn in oxygen, how much heat is released?  Table I: CH 4 + 2O 2  CO 2 + H 2 O Δ H=-890.4kJ  2CH4 2(-890.4) = KJ

Table I examples  Reactions that release the most energy are the most stable. Which reaction becomes the most stable?  4Al + 3O 2  2Al 2 O 3 Δ H=-3351kJ  Where did these values come from?

Calculating Heat of reactions  q is the symbol for heat.  If q is positive, the heat is endo.  If q is negative, the heat is exo.  q is measure in Joules, (J) or kilojoules (kJ).  The heat of a reaction is based on:  the mass of the substance  the temperature change it undergoes  specific heat.

Specific Heat  Specific heat is the heat needed to raise the temperature of one gram of a substance one degree Celsius.

To calculate heat: q = mc Δ T 1. The temperature of 95.4g of copper increases from 25 to 48C and absorbed 849J. Calculate copper’s specific heat. 849= (95.4) (x) (48-25) Q = J/gC

q = mc Δ T 2. How much heat is needed to raise the temperature of 100g of water 50C? q = (100) (4.18) (50) q = 20900J or KJ

q = mc Δ T 3. If 600J are needed to heat 50g of water to 100C, what is the initial temperature? 97.13C 600 = (50) (4.18) (x) X = 2.87

When can you use q=mc Δ T?  Only on the solid, liquid and gas only lines. (Where the temperature changes)  So, what equations do we use if the temperature is not changing?

Two more equations from Table T  Heat of vaporization: heat needed to change a substance from gas to liquid or liquid to gas. q=mH v  Heat of fusion: heat needed to change a substance from solid to liquid or liquid to solid. q=mH f  If the IMF is strong, the heats of vaporization and fusion is high.

q=mH v 1. Calculate the number of joules needed to vaporize 423g of H 2 O. 955, 980J or KJ q = (423) (2260)

q=mH f  How much heat is needed to melt ice at 0C if the sample weighs 255g? 85,170J or KJ Q = (255) (334)

Measuring heat in the lab You can measure the heat of physical and chemical changes in a calorimeter. The calorimeter acts like a styrofoam cup, it insulates the reaction (doesn’t let the overall heat change).

Measuring heat in the lab  The heat released by the reaction equals the heat absorbed by the water.  You will measure the change in heat of the water using q=mc Δ T.

Measuring heat in the lab  You will use a calorimeter more like this.  You must make sure you always stir the solution to make the heat equal throughout the cup.

RATES Video 6.2

Rate: A measure of the speed of any change.

Collision Theory: When two chemicals react, their molecules have to collide with each other with proper energy and orientation.

Temperature When temperature increases, the molecules move ___________ and collide ________. So the rate of reaction ____________, or the reaction moves _____________. faster more increases faster

Concentration When concentration increases, the molecules collide ________. So the rate of reaction ____________, or the reaction moves _____________. more increases faster

Surface Area When the surface area increases, the molecules collide ________. So the rate of reaction ____________, or the reaction moves _____________. more increases faster

Catalysts When catalysts are added, the make the reaction ___________ and collide ________. So the rate of reaction ___________. faster more increases

Nature of Reactants Ionic substances react faster than covalent substances because the easily break into ions when you dissolve them.

Pressure When the pressure on a gas increases, the gas has __________ volume. Therefore, there are _________ collisions and the rate __________, or moves _____________. less more increases faster

Activation Energy  There is a minimum amount of energy required for reaction: the activation energy, E a.  Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.

A + B C + D Exothermic Reaction Endothermic Reaction The activation energy (E a ) is the minimum amount of energy required to initiate a chemical reaction. 13.4

The activated complex is the highest part of the graph where the reaction can go to completion or revert back to reactants. Enthalpy, or heat, can be measured on the graph by subtracting reactants from products.

Energy Diagrams ExothermicEndothermic

Catalysts  Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction.  Catalysts change the mechanism by which the process occurs.

ACIDS AND BASES Video 6.3

What is an electrolyte?  A substance that dissolves in water and conduct electricity. Acids and bases are electrolytes.

Arrhenius  Acid:Substance that, when dissolved in water, increases the concentration of hydrogen H + (or hydronium H 3 O + ) ions.  Acids have a sour taste and can burn your skin.  Acids react vigorously with metals to make H 2  pH is less than 7  On table K

Arrhenius  Base: Substance that, when dissolved in water, increases the concentration of hydroxide ions.  Bases have a bitter taste and are corrosive.  pH>7  On Table L

Arrhenius  If an acid is added to a base, it results in neutralization, where all properties of the acid and base are lost. The products are a salt and water. NaOH + HCl  H 2 O + NaCl Salt is another name for solid ionic compounds containing elements other than H + and OH -. Salts are electrolytes with high mp and bp.

Label and name the acid, base and salt: 1. H 2 SO 4 + LiOH  H 2 O + Li 2 SO 4 2. KOH + HNO 3  KNO 3 + H 2 O AcidBase Salt BaseAcid Salt

Identify the salt produced: 1. NaOH + HF  2. Ca(OH) 2 + H 2 SO 4  H 2 O + NaF H 2 O + CaSO 4 Make sure you check your compounds with the criss cross rule for ions!

Acid and Base Strength  Strong acids and bases are completely dissociated in water to make a lot of H + or OH -.  Weak acids and bases only dissociate partially in water to make a small amount of H + or OH -.  How is strength different from concentration?

Strength versus Concentration  Strength refers to the amount of ions a substance makes when it breaks down.  Concentration refers to the amount of the substance initially, before it breaks down. This is usually measured in molarity (mol/L).

pH

How Do We Measure pH?  For less accurate measurements, one can use  Litmus paper “Red” paper turns blue above ~pH = 8 “Blue” paper turns red below ~pH = 5  An indicator

How Do We Measure pH? For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

Earth’s Natural Litmus  Hydrangeas are blue when the acidity of the soil is between and red if the acidity is between A mix of colors can be seen between

Table M

Indicators  If NaOH is tested with methyl orange, what color will it be?  At what pH will bromocrescol green turn yellow?  What type of solution will turn bromothymol blue, yellow?  At what pH will both bromothymol and thymol blue be yellow? yellow 3.8 acidic 7.6-8

Examples  Why won’t methyl orange be good at determining the difference between an acid and a base?  Which indicator is the best to test the difference between a strong and weak base?  What color change will be seen if NaOH is added to HCl with methyl orange?  What color change will be seen if nitric acid is added to lithium hydroxide using bromocrescol green? Bases and acids can both be yellow Thymol blue Red to yellow Blue to yellow