1 Physical and Chemical Changes Chapter 2

2 Changes in Matter Physical Changes are changes to matter that do not result in a change of the fundamental components that make that substance –State Changes – boiling, melting, condensing Chemical Changes involve a change in the fundamental components of the substance –Produce a new substance –Chemical reaction –Reactants  Products

3 Density Density is a property of matter representing the mass per unit volume For equal volumes, denser object has larger mass For equal masses, denser object has small volume Solids = g/cm 3 –1 cm 3 = 1 mL Liquids = g/mL Density : solids > liquids >>> gases Water: density = 1g/mlIron: density = 7.86 g/cm 3

4 Water Freezes at 0°C –At 1 atm, solid at 0°C or below –Normal freezing point = normal melting point Boils at 100°C –At 1 atm, liquid up to 100°C, then turns to steam –Normal boiling point –Boiling point increases as atmospheric pressure increases Temperature stays constant during a state change Relatively large amounts of energy needed to melt solid or boil liquid

5 Heating Curve As heat added to solid, it first raises the temperature of the solid to the melting point Then added heat goes into melting the solid –Temperature stays at the melting point –Heat of Fusion As more heat added it raises the temperature of the liquid to the boiling point Then added heat goes into boiling the liquid –Temperature stays at the boiling point –Heat of Vaporization As more heat added it raises the temperature of the gas

6 The heating/cooling curve for water heated or cooled at a constant rate

7 Both liquid water and gaseous water contain H 2 O molecules

8 Representations of the gas, liquid, and solid state

9 Intramolecular (bonding) forces exist between the atoms in a molecule and hold the molecule together

10 Sublimation Solids can change directly into gases without going through the liquid state Dry ice is solid CO 2 ; it never melts under normal conditions. Instead it sublimes to gaseous CO 2. The formation of frost is reverse of sublimation = vapor deposition

11 The Solution Process - Dissolving is not Melting When ionic compounds dissolve in water they dissociate into ions –ions become surrounded by water molecules - hydrated When solute particles are surrounded by solvent molecules we say they are solvated

12 When solid sodium chloride dissolves, the ions are dispersed randomly throughout the solution

13 Polar water molecules interact with the positive and negative ions of a salt

14 The Solution Process Covalent Molecules Covalent molecules that are small and have “polar” groups tend to be soluble in water The ability to H-bond with water enhances solubility O H H C O H H H H O H H

15 Attractive Forces and Properties Larger attractive forces between molecules in pure substance means –higher boiling point –higher melting point (though also depends on crystal packing ) Like dissolves Like –Polar molecules dissolve in polar solvents Water, alcohol Molecules with O or N higher solubility in H 2 O due to H- bonding with H 2 O –Non-polar molecules dissolve in non-polar solvents Oils and gasoline

16 Evaporation Requires overcoming intermolecular attractions Condensation is the reverse process In a closed container, eventually the rate of evaporation and condensation are equal –Equilibrium –In open system, evaporation continues until all liquid evaporated

17 Behavior of a liquid in a closed container. The system is at equilibrium.

18 Vapor Pressure Pressure exerted by a vapor in equilibrium with a liquid –Or solid Increases with temperature Larger intermolecular forces = Lower Vapor Pressure Liquid boils when its Vapor Pressure = Atmospheric Pressure –Normal boiling point –Raising external pressure raises boiling point, & visa versa Liquid just poured into open container, little vapor Evaporation faster than Condensation Evaporation as fast as Condensation Equilibrium

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20 Chemical Reactions Reactions involve chemical changes in matter resulting in new substances Reactions involve rearrangement and exchange of atoms to produce new molecules –Elements are not transmuted during a reaction Reactants  Products

21 Evidence of Chemical Reactions a chemical change occurs when new substances are made visual clues (permanent) –color change, precipitate formation, gas bubbles, flames, heat release, cooling, light other clues –new odor, permanent new state

22 Bubbles of hydrogen and oxygen gas form when an electric current is used to decompose water

23 Hot and cold pack reactions

24 Chemical reactions

25 Chemical reactions

26 Chemical reactions

27 Chemical reactions

28 Chemical Equations Shorthand way of describing a reaction Provides information about the reaction –Formulas of reactants and products –States of reactants and products –Relative numbers of reactant and product molecules that are required –Can be used to determine weights of reactants used and of products that can be made

29 Conservation of Mass Matter cannot be created or destroyed In a chemical reaction, all the atoms present at the beginning are still present at the end Therefore the total mass cannot change Therefore the total mass of the reactants will be the same as the total mass of the products

30 Writing Equations Use proper formulas for each reactant and product proper equation should be balanced –obey Law of Conservation of Mass –all elements on reactants side also on product side –equal numbers of atoms of each element on reactant side as on product side balanced equation shows the relationship between the relative numbers of molecules of reactants and products –can be used to determine mass relationships

31 Balancing by Inspection 1.Count atoms of each element polyatomic ions may be counted as one “element” if it does not change in the reaction Al + FeSO 4  Al 2 (SO 4 ) 3 + Fe 1 SO 4 3 if an element appears in more than one compound on the same side, count each separately and add CO + O 2  CO O 2

32 Balancing by Inspection 2.Pick an element to balance 3.Find Least Common Multiple and factors needed to make both sides equal 4.Use factors as coefficients in equation if already a coefficient then multiply by new factor 5.Recount and Repeat until balanced

33 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 1.write the equation in words –identify the state of each chemical magnesium + oxygen  magnesium oxide 2.write the equation in formulas –identify diatomic elements –identify polyatomic ions –determine formulas Mg + O 2  MgO

34 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 3.count the number of atoms of on each side –count polyatomic groups as one “element” if on both sides –split count of element if in more than one compound on one side Mg + O 2  MgO 1  Mg  1 2  O  1

35 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 4.pick an element to balance –avoid element in multiple compounds 5.find least common multiple of both sides & multiply each side by factor so it equals LCM Mg + O 2  MgO 1  Mg  1 1 x 2  O  1 x 2

36 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 6.use factors as coefficients in front of compound containing the element 3if coefficient already there, multiply them together Mg + O 2  2 MgO 1  Mg  1 1 x 2  O  1 x 2

37 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide –burning in air means reacting with O 2 7.Recount Mg + O 2  2 MgO 1  Mg  2 2  O  2 8.Repeat 2 Mg + O 2  2 MgO 2 x 1  Mg  2 2  O  2

38 Combustion of Methane methane gas burns to produce carbon dioxide gas and liquid water –whenever something burns it combines with O 2 (g) CH 4 + O 2  CO 2 + H 2 O H H C H H OO + O O C + O HH 1 C + 4 H + 2 O1 C + 2 O + 2 H + O 1 C + 2 H + 3 O

39 Combustion of Methane Balanced to show the reaction obeys the Law of Conservation of Mass it must be balanced CH O 2  CO H 2 O H H C H H OO + O O C + O HH OO + O HH + 1 C + 4 H + 4 O

40 Carbon Monoxide Incomplete combustion of carbon compounds occurs when there is not enough oxygen to fully make CO 2. 2 C 4 H O > 8 CO H 2 O 2 C 4 H O > 8 CO + 10 H 2 O CO is called a by-product (not the intended product)

41 Carbon Monoxide CO concentrations can be reported in the parts per million scale (ppm) –ppm tells how many particles out of a million are the compound in question –Air normally has 2 ppm CO CO is a common pollutant from burning fossil fuels like gasoline or coal –Cars must have a catalytic converter, which helps ensure all C is converted to CO 2

42 Carbon Monoxide Effects of CO on humans – ppm leads to headache, fatigue, dizziness in a few hours –Death would occur under these conditions if exposed for several hours –1000 ppm exposure can lead to death in < 1 h –CO interferes with oxygen (O 2 ) transport –The shape and size is about the same as O 2 –Hemoglobin is tied up with CO

43 Reactions of Metals Corrosion reactions can weaken structural metal 4 Fe + 3 O > 2 Fe 2 O 3 (rust) Tarnishing of Silver 2 Ag + H 2 S ----> Ag 2 S + H 2 Removing tarnish from silver 2 Al + 3 Ag 2 S ----> 6 Ag + Al 2 S 3

44 Nitrogen Reactions Ammonia Fertilizer N H > 2 NH 3 Liquid ammonia is easily stored and applied (-33 o C) Ammonia contains high nitrogen content Vehicle Airbags 2 NaN3 ----> 2 Na + 3 N 2 Rapid formation of nitrogen gas inflates the airbag