Chemistry 068, Chapter 3

Matter and Chemistry

• Chemistry is the study of the composition, structure, properties, and changes of matter.

• Matter is physical material that has mass and occupies space. Each type of matter has unique properties that distinguish it from other types of matter.

• Matter can exist in different states.

Physical States

• A sample of matter can exist in three states: solid, liquid, or gas.

• A solid has a definite shape and volume.• A liquid has a definite volume but no specific

shape.• A gas has neither definite volume nor shape.• Matter changes between states in response to

changes in its environment (pressure, temperature, etc.).

Properties of the Three States

Property Solid Liquid Gaseous

Atomic/Molecular Motion Vibration Free Motion Free Motion

Atomic/Molecular Spacing Close Close Distant

Shape Definite Indefinite Indefinite

Volume Definite Definite Indefinite

Compressibility Minor Minor Very High

Density High Lower Very Low

Thermal Expansion 0.01% 0.1% 0.3%

(per oC)

Classification of Matter

• Aside from its state, matter can be said to be a pure substance or a mixture.

• Pure substances have distinct properties and composition that do not vary between samples. They cannot be physically separated.

• The components that make up mixtures retain their individual properties. Mixtures can be physically separated into their individual components.

Pure Substances

• Elements are composed of only one kind of atom.

• Elements are summarized on the periodic table.• Atoms are the smallest representative particle of

an element.• Molecules are arrangements of two or more

atoms (which need not all be the element).• Compounds are made up of molecules

consisting of two or more elements. They cannot be physically separated.

Mixtures

• Mixtures are made up of two or more elements or compounds that can be physically separated.

• A variety of methods can be used to separate mixtures but the most common are decanting, distillation, and filtration.– When decanting, a solution with two or more distinct

layers, such as oil on water, is separated by pouring off one layer.

– In distillation, a more volatile (easier to boil) portion of a mixture is boiled off by heating at a temperature high enough to boil the volatile but low enough not to boil the other component.

– In filtration, a solid is removed a liquid mixture by pouring it through a filter, leaving the solid on the filter.

Mixtures (Cont’d)

• Homogeneous mixtures are uniform throughout the mixture.

• Heterogeneous mixtures vary throughout.• The terms homogeneous and heterogeneous

apply to both phase (solid, liquid, gas) and chemical component.– A mixture can be homogeneous physically but

heterogeneous chemically, or vice versa.

Pure vs. Mixtures Problems

• For each of the following mixtures state weather they are homogeneous or heterogeneous chemically and physically.– Salt water.

– Ice floating in water.

– Sand.

– Oil on water.

Elements vs. Compounds

• Pure substances are either elements or compounds.

• Elements cannot be decomposed into simpler substances.

• Compounds are substances composed of two or more elements.– Compounds have different properties from either

element alone.– Compounds differ from mixtures in that they cannot

be physically separated.– Additionally, in a mixture the components maintain

their individual properties.• Mixtures are made up of two or more

compounds or elements.

Determining Weather a Substance is an Element, Compound, or

Mixture• Unknowns with non-uniform appearance are

heterogeneous mixtures.• Unknowns with a uniform appearance are either

homogeneous mixtures, elements, or compounds.• Recall that mixtures can be physically separated by

heating, shifting, etc. – elements and compounds cannot.– Thus, if a substance can be physically separated it must be a

mixture and not an element.

• Elements cannot be broken up chemically.– Compounds however, can.

• Both elements and compounds can chemically react.• The individual components of a mixture can also react.

Determining Weather a Substance is an Element, Compound, or

Mixture Problem• A scientist performs the following tests to

try to identify an unknown as an element, compound, or mixture.– The unknown has a uniform color and

appearance.– Upon heating, the unknown does not break

down into simpler substances.– The unknown cannot be broken down

chemically.

Properties of Matter

• Physical properties can be measured without changing the identity and composition of matter.

• Chemical properties describe the ways that a substance may react with other substances to form new substances.

• Intensive properties, such as density or melting point, do not depend on the amount of sample present.

• Extensive properties, such as mass or volume, depend on the amount of sample present.

Classification of Properties Problems

• Which of the following are physical properties? Which are chemical properties?– Mercury is a liquid at room temperature?

– Sodium hydroxide reacts with hydrochloric acid.

– Sodium metal burns in water.

– Gold is a shiny yellow color.

Classification of Properties Problems (Cont’d)

• Which of the following are intensive properties? Which are extensive?– Mass.

– Volume.

– Density.

– Boiling point.

Physical and Chemical Changes

• In physical changes, a substance changes its state or appearance but not its composition. All changes of state are physical changes.

• In chemical changes, a substance is transformed into a chemically different substance.

Classification of Changes Problems

• Which of the following are physical changes? Which are chemical?– Ice melts when heated.

– Cooking an egg.

– Metal rusting.

– Water boiling.

Conservation of Mass

• Mass is conserved through any process – it is neither destroyed nor created in any process.– With the exception of nuclear processes.

• During any physical or chemical change the total mass of matter of all materials must be the same at the beginning and end.

Conservation of Mass Problems

• During a chemical reaction, 98g of potassium reacts with 199g of bromine to form the salt potassium bromide. What mass of potassium bromide is formed?

Energy and Energy Units

• Energy is defined as the potential or capacity to do work.

• Work is the result of a force acting over a distance.

• Like matter, energy is conserved – it is neither created nor destroyed.– And again, nuclear processes are an exception.

• Matter typically has two types of energy, kinetic (the energy of motion) and potential (the energy of its position in a force field, such as gravity or a magnetic field).

Energy and Energy Units (Cont’d)

• Energy does not have a specific form and can be converted from one type to another.

• Some common forms of energy include electrical (due to the flow of electric charge), thermal energy (associated with the random motion of molecules), and chemical (potential energy due to the position of particles in a chemical system).

• Energy has SI units of joules, or J. They are defined as a N*m or kgm2/s2.– Other energy units are calories (cal), electron volts

(eV), and Btu.– 1J = 0.2390cal = 9.4781x10-4Btu.– 1eV = 1.6022x10-19J.– 1 killowatt-hour or kWh = 3.60*106J– Watts are power units, they are defined as J/s.

Heat

• Heat is the energy flow into or out of a system due to a temperature difference between a thermodynamic system and its surroundings.

• It is given the symbol Q or q.• Heat can only flow between systems in thermal

contact.– In other words, touching.

• Heat flows to attain thermal equilibrium (equal temperature).– Thus it flows from high to low temperature.

Phase Changes

• The three standard states, or phases, of matter are solid, liquid, and gas.

• When a substance changes from one state to another it is called a change of state or phase transition.

• Melting is the change from solid to liquid.• Freezing is the change from liquid to solid.• Vaporization (or boiling) is the change from liquid to gas.• Condensation is the change from gas to liquid.• Sublimation is the change from solid directly to gas,

skipping the liquid phase..• Deposition is the change from gas directly to solid,

skipping the liquid phase..

Exothermic vs. Endothermic Processes and Phase Changes

• An endothermic process or phase change requires an input of energy.– For example, melting, boiling, chemical cold

packs.

• An exothermic process or phase change releases energy.– For example, freezing, condensation, TNT

exploding.

• Heat is the type of energy used in either sort of process.

Enthalpy

• Enthalpy, given the symbol H, is an extensive (depends on amount) property of a substance that can be used to obtain the heat absorbed of released as part of a chemical reaction.

• Enthalpy is a state function and as such depends only on its present state (such as temperature or pressure) rather than the path used to get there.

• The heat and enthalpy change of a process are usually equal.– Thus, in general, the terms heat and enthalpy are

used interchangeably.

Heat Capacity and Specific Heat

• The heat capacity of a substance is the quantity of heat needed to raise the temperature of a substance by one degree Celsius or Kelvin.– The molar heat capacity is the quantity of heat

needed to raise the temperature of mole of a substance by one degree Celsius or Kelvin.

– Specific heat (capacity) is the quantity of heat needed to raise the temperature of one gram of a substance by one degree Celsius of Kelvin at constant pressure.

– Specific heat, C, is given by:

q = mCΔTWhere C has units of J/gK or J/goC.

Heat Capacity and Specific Heat Problems

• Which of the following substances will have the highest temperature after adding 50.0J of heat? Assume equal masses.– Al (.901J/goC), Cu (.384J/goC), Fe (.449J/goC)

Heat Capacity and Specific Heat Problems (Cont’d)

• 100.0g of water (C=4.184J/goC) at 50oC is heated with 300J. What will its final temperature be?

Heat Capacity and Specific Heat Problems (Cont’d)

• 50.0g of water (C=4.184J/goC) at 50oC is heated to a final temperature of 55oC. How much heat was added?

Heat Capacity and Specific Heat Problems (Cont’d)

• A 55.0g sample has a heat capacity of 15.0 J/oC. What is its specific heat?

Heat Transfer

• Not in text.• Another common problem relies on the

transfer of heat from one object to another.• Heat always flows from the colder object

to the hotter object.• Energy is conserved, so the total energy

does not change.

qhot + qcold = 0Or:

- qhot = qcold

Heat Transfer Problems

• A 45.0g piece of aluminum (C=0.908) is heated to 90oC and then placed in 100.0g of water (C=4.184J/goC) initially at 25oC. What is the final temperature of the water?

Heat Transfer Problems (Cont’d)

• A 150.0g piece of metal is heated to 90oC and then placed in 50.0g of water (C=4.184J/goC) initially at 25oC. The water raises to 35oC. What is the specific heat of the metal?