Chem notes 5

Physical and Chemical Properties

• Physical Property: A property that a substance displays without changing its composition.

  • The substance remains the same while the property is observed.
  • Examples: color, smell, taste, solubility.
  • How well something dissolves is a physical property.
    • A marker has zero solubility.
    • Sugar is very soluble.
  • Phase changes (boiling, condensing, melting, freezing) are physical properties.
  • Density: mass per volume of a substance.

• Examples of Physical Properties

  • Salt dissolves in water: it's still salt after dissolving.
  • A substance boils or condenses.
  • A substance's melting and freezing point.
  • Density: mass per volume of a substance

Chemical Properties

• Chemical Property: A property of a substance that is displayed only when the substance is changing its composition.

• Examples:

  • Flammability: Knowing gasoline is flammable without it burning.
  • Corrosion/Rusting: Plastic is not corrosive, but metal can be.
  • Toxicity: How something affects your system if ingested.
  • Reactivity with other chemicals.
  • Stability of a chemical (inverse of reactivity).
    • Metals on the left side of the periodic table are very reactive.
    • Argon is very stable and can be used to preserve reactive metals.
  • Acidity or basicity
    • Vinegar on your arm is not a big deal unlike concentrated hydrochloric acid.
  • Luster, color, streak, hardness, cleavage, and specific gravity (density) of a mineral sample are examples of physical properties.

• Volatility: How easily a liquid vaporizes.

• Volatile substances

  • Vaporize easily.
  • Often flammable.
  • Have low melting and boiling points.
  • Have a lot of vapor right above the liquid and tend to be flammable.
  • Naphthalene (mothballs) is an example.
  • Paint thinner is also volatile.

Physical and Chemical Changes

• Physical Change: Matter changes its appearance but not its chemical composition.

  • Change of state (solid to liquid to gas).
    • H2O as ice, liquid, or vapor is still H2O.
  • Dissolving.
    • Sugar in water: the sugar molecules separate but remain sugar.
  • Changing shape by bending or breaking.
    • Breaking a plate or tearing paper doesn't change what it's made of.

• Chemical Change: Bonds are broken and reformed to make something completely different.

  • Represented by chemical equations with an arrow.

    • Reactants (left side of the arrow): substances before the reaction.
    • Products (right side of the arrow): substances made in the reaction.

  • Indications of chemical change:

    • Heat given off or absorbed.
    • Odor or color change.
    • A completely new substance is made, with different properties.
    • New set of physical properties.
    • Not every color change indicates a chemical change (e.g., mixing paints).

  • Examples of chemical change

    • A gas is produced because new gas is created.
    • A precipitate forms (a solid forms when two solutions are combined).
    • Burning or rusting.

Mixtures

• Mixtures: Two or more things in the same area that are not chemically bonded.

• Mixtures can be separated by physical means.

• Separation Techniques:

  • Decanting: Separating a solid from a liquid by pouring off the liquid (e.g., draining spaghetti).
  • Distillation: Separating two liquids with different boiling points.
    • The liquid with the lowest boiling point vaporizes first.
    • Can also separate salt water by boiling off the water.
  • Filtration: Using a filter to catch large particles while small particles pass through (e.g., coffee).
  • Using a magnet to separate magnetic substances (e.g., iron).

Oil Refinery Example of Distillation

• Crude oil is separated in a tall column based on boiling points.

• Crude oil is superheated to 600^\,\textdegree F, and as they travel up the column gets cooler.

• Heaviest fuels (20-70 carbons) liquefy at the bottom (bunker fuel, waxes, lubricants).

• Lighter fuels (diesel, jet fuel, kerosene) condense at higher levels.

• Gasoline is lighter still, and propane (3-carbon gas) is at the top.

Conservation of Mass

• Matter cannot be created or destroyed.
• In chemical reactions, the total mass on the left side of the equation must equal the total mass on the right side.

Methane + Oxygen results in Carbon Dioxide and Water

Matter and Energy

• Matter can be converted to energy ( E = mc^2 ).

• A tiny bit of mass can be converted into an enormous amount of energy because the speed of light is a really big number--10^8.

Energy

• Energy: The capability to do work.

• Work: Force times distance.

  • If there is no movement (distance = 0), no work is done, even if force is exerted.

• Kinetic Energy: The energy of motion.

  • KE = \frac{1}{2}mv^2, where v = velocity

• Potential Energy: Stored energy; position matters.

  • PE = mgh, where h = height

• Total Energy: Sum of kinetic and potential energy.

• When a hammer is raised (potential energy), as it moves (kinetic energy), potential energy is converted to kinetic energy as the velocity increases. The kinetic energy equals more toward the bottom because total energy stays the same.

• Law of Conservation of Energy: Energy cannot be created or destroyed, but can be converted.

Forms of Energy

• Thermal Energy: Associated with the random motions of atoms and molecules (heat).

  • Higher temperature means higher thermal energy.

• Types of Thermal Energy: You won't be tested on them, but it's good to understand.

  *   Radiation: Heat that we feel from the sun does not require any kind of medium to travel through.
  *   Conduction: A pan sitting on the stove and hot, the molecules that get hot cause neighboring molecules to increase temperature too.
  *   Convection: Fluid movement, and so here we have hot water at the bottom, and so the hot is where the movement is. It'll come up and force the cold to come down, and so it's like a circular turning, and it's what's happening under the Earth's crust, um, with magma.
  

• Electrical Energy: Associated with the flow of electrons.

  • Metals have electrons to spare, which is why metal wires are used for electricity.
  • Electrons jump from one atom to the next.

• Chemical Energy: Associated with the position of particles in a chemical system.

  • High-energy bonds have more potential.
  • ATP (adenosine triphosphate) is an energy molecule with high-energy bonds.
    • ATP loses a phosphate and turns into ADP (adenosine diphosphate), releasing energy.
    • ATP helps muscles relax, and after death, the lack of ATP causes stiffness.

• Exothermic Reactions: Energy is released (EXO = exit). Bonds go from high to low potential energy and become more stable.

  • Exothermic graph

    • Y-axis: Energy
    • X-axis: Time
    • Reactants start at a certain energy level.
    • Activation energy: Spark needed to start the reaction (the hill on the graph).
    • Products have lower energy than reactants; energy is lost to the environment (heat or light).
    • Enzymes lower the activation energy.

• Endothermic Reactions: Energy is absorbed.

  • Endothermic graph
    • Y-axis: Energy
    • X-axis: Time
    • Reactants start at a certain level.
    • Activation energy: Spark needed to start the reaction (the hill).
    • Products have more energy than reactants because energy was absorbed.
    • Examples: photosynthesis, cold packs, ice melting.