Unit 5: Phases of Matter Flashcards
What is thermal energy?
The type of energy that gives matter the ability to move
Temperature of a substance tells you how much thermal energy it has - cold less, hot more
When the amount of thermal energy that matter has changes, its state changes as well (e.g. adding energy moves toward gas)
What are exothermic changes?
Phase changes that release energy - freezing, condensation and deposition.
The gas phase has the most energy and solid the least (liquid is between but closer to solid) so as changes occur to move from gas to liquid or solid, energy is released.
What are endothermic changes?
Phase changes that require energy - melting, vaporization and sublimation.
The solid state has the least energy and gas the most (liquid is between but closer to solid) so as changes occur to move from solid to liquid or gas, energy is needed (e.g. heat needs to be added to liquid for it to become a gas).
What are the physical phase changes?
Ex of sublimation is CO2 (dry ice) - goes directly from solid to gas
Ex of deposition is snow - goes directly from gas to solid (without becoming liquid in between)
Note: If a substance gains energy, the molecules are able to pull themselves apart (e.g. gas) and if it loses energy, the molecules aren’t able to stay apart and get stuck together (e.g. solid).
What does the heating curve of a substance illustrate?
The heating curve of a substance illustrates the relationship between the heat added to a system, its temperature, and its phase.
During the sloped parts of the graph, heating a substance changes it temperature. During the flat parts of the graph, heating a substance changes its phase
Note that, at the melting and boiling points, the temperature does not increase until the phase change is complete
Ex: Ice water remains at 0oC until all of the ice has melted
What is an intermolecular force?
What are the different types?
Attraction between molecules which arise from the interaction of positive and negative charges
3 types (strength assuming similar sized molecules):
- London dispersion forces (weak)
- Dipole-diple forces (medium)
- Hydogen bonding (strong but still 10 times weaker than a covalent bond)
Note: Much weaker than chemical bonds but influence the bulk behavior of the substance; strength of intermolecular forces determines temp at which a substances undergoes a phase change
What are London dispersion forces?
Describes the interactions between a molecule with a temporary dipole moment and the induced dipole moment of it neighbors
Occurs between all atoms and molecules, and it is the only intermolecular force at work in nonpolar substances
Tend to be stronger, the larger the atom or molecule (the larger the electron cloud, the easier it is to distort it, resulting in stronger London dispersion forces)
Relatively weak forces
What are dipole-dipole forces?
Describe the interactions between polar molecules where the partially positive end of one molecule attracts the partially negative end of the other
Stronger than dispersion forces - attractions between polar molecules are stronger than those between nonpolar molecules of similar size
More polar molecules experience larger dipole-dipole forces
Ex: SH2
What is hydrogen bonding?
Special type of dipole-dipole force
An attractive intermolecular force that occurs when a hydrogen atom bonded to O, N, or F (must be a direct covalent bond) is electrostatically attracted to an O, N, or F atom in another molecule (e.g. water molecules being attractracted to each other)
Hydrogen bonds are stronger than normal dipole-dipole forces
What is a temporary dipole moment?
When the electrons of an atom or molecule happen to be more on one side at an instant in time, that side will be more negative than normal, and the other side will be more positive. This situation leads to the formation of a temporary dipole, or instantaneous dipole, which—like any dipole—contains partial charges.
Ex: Both of the electrons on He move to the same side of the atom
What is an induced dipole moment?
Reaction to a temporary dipole moment
When the positive end of the temporary dipole exerts an attractive force on nearby electrons, causing an adjacent atom to develop into another temporary dipole, this is called an induced dipole. This effect passes on to more atoms, resulting in a kind of electron “choreography” in which the movements of the electrons in nearby atoms correlate.
What intermolecular force accounts for the secondary structure of biological molecules?
The term secondary structure refers to the interaction of the hydrogen bond donor and acceptor residues of the repeating peptide unit.
What is a boiling point?
Which molecules have higher boiling points?
The amount of energy needed to overcome the intermolecular forces
The temperature at which boiling occurs—where the vapor pressure equals the external pressure of the atmosphere
Boiling points generally increase with the size of the molecule but some relatively small molecules have high boiling points due to hydrogen bonding (Ex: H2O has a strong hydrogen bond so requires a lot more energy than H2S; thus has a higher boiling point)
Study note: understand pattern - be able to determine which from graph has higher boiling point and why
What are the condensed phases?
Liquids and solids
Intermolecular forces keep the molecules close together
What is the viscosity of a liquid?
Its resistance to flow
Viscosity increases as temp decreases
Stronger intermolecular forces create greater viscosity
Gases have almost no viscosity, liquids have intermediate viscosity, and solids have very high viscosity
Ex: shampoo has relatively high viscosity - pours very slowly
What is surface tension?
The energy required to increase the surface area of a liquid by a unit amount
Surface tension increases as intermolecular forces becomes stronger
Ex: This is why a belly flop hurts and why high divers try to keep themselves as small as possible when entering the water
What is a surfactant?
A substance added to a liquid to disrupt intermolecular forces and lower surface tension
Ex: laundry detergent - reduces the surface tension so the water can wet the clothing better, allowing the detergent to surround the dirt particles
What is capillary action?
The ability of a liquid to climb up narrow spaces
Ex: Water creates a miniscus when you put it in a tube
The meniscus (curved surface of a liquid in a container) may be either concave or convex, depending on whether the intermolecular forces among liquid molecules are greater than or less than the forces of attraction between the liquid and the container walls. Concave for water because the water-glass interactions are stronger than the water-water interactions. In contrast, the intermolecular forces in mercury are stronger than mercury-glass interactions, so mercury tends to withdraw into itself and repel the glass, forming a convex surface.
What are cohesive forces of a liquid?
The forces pulling the liquid molecules together
A cohesive force that is stronger than the adhesive force with the container will cause the liquid form a convex surface or meniscus (e.g. mercury in a glass tube)
What are the adhesive forces of a liquid?
The forces pulling the liquid toward the container
An adhesive force with the container that is stronger than cohesive force of the liquid will cause the liquid form a concave surface or meniscus (e.g. water in a glass tube)
