SOL Review 7 Matter and Thermochemistry

Matter Anything that has mass and takes up space.

Elements Made up of one type of atom. Cannot be chemically broken down. Have a symbol on the periodic table. Compounds Two or more elements chemically combined. The elements do not retain their own properties when they form compounds. Examples: NaCl, H 2 O, CH 4

Pure Substance Consists of one chemically distinct material – could be an element or a compound Every sample of a particular substance has the same set of properties. It cannot be separated by physical means. Example: A bag of sugar is a pure substance. Every sample you take out of the bag consists of only sucrose (C 12 H 22 O 11 )

Mixtures Two or more substances physically combined. Substances retain their own properties. Composition can vary from one mixture to another. Example: sweet tea

Homogeneous Mixture Has a uniform composition throughout the mixture Also called a “solution” A solution can be: – Liquid, like sweet tea – Gas, like air (O 2, CO 2, N 2 and other gases) – Solid, like stainless steel (iron, chromium, and nickel)

Solutions (Homogeneous mixtures) Solvent – Substance doing the dissolving. Usually present in larger quantities. Solute – particles dissolved in a solution A solvent dissolves the solute. The solute becomes dispersed in the solvent. Solvents and solutes can be solid, liquid, or gas

Heterogeneous Mixture Not uniform. Areas of distinctly different composition. Example: sand on the beach

Properties are used to describe and identify a substance. Two types: 1. Physical properties: Properties describing a substance that can be observed without changing the substance’s composition – melting point, freezing point, malleability 2. Chemical properties: Properties that describe how a substance undergoes a change – rusting, burning, decomposing

Physical properties Extensive: Depend on the amount of substance present. Examples: mass, volume Intensive: Characteristic of the substance itself. Do not depend on the amount. Examples: hardness, color, density

Physical vs Chemical Changes in Matter In a physical change, the original substance still exists. It has just changed form. Some properties change, but the composition of the substance is the same. Boiling, freezing, melting, and condensing are used to describe physical changes. These are reversible physical change. Grinding, breaking, crushing, and cutting also describe physical changes, but these are not reversible.

Physical vs Chemical Changes in Matter During a chemical change, the composition of matter changes. Bonds are broken and new bonds are formed. Chemical changes occur when a substance undergoes a chemical reaction. Example: rusting Fe + O 2 → Fe 2 O 3 What are the signs that a chemical change (reaction) has taken place?

Separation of Mixtures Differences in physical properties can be used to separate mixtures. Some mixtures can be separated based on chemical properties as well. 3 methods of separating mixtures are: 1.Filtration 2.Distillation 3.chromatography

Filtration Method for separating a solid from a liquid. Uses holes in filter paper to separate parts of a mixture based on size of particles (think of using a strainer on beach sand, the larger sand crystals stay in the strainer)

Distillation: Method for separating or purifying liquids based on boiling point. Uses the different boiling points of substances in a mixture to separate them We distill alcohol because the alcohol has a lower boiling point than water – it boils off first and we can re- condense it in a purer form.

Chromatography Method for separating components of a mixture The components are separated based on their attraction between two phases: a stationary phase bed and a mobile phase which percolates through the stationary bed. The phases may be liquid, solid, or gas (there are many types of chromatography!)

Kinetic Molecular Theory The theory that the behavior of substances can be explained by the motion of the molecules that make up the substance.

Assumptions KMT makes about gas molecules. 1. The molecules of a gas are so far apart that the volume of the molecules is negligible. 2.The molecules of a gas move in rapid straight line motion. 3.The molecules of a gas have no attraction or repulsion for each other. 4.The collisions between the molecules are perfectly elastic.

Heat in Changes of State When a change of state occurs (either by gain or loss of heat) the temperature of the substance undergoing the change remains constant. Ice absorbs heat as it melts, but its temperature remains constant until all the ice is melted

Heating and Cooling Curve (shows changes of state) During the flat parts of the curve, this substance is undergoing phase changes: first from solid to liquid, then from liquid to gas. During this time, the energy absorbed is being used to overcome the attractions between molecules, not to increase the kinetic energy. Therefore there is no change in temperature.

When a substance is changing temperature, the equation to use for heat flow is: q =mC  T Where: q is heat/energy (J) m is mass (g)  T is the change in temperature ( o C) C is the specific heat of the substance (J/g o C) Please note that C is different for different phases of the substance.

When a substance is undergoing a phase change, the equation to use for heat flow is: q =m  vap/fus/solid/cond Where: q is heat (J) m is mass (g) or moles*  vap is the Heat of vaporization, fusion, solidification or condensation.  gives the heat (or energy) per mole required to undergo a change of state

Phase Diagram Shows what state a substance will exist in at a given temperature and pressure The normal boiling point is the temperature at which the substance will boil at standard pressure (1 atm, kPa, or 760 mm Hg)

Enthalpy (H) Enthalpy is the heat content of the system – the change in enthalpy (ΔH ) is the heat absorbed or released during a chemical reaction – we use the terms “heat” and “enthalpy change” interchangeably q (heat) = ΔH (enthalpy) Remember, we measure heat (energy) in joules or kilojoules.

Endothermic Reaction When a reaction takes in energy as it occurs, the products will have more energy than the reactants.

Exothermic Reaction When a reaction releases energy to the environment as it occurs, the products will have less energy than the reactants

A catalyst lowers the activation energy needed to get a reaction going, but does not change the energy of the reactants or products – therefore it does not change the ΔH.