Chapter 21 Physical and Chemical Properties A. Physical properties 1. Describe look, feel of material 2. Physical change: looks different, but still same element 3. Water: ice melts, but still water

WATER BOX, “Liquid water”

Changes in State are physical Changes Gas--steam Liquid--water Solid--ice

Factors Affecting Changes in State Temperature Pressure Foreign materials (i.e. Salt depresses the freezing temperature of water)

Changes in State Why do cooking recipes have different directions for high altitude? There is less air pressure at high altitude. Less air pressure means a lower boiling point, as much as 10 degrees F

Factors Affecting Changes in State In other words, the higher the pressure, the higher the temperature will need to be for a phase change to happen. Example: the melting temperature of rocks (in general) is higher in the deep earth than at the surface.

Physical vs. Chemical Changes D. How to tell between physical/chemical change 1. Physical: restoring original conditions restores original appearance 2. Chemical: forms new materials with own characteristics

Chapter 21 Chemical Equations & Balancing A. Chemical properties 1. How a material transforms or reacts with other materials 2. Change in way atoms are bonded together 3. Chemical change: rearrangement of way atoms are bonded

B. Elements made of atoms C. Formula: 1. Element name followed by number of atoms bonded into molecules 2. 02, N2

D. Formula: symbols for elements written together 1.) H20, NaCl E. Compounds have different physical and chemical properties from original elements

F. Chemical reaction 1. Materials undergoing chemical change react with each other 2. New materials formed

2Na + Cl2 2NaCl

G. Chemical equation 1. Reactants  products 2. Coefficient = # of molecules 3. C + 02 CO2 No coefficient, assumed to be 1 like in algebra

Reactions represented by equations H. Law of conservation of mass 1. Matter not created or destroyed 2. Just rearranged 3. Never changes in total amount 4. Equations balance a) same number of atoms on each side b) 2 H2 + 02 --2 H20

Reactions represented by equations I. Balancing equations 1. Add or change coefficients (the numbers in front of the element symbols) 2. Do not change subscripts (the numbers down and to the right of element symbols) EVER!!!!!

Chapter 21 Endothermic vs. Exothermic & Reaction Rates Reactions can be categorized by whether they absorb energy or release it. Reactions that absorb energy from the environment in order to move forward are called endothermic reactions. Reactions that release energy into the environment as they move forward are called exothermic reactions.

Endothermic vs. Exothermic Reactions In a chemical reaction, energy is stored in the bonds of the elements and compounds in the reaction. Endothermic: 2NaCl + Energy  2Na + Cl2 Reactants less  Products more energy Energy Notice the energy is on the left as it is being put into the reactants to help the reaction go

Endothermic vs. Exothermic Reactions CH4 + 2O2 CO2 + 2H2O + Energy Reactants more  Products less energy energy Notice the energy is on the right as it is released after the products are formed.

Endothermic vs. Exothermic Reactions

Activation energy - the minimum quantity of energy that the reacting species must possess in order to undergo a specified reaction. Why would an energy-releasing reaction need energy to proceed? What actually happens to reactant molecules during a chemical reaction? Because the transition state is unstable, molecules don’t stay there long, but quickly form products.

In order for the reaction to take place, some or all of the chemical bonds in the reactants must be broken so that new bonds form. To get the bonds into a state that allows them to break, the molecule must be contorted (deformed, or bent) into an unstable state called the transition state. The transition state is a high-energy state, and some amount of energy – the activation energy – must be added in order for the molecule reach it.

Reaction Rates (Kinetics) Factors Affecting Reaction Rates: Concentration – Increase concentration and you increase the number of collisions thus speeding up the reaction rate. Surface Area – More surface area increases reaction rate. Example: sawdust burns faster than a whole log. Temperature – Increase the temperature and you increase the kinetic energy of the molecules leads to an increase in the number of collisions thus speeding up the reaction rate.

Reaction rates (kinetics) Factors Affecting Reaction Rates (cont.): Catalysts – speeds up the reaction by assisting. It does not enter into the reaction and it can be recovered at the end of the reaction. Example: Catalytic converters in cars convert harmful gases into harmless ones quick. Enzymes (proteins) are the human body’s catalysts assisting all kinds of reactions.

Reaction rates (Kinetics) Factors Affecting Reaction Rates (cont.): Pressure – Higher pressure increases reaction rate because a gas has been squeezed into a smaller volume thus increasing the number of collisions. Molecule Size – Bulky, big molecules move slower and collide less than smaller more nimble molecules.

Reaction rates (kinetics) Surface area Temperature