Chemical Reactions PESS 1

Observing Chemical Change Review: Physical properties: a characteristic that can be observed without changing the substance Chemical properties: a characteristic of a substance that describes a substance’s ability to change Physical change: a change that alters the appearance of a substance, but does not change the substance Chemical change: a change that occurs when the substance does change into a new substance (AKA chemical reaction)

Observing Chemical Change Chemical changes occur when bonds break and new bonds form the result is a new substance is produced Evidence that a chemical reactions has occurred The substance might change form A solid may appear when two solutions are mixed together (this is called a precipitate) A gas might be produced from solids or liquids Be careful because these changes might also occur in a physical change

Observing Chemical Change Changes in energy might also occur in a chemical reaction Energy can either be absorbed or released Endothermic: energy is absorbed & feels cool to the touch Exothermic: energy is released & feels warm to the touch

Describing Chemical Reactions Chemical equations A short, easy way to show a chemical reaction Symbols and numbers are used in placed of words Formulas in equations CO2, H2O, C6H12O6, NaCl, H2O2, NaHCO3 These are the formulas for common substances

Describing Chemical Reactions Structure of an Equation Reactants are the substances before a chemical reaction takes place Products are the substances after a chemical reaction takes place Reactant + Reactant  Product + Product CaCO3  CaO + CO2

Describing Chemical Reactions Conservation of Mass Review: Law of Conservation of Mass (LCM) – matter cannot be created or destroyed So this means that the same number of atoms that enter a chemical reaction must be the same after the chemical reaction occurs Open systems: matter can escape into the surroundings (e.g. fire place) Closed systems: matter cannot escape into the surroundings (e.g. baking bread)

Describing Chemical Reactions Balancing equations Due to the LCM in a chemical equation the number of atoms that in the reactants must be the same in the products First step: write the equation Second step: identify which atoms and how many are used in the equation Third step: Use coefficients (NOT subscripts) to even out the reactants and the products

Practice

Answer

Describing Chemical Reactions Classifying Chemical Reactions Synthesis: 2 or more substances combine to form a more complex substance H2 + O2  2H2O Decomposition: a more complex substance breaks apart to 2 or more simpler substances 2H2O2  2H2O + O2 Replacement: an uncombined element replaces an combined element 2Cu2O + C  4Cu + CO2 Double replacement: elements in one compound trade places with elements in another compound FeS + 2HCl  FeCl2 + H2S

Examples

Controlling Chemical Reactions Activation energy Remember: motion needs a force to get it to move, the same goes for chemical reaction Activation energy is the “push” that gets chemical reactions going Examples: a spark is needed for combustion, heat is needed to bake, etc.

Controlling Chemical Reactions Rates of chemical reactions Chemical reactions can be controlled by changing such factors as: surface area, temperature, concentration levels and using substances called catalysts or inhibitors Surface area: the smaller the surface area the quicker the chemical reaction (that is why we chew our food)

Controlling Chemical Reactions Temperature: the greater the temperature the quicker the chemical reaction (that is why we put food in the refrigerator because it slows bacteria growth) Concentration: the greater the concentration the quicker the chemical reaction Catalysts: a substance that increase the reaction rate by lowering the activation energy needed to get the reaction started Inhibitors: this is used to decrease the reaction rate by preventing the reactants from combining (preservatives are examples of inhibitors)