Chemical Reactions A chemical reaction is a process of breaking the bonds of the reactants rearranging the elements in the reactants and forming new bonds to create the products Reactants  Products

Chemical reactions are expressed in symbols because words are too cumbersome Skeleton Equation: a chemical equation that doesn’t indicate the amounts of each element or compound in the reaction - In other words it is a chemical equation that isn’t balanced Fe + O 2  Fe 2 O 3

Is there a way to tell what physical state a compound or element is in during a reaction? - To display the physical state of the material in a reaction a symbol is displayed after that material in the chemical equation Fe (s) + O 2 (g)  Fe 2 O 3 (s) The symbols have the following meanings s = solidg = gasl = liquid aq = aqueous solution or a substance dissolved in water ↑ = gas given off↓ = precipitate formed ∆ = heat

Sometimes reactions happen at a very slow pace. So we will not have to wait forever to see the results of a reaction we sometimes add things to speed up the reaction. Catalyst: a substance that speeds up the rate of reaction but is not used up in the reaction -In a chemical reaction the catalyst is written above the arrow H 2 O 2 (aq) > H 2 O (l) + O 2 (g) MnO 2

Balanced Chemical Reaction: For example lets consider putting a bike together a chemical reaction. Frame + Wheel + Handle Bars + Pedal  Bike or F + W + H + P  FW 2 HP 2 22 Important: If you change the subscripts in a chemical equation you change that chemical. To balance an equation use coefficients not subscript -The subscripts or little numbers mean the amount bonded while the coefficients or big numbers mean sets of that substance

Rules for Balancing Equations 1. Before you start make sure you know how many moles of each elements is in the products and reactants 2. The numbers of moles for each element in the products and the reactants must be the same. (trick make sure that the number of moles of an element is even or odd on each side) Law of Conservation of Mass: the mass of the products and the reactants in any reaction have to be equal. Mass can not be lost or created in a reaction

Enothermic vs. Exothermic Reactions Endothermic Reaction – a reaction that requires heat - ∆ is in the reactants because the reaction requires energy to happen - This reaction will feel cold because it will suck in all the heat around it

Exothermic Reaction – a chemical reaction that gives off heat - ∆ is in the products because the reaction gives off heat or energy - This type of reaction will feel hot because it gives off so much heat or energy

Types of Reactions (pg 338 – 339) - Double replacement (AB + CD  CB + AD) - Single replacement (A + BC  AC + B) - Synthesis or Combination (A + B  AB) - Decomposition (AB  A + B) - Combustion (C x H y + O 2  CO 2 + H 2 O)