Presentation on theme: "Unit 11: Energy in Chemical Reactions. The Universe Is made up of the system and the surroundings Energy can be transferred between the system and the."— Presentation transcript:
Chemical Reaction A chemical reaction is a chemical change. What you start with is different than what you end up with. They have different properties Most reactions happen in aqueous or gas state because it is easy for the particles to touch.
In order to have a chemical reaction Involves breaking of existing bonds (requires energy) and formation of new bonds (releases energy)
In a physical change: No new substances are formed No bonds are broken or formed
Intensive and Extensive properties Physical properties such as: Intensive Color Odor Luster Malleability Ductility Conductivity Hardness Boiling Point/Melting Point Density Temperature Extensive Mass Weight Volume Length Watch this (3:51) Watch this
Stop and Think Watch this Watch this (1:35 BN) Name some intensive properties of matter that are different after a chemical change.
parts of a chemical equation Reactants Products state of matter A chemical equation represents a chemical reaction.
Write an equation to represent this chemical change Nitrogen Chlorine
Collision Theory “Atoms, ions, or molecules must collide in order to react” 1.Reactant particles must collide. 2.Reactant particles must have the correct orientation when they hit. 3.Reacting substances must collide with enough energy to break bonds.
Every collision does not result in a chemical reaction. Watch This (0:54) Watch This
A potential energy diagram plots the change in potential energy that occurs during a chemical reaction. Watch This (1:16) Watch This
Activation Energy- Energy required to start a chemical reaction. Both endothermic and exothermic reactions need activation energy. – Measured from the starting point of the reaction to the highest point on the curve
What do exothermic and endothermic reactions look like? Watch this (4:35) Watch this
Enthalpy (H) A measure of heat content of a system. The heat lost or gained in a reaction. An extensive property Measured in units of Joules/mole (J/mol) or kJ/mol H = change in heat H = ∆H f ° (products) – ∆H f ° (reactants) This equation can be found on the bottom of your STAAR chart. “ ° “ refers to STP conditions
How to determine H Method 1: By using the ∆H f ° values for each reactant and product Method 2: By analyzing a graph Method 3: By looking at a chemical equation that includes the energy term Method 4: By making observations of a chemical reaction Method 5: Calorimetry Watch This L stop at 4:46 Watch This
Method 1: Enthalpy calculations Use the table of ∆H f ° values and the equation H = ∆H f ° (products) – ∆H f ° (reactants) to determine the enthalpy of the reaction. - H indicates that a reaction is exothermic + H indicates that a reaction is endothermic
Using enthalpies of formation, calculate the standard change in enthalpy for the thermite reaction: Fe 2 O 3 (s)+2Al(s) Al 2 O 3 (s)+2Fe(s) ΔH f Fe 2 O 3 = (-826 kJ/mol) ΔH f Al 2 O 3 = (-1676 kJ/mol) Hints: i. all elements have a ΔH f = 0. ii. if the substance is multiplied by a coefficient, multiply ΔHº f by the same coefficient.
Practice problem Is the reaction described by the following equation an endothermic or exothermic reaction? What is the H value? CH 4(g) + O 2(g) CO 2(g) + 2H 2 O(l) Substance H f (kJ/mol) CH 4 -74.8 CO 2 (g)-393.5 H 2 O(l)-285.8
Method 2: Analyzing Graphs Energy of reactants is bigger than energy of products Excess energy leaves to the surroundings. Exothermic (0:56) Exothermic
Method 2: Analyzing Graphs The energy of the products is bigger than the energy of the reactants Energy is needed from the surroundings Endothermic (5:45) Endothermic
Method 3: chemical equation EXOTHERMIC 4Fe + 3O 2 2Fe 2 O 3 + 1625 kJ or 4Fe + 3O 2 2Fe 2 O 3 + energy Heat is written on the product side because it exits the system.
Method 3: chemical equation ENDOTHERMIC 27 kJ + NH 4 NO 3 NH 4 + + NO 3 - or energy + NH 4 NO 3 NH 4 + + NO 3 - Heat is written on the reactant side because it enters the system.
Method 4: observations of chemical reaction Feels hot -----EXOTHERMIC Feels cold -----ENDOTHERMIC
Method 5: Calorimetry (10:44) Calorimetry Calorimetry - the act of measuring the heat of chemical reactions or physical changes, or the science of making such measurements Calorimeter – the instrument used in calorimetry Q = mΔCT Q is the same as ΔH
Reaction Rate Rate at which reactant is converted to product depends on: – Concentration (more particles … more collisions… more reactions) – Temperature (particles moving faster.. more collisions.. more reactions) – Presence of a catalyst (lowers the activation energy, speeds up reaction rate) or inhibitor (raises the activation energy, slows down the reaction rate) – Nature of reactants (think back to “reactivity trends” for metals and nonmetals…K is more reactive than Li so K will react faster than Li) – Watch This (6:24) Watch This – Watch This (1:20) Watch This
Catalyst They increase the frequency of collisions or change the orientation of the molecules so more collisions are effective
Inhibitor Pathway with inhibitor Inhibitors raise the activation energy. Food preservatives are inhibitors.
Equilibrium When a reaction starts, the reactants are used up and products are made. Reactants Products After awhile, the products re-form to make reactants. Reactants Products Processes that proceed in both the forward and reverse direction are said to be reversible. Reactants Products Watch ThisWatch This (1:47)
Equilibrium All reactions are reversible if they are in a closed system. That means there is a top on the container. Dynamic equilibrium is the condition wherein the rates of the forward and reverse reactions are equal. Once the reaction reaches equilibrium, the concentrations of all the chemicals remain constant because the chemicals are being consumed and made at the same rate. Watch This (4:18) Watch This