Do Now 1.Describe 3 different ways to transfer heat between substances. 2.A boy holds an ice cube in his hand and it melts. Describe the method and direction of heat transfer in this scenario. 3.Where do chemical compounds store their energy?

By the end of class today… We’ll identify and describe exothermic and endothermic reactions.

Today’s Agenda Do Now(7 min) System + Surroundings (15 min) Exothermic & Endothermic (20 min) Enthalpy (20 min) ACT Practice(15 min) Exit ticket (8 min)

System – reactants & products (Chemists take the system’s point of view) Surroundings – container & everything else around it Chemical Energy

Quick Check 1. Burning Logs The system is … The LOGS

Quick Check 2. Water evaporating The system is … The water

Quick Check 3. KBr is dissolved in water and the water gets warmer. The system is … KBr

Quick Check 4. Natural gas is burned. The system is … The gas

Today’s Agenda Do Now(7 min) System + Surroundings (15 min) Exothermic & Endothermic (20 min) Enthalpy (20 min) ACT Practice(15 min) Exit ticket (8 min)

Observations of Chemical Reactions We will be combining 2 chemicals together and recording the temperature before and after the reaction. Reactants Initial Temperature Temperature after Reaction Observations Calcium chloride + Water Baking Soda (Sodium Bicarbonate) + Vinegar (Acetic Acid)

Energy BOND FORMED

Releasing and Absorbing Energy Energy is released when bonds form

Energy

BOND BROKEN 

Releasing and Absorbing Energy Energy is absorbed to break bonds Are all bonds created equal? Explain!

Measuring Energy 2 ways to measure… Enthalpy Reaction Progress 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) kJ

Exothermic Exothermic – more thermal energy is released by the system – More energy was released when bonds formed than when bonds were broken in the reactants Check For Understanding: How are we able to see this conclusion in the picture?

Endothermic Endothermic – more thermal energy is absorbed by the system – More energy was absorbed to break the bonds than to form the bonds of the products IN EVERY CHEMICAL REACTION, BONDS ARE BOTH BROKEN AND FORMED.

Problem 1 (With your partner) Endothermic or Exothermic? 2 min Check For Understanding: How can you tell? Ex) 2H 2 + O 2  2H 2 O Reactants Products Energy absorbed Energy released

Enthalpy Enthalpy – amount of energy released or absorbed as heat ∆H rxn = H products – H reactants – Exothermic ⇒ ∆H = negative values – Endothermic ⇒ ∆H = positive values H reactants > H products Exothermic Enthalpy Reaction Progress H reactants < H products Endothermic Enthalpy Reaction Progress

Thermochemical Equation When energy is a product, the process is exothermic (releasing heat) When energy is a reactant, the process is endothermic (absorbing heat) 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) kJ

Thermochemical Equation Chemical equation including enthalpy Check For Understanding: Is the above example exothermic or endothermic? Is heat being absorbed or released? Ex) 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) kJ or 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) ∆H= –1625 kJ

Quick Check Talk to the person sitting next to you. Do you think this represents an exothermic or endothermic reaction? How do you know? H 2 +O 2 (g)  H 2 O(l) kJ Reactants Products Energy absorbed Energy released

Problem 2 (Try it yourself first) What type of chemical reaction absorbs energy and requires energy for the reaction to occur? 2 min

Problem 3 What type of reaction releases energy and does not require initial energy to occur? 2 min

Problem 4 Which of the following are examples of an exothermic chemical reaction? a.Photosynthesis b.Burning a piece of wood c.Cutting paper d.None of the above 2 min

Problem 5 Any type of reaction that involves burning (combustion) can be classified as which of the following types of reactions? a.Synthesis b.Endothermic c.Exothermic d.All of the above 2 min

Problem 6 What is enthalpy? a.Heat content b.Absolute amount of energy in a chemical system c.The reactants of the chemical reaction d.None of the above 2 min

Problem 7 Categorize the following reaction based on the graph 2 min

Problem 8 Categorize the following reaction based on the graph 2 min

Problem 9 Label the enthalpy on the following graph (Exo or Endo?) 2 min

Today’s Agenda Do Now(7 min) System + Surroundings (15 min) Exothermic & Endothermic (20 min) Enthalpy (20 min) ACT Practice(15 min) Exit ticket (8 min)

Enthalpy (ΔH) How much net energy (heat) is released or absorbed by a given reaction Puts a number with exothermic or endothermic

Enthalpy (ΔH) ∆H rxn = H products – H reactants Exothermic ⇒ ∆H = negative values Endothermic ⇒ ∆H = positive values

Thermochemical Equation A balanced chemical equation that includes the physical states of matter of all reactants and products and the energy change * Is this reaction exothermic or endothermic? 2H 2 O 2(l)  2H 2 O (l) + O 2(g) ΔH= -196 kJ

Reactants Products Energy absorbed Energy released

Class Practice Problem Note – the standard enthalpy of formation of an uncombined element is 0. Use the standard enthalpies of formation to determine the value of ΔH for: NaOH(aq) + HCl(aq)  NaCl(s) + H 2 O(l) Check For Understanding: Is the above example exothermic or endothermic? Is heat being absorbed or released? How do you know?

You Try Use the standard enthalpies of formation to determine the value of ΔH for: PCl 5 (g) → PCl 3 (g) + Cl 2 (g) Create a graph that could represent the reaction above. Label the reactants, products, and change in enthalpy. Check For Understanding: Is the above example exothermic or endothermic? Is heat being absorbed or released? How do you know?

Class Practice Problem Note – Multiply the coefficients by the enthalpy of formation for each compound. Use the standard enthalpies of formation to determine the value of ΔH for: 8 Al(s) + 3 Fe 3 O 4 (s) --> 4 Al 2 O 3 (s) + 9 Fe(s) Check For Understanding: Is the above example exothermic or endothermic? Is heat being absorbed or released? How do you know?

You Try Use the standard enthalpies of formation to determine the value of ΔH for: 2Na(s) + 2H 2 O(l)  2NaOH(aq) + H 2 (g) Create a graph that could represent the reaction above. Label the reactants, products, and change in enthalpy. Check For Understanding: Is the above example exothermic or endothermic? Is heat being absorbed or released? How do you know?

Today’s Agenda Do Now(7 min) System + Surroundings (15 min) Exothermic & Endothermic (20 min) Enthalpy (20 min) ACT Practice(15 min) Exit ticket (8 min)

Independent Practice Silently work on the ACT practice on your own for the first 10 minutes Work with a partner for 5 minutes – be prepared to defend your answers. 15 min

Today’s Agenda Do Now(7 min) System + Surroundings (15 min) Exothermic & Endothermic (20 min) Enthalpy (20 min) ACT Practice(15 min) Exit ticket (8 min)