Thermochemical Equations

Energy as an Entity Energy is gained or released during both physical and chemical changes. Like matter, energy cannot be created or destroyed; it is always conserved. Lavoisier’s law of conservation of matter also applies to energy Einstein later showed that energy and matter are related by his famous equation E = mc 2

Endothermic Reactions In physical or chemical processes where energy is absorbed, we can treat energy as a reactant. NH 4 Cl(s) NH 4 + (aq) + Cl - (aq) This was an endothermic process, heat flowed from the water into the ions. We can also represent this NH 4 Cl(s) + energy NH 4 + (aq) + Cl - (aq)

Representing Changes of Energy We could also say, NH 4 Cl(s) J/g NH 4 + (aq) + Cl - (aq) or NH 4 Cl(s) kJ/mol NH 4 + (aq) + Cl - (aq) Often the energy term is shown after the equation: NH 4 Cl(s) NH 4 + (aq) + Cl - (aq) ∆H soln = kJ/mol

Table I: Heats of Reaction On Reference Table I, a number of physical and chemical processes are represented.

Heats of Solution on Table I The heats of solution for a number of ionic compounds are shown.

Exothermic Reactions In exothermic processes, heat is released from the “system” into the “surroundings”. In a thermochemical equation, the heat is recognized as a product. CaCl 2 (s) Ca + (aq) + 2Cl - (aq) J/g CaCl 2 (s) Ca + (aq) + 2Cl - (aq) ∆H soln = kJ/mol

Endothermic and Exothermic Endothermic reactions have a (+) energy term Exothermic reactions have a (-) energy term

A hot pack contains chemicals that can be activated to produce heat. A cold pack contains chemicals that feel cold when activated. Based on energy flow, state the type of chemical change that occurs in a hot pack.

Energy Flow A hot pack contains chemicals that can be activated to produce heat. A cold pack contains chemicals that feel cold when activated. A cold pack is placed on an injured leg. Indicate the direction of the flow of energy between the leg and the cold pack.

Given the equation for the dissolving of sodium chloride in water: When NaCl(s) is added to water in a 250- milliliter beaker, the temperature of the mixture is lower than the original temperature of the water. Describe this observation in terms of heat flow. Explain, in terms of particles, why NaCl(s) does not conduct electricity.

Given the reaction: What is the overall result when CH 4 (g) burns according to this reaction? Energy is absorbed and ΔH is negative. Energy is absorbed and ΔH is positive. Energy is released and ΔH is negative. Energy is released and ΔH is positive.

Base your answer to this question on the reaction represented by the balanced equation below. 2H 2 (g) + O 2 (g) → 2H 2 O(l) kJ Identify the information in this equation that indicates the reaction is exothermic.

Thermochemical Equations and Mole Ratios When energy is expressed in kJ/mol, mole ratios can be used to predict the amount of energy absorbed or released in physical or chemical processes.

The combustion of 0.5 moles of methane (CH 4 ) produces how many kilojoules of heat?

How many kilojoules of heat are released when 2 moles of carbon dioxide are formed from its elements? Using information from Table I

Which of the salts have endothermic heats of solution?

How much heat is absorbed by hydrogen and iodine to produce 1 mole of HI?

How many moles of glucose (C 6 H 12 O 6 ) are needed to produce 1402 kilojoules of heat when burned in air?

In a laboratory experiment, grams of an unknown solid is added to milliliters of water and the temperature of the resulting solution is measured over several minutes, as recorded in the table. Given the statement: The unknown solid is either sodium hydroxide or lithium bromide, and both of these compounds dissolve in water exothermically. a) Explain how the experimental data support the statement. b) State specific information from Reference Table I to support the statement.