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Heat & Enthalpy Changes

Heat & Enthalpy Changes. Objectives: 1. Define the terms enthalpy and enthalpy changes. 2. Explain what information is given by the standard enthalpy change of a reaction. 3. Explain how a calorimeter is used to determine the quantity of heat transferred in a chemical reaction. Key Terms:

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Heat & Enthalpy Changes

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  1. Heat & Enthalpy Changes Objectives: • 1. Define the terms enthalpy and enthalpy changes. • 2. Explain what information is given by the standard enthalpy change of a reaction. • 3. Explain how a calorimeter is used to determine the quantity of heat transferred in a chemical reaction. Key Terms: • Enthalpy, standard enthalpy change, calorimetry

  2. Calorimetry • Calorimetry is the measurement of the heat gained or lost in a chemical reaction. • A calorimeter is a device that isolates the outside environment. • Any loss or gain of energy in the products results in a change in temperature only in the calorimeter. • The rise (or drop) of temperature in the calorimeter is dependent on the amount of heat released or gained and the heat capacity of the surroundings • Bomb calorimeter • q = (masswater)(Cwater)(DT)

  3. Enthalpy • The difference in the energies related to products and reactants along is called enthalpy. • Enthalpy is represented by the capital letter H • DH = -qenvironment DH = Hproducts – Hreactants • if exothermic then enthalpy is negative (-DH) • if endothermic then enthalpy is positive (+DH) • Standard Enthalpy change (DHo) occurs at constant temperature (25oC) and pressure (1atm) • Represents a single mole • Is calculated using the most stable allotrope of an element

  4. Interpreting enthalpy equations • C(s) + H2O(l) + 113kJ g CO(g) + H2(g) DHo=+ 113kJ • In this reaction 113kJ of heat energy is absorbed in the endothermic reaction. • Since the products have more energy than the reactants the products are said to have undergone positive enthalpy (a positive gain in chemical potential energy). • endothermic reactions result in positive enthalpy (+H)

  5. Interpreting enthalpy equations • C3H8(g) + 5O2(g)g 3CO2(g) + H2O(g) + 2043kJ DHo =- 2043kJ • In this reaction 2043kJ of heat energy is released in the exothermic reaction. • Since the products have less energy than the reactants the products are said to have undergone negative enthalpy (a negative gain in chemical potential energy). • exothermic reactions result in negative enthalpy (-H)

  6. Solving Enthalpy Consider this question. • C3H8(g) + 5O2(g)g 3CO2(g) + H2O(g) + 2043kJ DHo=- 2043kJ • If 2.50 moles of oxygen are reacted with 1.00 molof C3H8, how much heat is produced in the reaction? • What is the enthalpy? • (2.5/5) x 2043kJ = qrxn • DH = -q • Since you start out with 1/2 the quantity of O2 required for the reaction, the heat produced is also halved. (1021.5kJ) • DH = -1020kJ Ratio of coefficients

  7. Using Calorimetry to solve enthalpy • Examine the reaction of sodium hydroxide water • NaOH(s)gNa+(aq) + OH-(aq) • 0.050 moles of sodium hydroxide reacts in a calorimeter containing 75.0g of water. The temperature of the water is raised from 19.8oC to 26.7oC. What is the enthalpy of the rxn? • q = (masswater)(Cwater)(DT) • qNaOH= (75g )x(4.184J)/goC)x(26.7oC-19.8oC) = +2170J • Denthalpy (DH)= -Denergy (-Dq) • Enthalpy (remember that the sign must change for enthalpy) • DH = -2.170kJ • Since the water in the calorimeter represents the environment, the reaction is exothermic • DHo= (1 molNaOH / 0.050 molNaOH) x -2.17kJ = -4.3kJ

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