$$$ Review $$$ Thermochemistry

Gives off heat (emits) exothermic

Absorbs heat endothermic

It flows from hot to cold objects and is known by the letter “q” heat

The study of energy changes that occur during chemical reactions thermochemistry

What is a calorie or joule? Unit of heat (q)

Defined as the amount of heat needed to increase the temperature of an object by 1 o C. Heat capacity

Defined as the amount of heat needed to increase 1 gram of an object by 1 o C. Specific heat or specific heat capacity

The formula for q? q = C x m x  T

Solve the previous equation for the other three variables. C = q/m  T m = q/C  T  T = q/Cm

A balance bar has 200 Calories. How many kilojoules is this? How many joules is this? kj 836,800 joules

True or False. cal/ o C is an acceptable unit for specific heat. False, cal/g o C

True or False. Metals generally have a higher specific heat capacity than water. False

True or False. Metals generally have a higher specific heat capacity than molecular compounds. False

You measure 1200 joules of heat during a 30 o C temperature change with a substance that weighs 100 g. What is the specific heat of the substance J/g o C

During a phase change, the temperature of a substance ________. Remains constant

A calorimeter can effectively measure the heat of another substance because of the ____________? Law of Conservation of Energy

 H of fusion involves which phase change? Melting

 H of solidification involves which more commonly known phase change? Freezing

If the percent mass of a solution weighing 300 g is 6%, what is the mass of the solute?.06 = x/300g x = 18 g

The heat content of a system at a constant pressure is known as the ________ of that system. enthalpy

What is the enthalpy change in a chemical reaction known as? Heat of reaction,  H

If the  H of a reaction is negative then the reaction is _______. exothermic

The heat of the reaction for the complete burning of one mole of a substance. Heat of combustion

The enthalpy change when a mole of solute is dissolved in a solvent. Heat of solution

True or False. The quantity of heat absorbed when a solid melts is the same as the quantity released when the substance freezes. True.  H fus = -  H solid.

What makes Hess’s law useful? It allows you to determine heats of reactions indirectly

The change in enthalpy that involves the formation of one mole of a compound from its elements (at 25 o C) is known as? Standard heat of formation, (  H f o )

True or False. The standard heat of a reaction can be calculated by the following equation …  H o =  H f o (products) -  H f o (reactants) True

How to solve a phase change problem You just multiply  H of fusion/vaporization by mass Water is vaporizing  H vap = 2260 J/g of H 2 O q = m x  H vap Water is melting  H fus = 334 J/g of H 2 O q = m x  H fus (J)

Heating/Cooling Curve: No Phase Changes What happens at A, C, and E?? (J) We use our old formula q = m x C x  T q = m x C liq x  T q = m x C solid x  T q = m x C gas x  T

Heating/Cooling Curve: Putting it all Together (J) You should be able to calculate the total heat going all the way from heating a substance from its solid to its gas q = m x C liq x  T q = m x C solid x  T q = m x C gas x  T q = m x  H fus q = m x  H vap

Sample Problem You have a 4.30 grams of ice at o C. You heat it until it completely vaporizes. How much heat was needed to complete this process? Here are some numbers you might need. (C ice = 2.10 J/g o C) (C water = 4.18 J/g o C) (C steam = 1.70 J/g o C) (  H fus = 334 J/g) (  H vap = 2260 J/g) q = m x C ice x  T (4.30)(2.10)( )= J q = m x  H fus (4.30)(334)= J q = m x C wat x  T (4.30)(4.18)( )= J q = m x  H vap = (4.30)(2260)= 9718 J To get answer you simply add these 4 numbers together: J or 13.1 kJ (3 sig. figs)