25 Calorimetry If two reactants at the same temperature are mixed and the

25 calorimetry if two reactants at the same

This preview shows page 25 - 33 out of 33 pages.

25
Image of page 25
Calorimetry If two reactants at the same temperature are mixed and the resulting solution gets warmer, this means the reaction taking place is exothermic. An endothermic reaction cools the solution. 26
Image of page 26
Calorimetry Energy released (heat) = s × m × Δ T s = specific heat capacity (J/ ° C·g) m = mass (g) Δ T = change in temperature ( ° C) 27
Image of page 27
28 Q.: A 100.0 g sample of water at 90.°C is added to a 500.0 g sample of water at 10.°C. Calculate the final temperature of the water: q lost + q gained = 0 q = s × m × Δ T 4.18(100)(T f - 90) + 4.18(500)(T f - 10)=0 T f = 23 ° C
Image of page 28
Hess’s Law: In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. This idea is called Hess’s law. 29
Image of page 29
N 2 ( g ) + 2O 2 ( g ) → 2NO 2 ( g ) Δ H 1 = 68 kJ This reaction also can be carried out in two distinct steps, with enthalpy changes designated by Δ H 2 and Δ H 3 . N 2 ( g ) + O 2 ( g ) → 2NO( g ) Δ H 2 = 180 kJ 2NO( g ) + O 2 ( g ) → 2NO 2 ( g ) Δ H 3 = 112 kJ N 2 ( g ) + 2O 2 ( g ) → 2NO 2 ( g ) Δ H 2 + Δ H 3 = 68 kJ Δ H 1 = Δ H 2 + Δ H 3 = 68 kJ 30
Image of page 30
The Principle of Hess’s Law 31 The same change in enthalpy occurs when nitrogen and oxygen react to form nitrogen dioxide, regardless of whether the reaction occurs in one (red) or two (blue) steps
Image of page 31
Characteristics of Enthalpy Changes If a reaction is reversed, the sign of Δ H is also reversed. The magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of Δ H is multiplied by the same integer. 32
Image of page 32
Standard Enthalpy of Formation ( Δ H f ° ) Change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states. 33
Image of page 33

You've reached the end of your free preview.

Want to read all 33 pages?

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes