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Ch. 15: Energy and Chemical Change

Ch. 15: Energy and Chemical Change. 15.2 Heat. Objectives. Describe how a calorimeter is used to measure energy absorbed or released. Explain the meaning of enthalpy and enthalpy change in chemical reactions and processes. Measuring Heat.

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Ch. 15: Energy and Chemical Change

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  1. Ch. 15: Energy and Chemical Change 15.2 Heat

  2. Objectives • Describe how a calorimeter is used to measure energy absorbed or released. • Explain the meaning of enthalpy and enthalpy change in chemical reactions and processes.

  3. Measuring Heat • Heat changes that occur during chemical and physical processes can be measured accurately and precisely using a calorimeter. • A calorimeter is an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.

  4. Calorimetry • A known mass of water is placed in the insulated chamber. • The amount of energy that this water absorbs or releases can be calculated from the change in temperature that occurs.

  5. Example qw = mCwT q = (50.0 g)(4.184 J/g 0C)(6.8 0C) = 1,400 J

  6. Practice Problems • A sample of metal is heated and put into a calorimeter containing 125 g of water at 25.6 0C. The final temperature of the water is 29.3 0C. How much heat in J is absorbed by the water? • If 335 g of water at 65.5 0C lost 9750 J of heat, what was the final temperature of the water? • The temperature of a sample of water increases from 20 0C to 46.6 0C as it absorbs 5650 J of heat. What is the mass of the sample?

  7. Determining Specific Heat Remember: we calculated the water absorbs 1,400 J. How much energy does the lead release?

  8. Determining Specific Heat • The lead releases 1, 420!! qPb = -1,420 J. • If we know the mass of lead (150 g) and the change in temperature of the lead (28.8 0C - 100 0C = -71.2 C0), we can calculate the specific heat of lead. qPb = mCPbT CPb = q/mT C = -1,400 J/(150 g)(-71.2 0C) C = 0.13 J/g 0C

  9. Specific Heat Practice Problems • To solve: • Use Q = mwCwΔTw to determine the amount of heat the water absorbs. • This is the same amount of heat the metal releases. (Just change the sign!) • Use the heat, mass of metal (mm), and temperature change of metal (ΔTm) to find the specific heat of the metal (Cm). • Determine the identity of the metal through its specific heat.

  10. Chemical Energy and the Universe • Every chemical reaction and change of physical state either releases heat (is exothermic) or absorbs heat (is endothermic).

  11. Chemical Energy and the Universe • Thermochemistryis the study of heat changes that accompany chemical reactions and phase changes. • The system is the specific part of the universe that contains the reaction or process you wish to study. • Everything else in the universe is considered the surroundings. • The universe = system + surroundings

  12. Chemical Energy and the Universe • Consider heat flow in exothermic and endothermic reactions or processes • In summary, (EXOTHERMIC) (ENDOTHERMIC)

  13. Enthalpy and Enthalpy Changes • The total amount of energy a substance contains is impossible to measure. • But, for many reactions, the amount of energy lost or gained (CHANGE in energy) can be measured conveniently in a calorimeter. • Reactions in a calorimeter or any lab take place at a constant atmospheric pressure.

  14. Enthalpy and Enthalpy Changes • Chemists use the term enthalpy (H) to represent the heat content of a system at constant pressure. • Therefore, the change in enthalpy (H) is the heat absorbed or released in a chemical reaction at constant atmospheric pressure. • Hrxnis the difference between the enthalpy of the substances present at the end of a reaction and the enthalpy of the substances present at the beginning.

  15. Enthalpy and Enthalpy Changes • In other words, the difference between the heat contained in the products and the heat contained in the reactants is the enthalpy (heat) of reaction. • Hrxn = Hproducts - Hreactants

  16. Note • Recall q was defined as the heat lost or gained in a chemical reaction or process. • If the reaction takes place at constant pressure, q = Hrxn. • (Therefore,Hrxn = mCT)

  17. Enthalpy and Enthalpy Changes 4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJ • According to the equation, the reaction is exothermic - energy is written as a product. • Therefore, Hreactants has to be greater than Hproduct and Hrxn has to have a negative sign. • The enthalpy change for this reaction can then be indicated by the notation: Hrxn = -1625 kJ

  18. Enthalpy and Enthalpy Changes

  19. Enthalpy and Enthalpy Changes NH4NO3 + 27 kJ NH4+ (aq) + NO3- (aq) • This reaction is endothermic since energy is a reactant. • Therefore, Hproducts has to be greater than Hreactant and Hrxn has to have a positive sign. • Hrxn = 27 kJ

  20. Endothermic Enthalpy Change

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