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Chapter 11 Thermochemistry - Heat and Chemical Change

Chapter 11 Thermochemistry - Heat and Chemical Change. 11.1 The flow of energy-heat. Things you will learn. You will be able to explain the difference between energy and heat You will know the difference between heat capacity and specific heat

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Chapter 11 Thermochemistry - Heat and Chemical Change

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  1. Chapter 11Thermochemistry-Heat and Chemical Change 11.1 The flow of energy-heat

  2. Things you will learn • You will be able to explain the difference between energy and heat • You will know the difference between heat capacity and specific heat • You will know how to convert joules to calories

  3. Thermochemistry • In all our writing formulas correctly and balancing equations, we never paid any attention to one of the fundamental characteristics of chemical reactions: • All chemical reactions and changes of state absorb or release heat • Thermochemistry is the study of heat changes during chemical reactions

  4. Relationship between heat and energy • Energy is the capacity to do work or supply heat • Energy can only be observed by its effects • Energy stored in the structural units of chemicals (bonds) is chemical potential energy • Gasoline and dynamite have lots of chemical potential energy

  5. Relationship between heat and energy • Heat (q) is energy that transfers from one object to the next because of a temperature difference; this can be measured in calories (c) or Joules (J) • Heat is undetectable; only the effects of changes of heat are evidence of heat • Heat always flows from a warmer object to a cooler object until they are the same temperature (have the same average kinetic energy)

  6. Exothermic and endothermic processes • Exothermic reactions release energy to the surroundings; the products have less energy than the reactants • Endothermic reactions absorb energy from the surroundings; the products have more energy than the reactants

  7. Exothermic or endothermic?

  8. Exothermic or endothermic?

  9. Exothermic or endothermic?

  10. Exothermic or endothermic?

  11. Some defiinitions • Heat flowing into a system from the surroundings is called endothermic; q has a positive value! The surroundings cool down. • Heat flowing out of a system (burning match example) is called exothermic; q has a negative value! The surroundings heat up. • Remember that it is the direction of heat flow with regards to the system which determines the sign of q!

  12. In chemical equations… • CaO + H2O Ca(OH)2 + 65.2 kJ • CaO + H2O Ca(OH)2 ∆H = -65.2 kJ • These are exothermic; the products have less energy than the reactants • C + 2S + 89.3 kJ CS2 • C + 2S CS2 ∆H = 89.3kJ • These are endothermic; the products have more energy than the reactants

  13. Measures of heat • What is a Calorie?

  14. Measures of heat • What is a Calorie? • What is a calorie

  15. Measures of heat • What is a Calorie? • What is a calorie • What is a joule?

  16. Measures of heat • What is a Calorie? • What is a calorie • What is a joule? • These are all measures of energy

  17. Measures of heat • What is a Calorie? • Used in dietary language; = 1000 calories • What is a calorie • The amount of energy/heat needed to raise 1 gram of water by 1°C • What is a Joule? • The SI units we use in chemistry to measure energy/heat; 4.184 Joules =1 calorie 4.184 Joules/g/°C is the specific heat of water

  18. Heat capacity and specific heat • Heat capacity - the amount of heat needed to raise the temperature of an object by 1°C • This depends on the mass and composition of the object • Specific heat – the amount of heat needed to raise the temperature of 1 g of a substance by 1°C • To calculate specific heat (C), divide the heat (q) of the substance by (the mass (m) times the change in temperature(∆T)) q m x ∆T C x m x ∆T q= C = Find specific heat Find amount of heat

  19. The temperature of a piece of copper with a mass of 95.4 g increases from 25°C to48°C when the metal absorbs 849 J of heat. What is the specific heat of copper? • Knowns: • mCu = 95.4 g • ∆T = 23°C • q = 849 J • Unknown: • Ccu= J/(g x °C) • Caclulate: • Ccu= 849 J/(95.4 g x 23°C) = .387 J/g x °C

  20. Calculate the amount of heat needed to increase the temperature of 250g of water from 20oC to 56oC. C x m x ∆T q= Find amount of heat

  21. Calculate the amount of heat needed to increase the temperature of 250g of water from 20oC to 56oC. C= M= ∆T= q=

  22. Calculate the specific heat capacity of copper given that 204.75 J of energy raises the temperature of 15g of copper from 25o to 60o. q m x ∆T C = Find specific heat

  23. Calculate the specific heat capacity of copper given that 204.75 J of energy raises the temperature of 15g of copper from 25o to 60o. q= M= ∆T= C=

  24. 216 J of energy is required to raise the temperature of aluminium from 15o to 35oC. Calculate the mass of aluminium. CAl is 0.90 JoC-1g-1). q C x ∆T m= Find amount of mass

  25. 216 J of energy is required to raise the temperature of aluminium from 15o to 35oC. Calculate the mass of aluminium. CAl= 0.90 JoC-1g-1 q= C= ∆T= M=

  26. The initial temperature of 150g of ethanol was 22oC. What will be the final temperature of the ethanol if 3240 J was needed to raise the temperature of the ethanol? Cethanol= 2.44 JoC-1g-1 q C x m ∆T = Find ∆ T

  27. The initial temperature of 150g of ethanol was 22oC. What will be the final temperature of the ethanol if 3240 J was needed to raise the temperature of the ethanol? Cethanol = 2.44 JoC-1g-1 q= C= M= TI= TF=

  28. Homework (HW 12) • Chapter 11.1 • Questions 4-10 • Page 299 • Due: April 7 (red) April 8 (gold)

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