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Energy changes
 

Energy changes

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CHEM

CHEM

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  • http://chemistry.about.com/od/chemistrydemonstrations/a/elephanttooth.htmhttp://chemistry.about.com/cs/howtos/ht/endothermic.htm

Energy changes Energy changes Presentation Transcript

  • Energy Changes
  • Learning Objectives… • Describe the term exothermic as a process or chemical reaction which transfers energy, often in the form of heat, to the surroundings and may be detected by an increase in temperature, e.g. the reaction between sodium hydroxide and hydrochloric acid • Describe the term endothermic as a process or chemical reaction which takes in energy, often in the form of heat, from the surroundings and may be detected by a decrease in temperature, e.g. the dissolving of ammonium nitrate in water
  • Introduction • Endo = In • Exo = Out • Thermic = Heat
  • What is an ENDOTHERMIC reaction? • A reaction that takes in heat (thermal energy) • Causes a drop in temperature in the surroundings.
  • What is an EXOTHERMIC reaction? • A reaction that gives out heat (thermal energy) • Cause a temperature rise in the surroundings.
  • Heat of reaction
  • ΔH Heat of reaction Unit: kilojoules (kJ) The amount of heat energy given out or taken in (absorbed) during a chemical reaction.
  • Exothermic reaction Heat energy lost - ΔH Endothermic reaction Heat energy gained + ΔH
  • Example 1 Reaction: Equation: ΔH Exothermic/Endo thermic reaction: Explanation: Magnesium and hydrochloric acid Mg + 2HCl  MgCl2 + H2 [ΔH = -462 KJ] ΔH = -462 KJ Negative value of ΔH shows that the reaction is exothermic. 462 kJ means that 462 kilojoules of energy is given out.
  • Example 1 Reaction: Equation: ΔH Exothermic/Endo thermic reaction: Explanation: Magnesium and hydrochloric acid Mg + 2HCl  MgCl2 + H2 [ΔH = -462 KJ] ΔH = -462 KJ exothermic reaction Negative value of ΔH shows that the reaction is exothermic. 462 kJ means that 462 kilojoules of energy is given out.
  • Example 2 Reaction: Equation: ΔH Exothermic/End othermic reaction: Explanation: Decomposition of calcium carbonate CaCO3  CaO + CO2 [ΔH = +222 KJ] ΔH = +222 KJ endothermic reaction Positive value of ΔH shows that the reaction is endothermic. 222 kJ means that 222 kilojoules of energy is given out.
  • CHECKPOINT!
  • 1. DETERMINE IF THE REACTIONS STATED BELOW ARE ENDOTHERMIC OR EXOTHERMIC REACTIONS. (A) COMBUSTION OF METHANE: EXOTHERMIC/ENDOTHERMIC REACTION CH4 + 2O2  CO2 + 2H2O *ΔH = -882 KJ] (b) Decomposition of copper(II) carbonate: exothermic/endothermic reaction CuCO3  CuO + CO2 *ΔH = +46 kJ] (c) Formation of nitrogen oxide in a car engine: exothermic/endothermic reaction N2 + O2  2NO *ΔH = +181 kJ] (d) Reaction of sodium hydroxide and hydrochloric acid: exothermic/endothermic reaction NaOH + HCl  NaCl + H2O *ΔH = -57 kJ]
  • 1. DETERMINE IF THE REACTIONS STATED BELOW ARE ENDOTHERMIC OR EXOTHERMIC REACTIONS. (A) COMBUSTION OF METHANE: EXOTHERMIC/ENDOTHERMIC REACTION CH4 + 2O2  CO2 + 2H2O *ΔH = -882 KJ] (b) Decomposition of copper(II) carbonate: exothermic/endothermic reaction CuCO3  CuO + CO2 *ΔH = +46 kJ] (c) Formation of nitrogen oxide in a car engine: exothermic/endothermic reaction N2 + O2  2NO *ΔH = +181 kJ] (d) Reaction of sodium hydroxide and hydrochloric acid: exothermic/endothermic reaction NaOH + HCl  NaCl + H2O *ΔH = -57 kJ]
  • Exothermic reaction Exothermic reaction only occurs when the chemical energy present in the substance is converted to thermal energy (and sometimes light and sound energy).
  • 1. Change of state condensation freezing • Particles move closer together, at a lower energy level • Excess energy is lost in the form of heat energy
  • 2. Combustion reactions • Combustion is the reaction between a substance and oxygen. • Energy is released in the form of heat (sometimes light and sound energy). • Respiration is also a type of combustion reaction.
  • 3. Neutralisation • Acid + Alkali  Thermal energy given out – Combining of H+ and OH- ions to form H2O – H+ (aq)+ OH- (aq) H2O (l)
  • Endothermic reaction occurs when heat energy is absorbed. Endothermic reaction
  • 1. Change of state • Particles move further apart, breaking forces of attraction holding them together • Energy is absorbed and used to break bonds • Surrounding temperature drops.
  • 2. Decomposition of compounds • Most compounds require heating to be decomposed. – Thermal energy absorbed – Compound is decomposed
  • bond-making vs. bond-breaking
  • Heat changes in a reaction
  • heat of reaction, ΔH Overall heat change (heat of reaction, ΔH) = heat given out in making bonds + heat absorbed in breaking bonds