Heat changes in reactions 5.4.0 Thermochemistry Heat changes in reactions

Endothermic Reactions Exothermic Reactions Produce heat [give out heat] Feel hot to the touch  H is negative (-) Endothermic Reactions Use heat [take in heat] Feel cold to the touch  H is positive (+)

Reactions usually result in a change of temperature. Temperature will rise if heat is produced Temperature will fall if heat is used

Changes of Phase [state] These add or subtract from actual heat change values. Melting and Evaporation use heat they have a positive value [endothermic] Condensing and Solidifying release heat they have a negative value [exothermic]

Test What does exothermic mean? [1] How can you tell a reaction is exothermic? [2] Give an example of an exothermic reaction. [2] What does endothermic mean? [1] How can you tell a reaction is enothermic? [2] Give an example of an endothermic reaction. [2] What does ΔH mean? Give the sign of ΔH for exothermic and endothermic reactions. What is ΔH for solid to liquid?

Test What does exothermic mean? [1] gives out heat How can you tell a reaction is exothermic? [2] feels / hot Give an example of an exothermic reaction. [2] burning/coal What does endothermic mean? [1] takes in heat How can you tell a reaction is endothermic? [2] feels / cold Give an example of an endothermic reaction. [3] dissolving / sherbet [or ammonium chloride] / in water What does ΔH mean? [2] Heat change Give the sign of ΔH for exothermic and endothermic reactions. [2] –ve and + ve What is ΔH for solid to liquid? [1] -ve [16]