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Energy in Reaction
• Some reactions give off energy– Called _________– Examples:
– They feel _______
Energy continued
• Some reactions take in energy– Called ___________– Examples:– They feel _________
From your lab….
• What type of reaction was it?
• Where did the energy come from?
• What do we need to know to figure out how much energy was transferred?
Energy Calculations
q = mCT
Specific Heat
• Tells you how easily something heats up
Specific heats
Calculate your Q
Heat of Combustion
• How much energy is given off when a substance is burned.
• Measured in kilojoules or kilocalories
Heat of combustion
• Thermal energy/mass of substance burned
• Q/mass of candle burned
Molar heat of combustion
• Amount of energy to burn 1 mole of a substance
• kJ/mol• Q/moles burned
What is efficiency???
What are some ways your car is inefficient?
Where are some places your lab set up could have lost heat??
Step 1
• Bonds need to be broken so that they can be rearranged– Endothermic - energy is needed
• Energy + CH4 + 2O2 C + 4H + 4O
• Energy must be added to “pull apart” the compounds
Step 2
• The separate atoms need to form new bonds to make our products.– Exothermic change - energy is given off
• C + 4H + 4O CO2 + 2H2O + energy
Ene
rgy
Reaction coordinate
Reactants
Products
2 Hydrogens1 Oxygen
2 Waters
Overall Reaction
• The energy released when bonds are formed in CO2 and H2O is more than the energy needed to break the bonds of CH4 and O2
– Therefore, the total reaction is exothermic (gives off energy)
Exo Vs Endo
• If the total reaction GIVES OFF heat it is exothermic
• If the total reaction REQUIRES HEAT it is endothermic
• If one reaction is exothermic, the complete opposite would be endothermic
Ene
rgy
Reaction coordinate
Reactants
Products
Activation Energy Energy required to make the reaction happen
Ene
rgy
Reaction coordinate
Reactants
Products
Heat released
Exothermic Reaction
Ene
rgy
Reaction coordinate
Reactants
Products
Heat absorbed
Endothermic Reaction
Practice Problems
• How many calories of heat are taken in when 10.0 g of water cool from 50.0 oC to 25.0 oC?
• Iron has a specific heat of 0.11 cal/gºC. How much heat will it take to change the temperature of 48.3 g of iron by 32.4ºC?