7.1 Exothermic and endothermic reactions + RP
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Exothermic reaction def
A reaction that transfers energy to the surroundings
Endothermic reaction def
A reaction that takes in energy from the surroundings
Do endothermic and exothermic reactions cause rises or falls in temperature
Exothermic: temperature rise Endothermic: temperature fall
Examples of exothermic reactions
• burning of fuel (eg combustion of methane) • neutralisation of acids and alkali
Do the products in exothermic reactions have lower or higher energy content than the reactants
Lower
How are actual differences in energy measured
Kg/mol
Examples of endothermic reactions
• citric acid and sodium hydrogen carbonate • thermal decomposition
Example of thermal decomposition
• decomposition of calcium carbonate • when heated, it forms calcium oxide and carbon dioxide • needs to be heated and absorb energy from the surroundings to be broken down
Do products in endothermic reactions have a higher or lower energy content than the reactants
Higher
Investigating temperature changes method
• Use the first measuring cylinder to measure 25 cm3 of the sodium hydroxide solution and pour this into the polystyrene cup. • Stand the polystyrene cup in the 250 cm3 beaker. • In the second measuring cylinder, measure 25 cm3 of hydrochloric acid. • Using the thermometer, measure the temperature of the sodium hydroxide every 30 seconds whilst gently stirring. • After exactly 2 minutes add the hydrochloric acid and continue to stir and to record the temperature of the solution every 30 seconds for 10 minutes. • Repeat this with with 25 cm3 of copper(II) sulfate and iron filings
Why is a polystyrene cup used
• polystyrene is an insulator • so less energy transfer to surroundings
Suggest one reason for random errors in the experiment
Inconsistent stirring/use of lid
The reaction between hydrogen and chlorine is exothermic. Explain why this reaction releases energy to the surroundings
• energy needed to break bonds and energy is released when bonds form • the energy released is greater than the energy needed
Explain why methane and polyethene exist in different states at room temperature
• methane has much smaller molecules • so has weak intermolecular forces • so intermolecular forces need less energy to overcome • so boiling point is lower and methane is a gas at room temperature