Energy changes
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Define the word exothermic
Gives out heat energy
Define the word endothermic
Take in heat energy
Give examples of types of reaction that are exothermic
Combustion Neutralisation Displacement
Give examples of types of reaction that are endothermic
Thermal decomposition
What is the symbol for enthalpy change
TriangleH
Define enthalpy change
The amount of heat energy taken in or given out during a reaction, per mole of reactant i.e. the difference in energy between products and reactants
What are the units for enthalpy change
kJ/mol
For exothermic reactions is the enthalpy change negative or positive and why?
Negative The reaction gives out energy so the products have less energy than the reactants
For endothermic reactions is the enthalpy change negative or positive and why?
Positive The reaction takes in energy so the products have more energy than the reactants
Draw an energy level diagram for an exothermic reaction
__________ Reactants \_\_\_\_\_\_\_\_\_ Products
Draw an energy level diagram for an endothermic reaction
X axis: Progress of reaction Y axis: Energy \_\_\_\_\_\_\_\_\_\_\_\_ products _____________ reactants
Compare the energy stability in exothermic reactions
The products are more energetically stable than the reactants
Describe a practical to investigate temperature changes for displacement reactions
• Place the polystyrene cup into the beaker to keep it stable • Measure out 25cm3 of hydrochloric acid solution using a measuring cylinder and pour this into the polystyrene cup • Place the thermometer into the acid • Carefully weigh out 0.3g of magnesium powder into a weighing boat using a mass balance • Record the starting temperature of the solution • Add the magnesium to the solution quickly and stir the mixture gently with the thermometer • Check the temperature regularly until the temperature stops rising. Record the maximum temperature
Describe a practical to investigate temperature changes for neutralisation reactions
• Place the polystyrene cup into the beaker to keep it stable • Measure out 25cm3 of hydrochloric acid solution using a measuring cylinder and pour this into the polystyrene cup • Place the thermometer into the acid • Measure out 25cm3 of sodium hydroxide using a measuring cylinder • Record the starting temperature of the solution • Add the magnesium to the solution quickly and stir the mixture gently with the thermometer • Check the temperature regularly until the temperature stops rising. Record the maximum temperature
Describe a practical to investigate temperature changes for dissolving salts in water
• Place the polystyrene cup into the beaker to keep it stable • Measure out 25cm3 of distilled water using a measuring cylinder and pour this into the polystyrene cup • Place the thermometer into the acid • Carefully weigh out 1.35g of ammonium nitrate into a weighing boat using a mass balance • Record the starting temperature of the solution • Add the magnesium to the solution quickly and stir the mixture gently with the thermometer • Check the temperature regularly until the temperature stops rising. Record the maximum temperature
Describe a practical to investigate temperature changes for combustion reactions
• Measure out 100cm3 of water using a measuring cylinder and pour this into the conical flask • Clamp the flask approximately 1cm above the spirit burner with the lid on • Clamp the thermometer into the water so that. the bulb of the thermometer is submerged in the water but is not touching the bottom of the flask • Record the mass of the spirit burner with the lid on • Record the starting temperature of the solution • Light the spirit burner and then move it under the conical flask. • Allow the burner to heat the water until the temperature of the water rises by approximately 40C • Move the burner out from under the conical flask and extinguish • Check the temperature regularly until the temperature stops rising. Record the maximum temperature reached in the observations • After cooling, record the mass of the burner
What equations should you use for calculating enthalpy change of combustion reactions
• Q=mcdeltaT = J (mass of water) • moles=mass/Mr (mass difference of spirit burner) • (E/1000)/moles = kJ/mol
Give possible sources of error in the combustion experiment (and explain)
1) Heat energy lost to surroundings 2) Incomplete combustion (less exothermic than complete) 3) Boiling/evaporating water - mass of water incorrect 4) Lid off spirit burner; fuel evaporate –> fuel mass burned bigger
Describe the bonds in exothermic reactions
Bond making Energy released
Describe the bonds in endothermic reactions
Bond breaking Requires energy
Define bond energy
The amount of energy required to break a particular bond in kJ/mol
How do you find out the total energy change for a chemical reaction?
DeltaH = total bond energy of reactants-total bond energy of products
What determines whether a reaction is exothermic overall?
The energy released when the bonds form in the products is greater than the energy required to break bonds in the reactants
What determines whether a reaction is endothermic overall?
The energy required to break bonds in the reactants is greater than the energy released when bonds form in the products