Unit 1 Section 4: Energetics

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What is enthalpy change?
It is the heat energy transferred in a reaction at constant pressure
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What are standard conditions?
100kPa and 298K (25 degrees)
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What are exothermic reactions?
Bond breaking
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What are endothermic reactions?
Bond making
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Do the products or reactants have more energy in an exothermic reaction?
The reactants have more energy
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What is the enthalpy change in exothermic reactions?
Negative
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What is the enthalpy change in endothermic reactions?
Positive
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What are the units of enthalpy change?
kj mol-1
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What is an example of an exothermic reaction?
Combustion, (oxidation reactions)
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Do the products or reactants have more energy in an endothermic reaction?
The products have more energy
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Name an endothermic reaction
Photosynthesis
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What are the differences between endothermic and exothermic reactions?
Exo give out energy, endo absorb energy. In exo reactions the enthalpy change is negative, in endo it is positive.
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Define bond enthalpy
The energy needed to break a bond
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What is mean bond enthalpy?
The average energy needed to break a certain type of bond, over a range of compounds
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How do you work out the overall enthalpy change of a reaction?
Total energy absorbed - total energy released
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Define standard enthalpy change of formation
It is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
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Define standard enthalpy change of combustion
It is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their natural states
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Define standard enthalpy change of reaction
It is the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states
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What experiments measure the heat given out by certain reactions?
Calorimetry experiments
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How do you find the enthalpy change of combustion of a flammable liquid?
You burn it inside a calorimeter
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What is the equation for enthalpy change?
Mass X specific heat capacity X change in temperature
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What is q in q=mc∆T?
Heat lost/gained in joules (this is the same as enthalpy change if the pressure is constant)
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How do you calculate the standard enthalpy change of combustion?
Calculate heat lost/gained in q=mc∆T and change the units of q to kilojoules, then calculate the number of moles of fuel that caused the enthalpy change, then divide q (kj) by number of moles
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What is the final equation used to calculate the standard enthalpy change of combustion?
Actual heat change / number of moles
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What is the enthalpy change of neutralisation?
It is the energy change when one mole of water is formed by the reaction of an acid and an alkali
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How do you find the enthalpy change for a neutralisation reaction?
You add a known volume of acid to an insulated cup and measure the temperature, then add a known volume of alkali and record temperature rise. Stir to distribute heat evenly.
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What are two problems with calorimetry?
Some heat will be absorbed by the container, instead of the water & some heat is always lost to surroundings
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What are 2 problems with calorimetry that only apply to flammable liquid calorimetry?
Some combustion may be incomplete (meaning less energy will be given out) & some of the flammable liquid may escape by evaporation (they're usually volatile)
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What is hess's law?
The total enthalpy change for a reaction is independence of the route taken
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Card 2

Front

What are standard conditions?

Back

100kPa and 298K (25 degrees)

Card 3

Front

What are exothermic reactions?

Back

Preview of the front of card 3

Card 4

Front

What are endothermic reactions?

Back

Preview of the front of card 4

Card 5

Front

Do the products or reactants have more energy in an exothermic reaction?

Back

Preview of the front of card 5
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