Enthalpy changes

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What is enthalpy?
Heat content that is stored in a system
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What is enthalpy change and it's unit?
The heat energy change of a reaction at a constant pressure, kJmol^-1
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What happens in exothermic reactions?
Heat energy is given out to the surroundings so the temperature of the surroundings rise
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What is the value of Delta H for exothermic reactions?
Negative, as the system looses heat to the surroundings
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What are 2 exothermic reactions?
Combustion of methane and respiration
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What happens in endothermic reactions?
heat energy is taken from the surrounding so the temperature of the surroundings falls
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What is the Delta H value of endothermic reactions?
Positive as the system is gaining heat energy
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What are two examples of endothermic reactions?
Thermal decomposition of calcium carbonate and photosynthesis
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What is activation energy?
Minimum amount of energy required to start a reaction by breaking the bonds
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Does every chemical reactions energy value have to be higher than that of the reactants?
Yes
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What is standard enthalpy change of a reaction?
Enthalpy change when a chemical reaction has happened at standard conditions when reactants and products are in standard states and molar quantities
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What is the standard enthalpy of formation?
Enthalpy change when one mole of a substance is formed from its elements in their standard states in standard conditions
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The more negative the standard enthalpy change of formation, the more stable the compound. True or false?
True
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What is the standard enthalpy of combustion?
Enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states
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What is enthalpy change of neutralisation?
Enthalpy change when one mole of water is formed from neutralisation reactions between an acid and a base, all reactants and products being in their standard states.
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What is the name of the method we use to calculate enthalpy changes by experiment?
Calorimetry
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How do you measure the enthalpy change?
Measure temperature change of given amount of water as reaction occurs, and converting this temperature change to a heat energy value
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What does the specific heat capacity mean?
Amount of energy in joules required to increase temperature of 1g of substance by 1 degree
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What is the equation for heat change?
Q (heat change) = m (mass of water in g) x c (specific heat capacity of water) x delta t (change in temperature
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How do we change the value of heat change from J to kJ?
Divide by 1000
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How do you calculate amount of heat energy given out by 1 mole of a substance and what is its unit?
Q (heat change IN KJ) / no. of moles, kJmol^-1
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What must we always remember to do when we get our final kJmol^-1 value
Give it a negative or positive value (negative for exothermic, positive for endothermic)
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Why may a calculated Delta H differ from the data book value?
Incomplete combustion, slightly different conditions to standard conditions, heat loss to environment
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What is bond enthalpy?
Energy required to break by homolytic fission, one mole of a given bond in the molecules in a gaseous state
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Why are bond enthalpy values always positive?
Energy is always needed to break bond (endothermic process)
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What is the average bond enthalpy?
Average energy required to break one mole of a given bond in molecules at a gaseous state
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How can you calculate Delta H by enthalpies of bond breaking and making?
(Sumo of enthalpy of bond breaking) - (some of enthalpy of bond forming)
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What are two limitations of using bond energy calculations?
Using averages and only in gaseous state
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What is the energy law?
Energy cannot be created nor destroyed, just transferred from one form to another
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What does Hess' law state?
If a reaction can take place by more than one route, the enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same
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What does sum of Delta H of the indirect route equal?
The delta H of the direct route
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Why do we sometimes make reactions go the indirect way?
Direct route may be too slow or the activation energy may be too high
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How do we calculate the delta of a reaction from enthalpy change of COMBUSTION VALUES?
sum of delta h reactants - sum of delta h products
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How do you calculate the Delta H values with FORMATION VALUES
sum of delta h products - sum of delta h reactants
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Other cards in this set

Card 2

Front

What is enthalpy change and it's unit?

Back

The heat energy change of a reaction at a constant pressure, kJmol^-1

Card 3

Front

What happens in exothermic reactions?

Back

Preview of the front of card 3

Card 4

Front

What is the value of Delta H for exothermic reactions?

Back

Preview of the front of card 4

Card 5

Front

What are 2 exothermic reactions?

Back

Preview of the front of card 5
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