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23 Cards in this Set

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Enthalpy Change

The amount of heat energy taken in or given out during any change in a system provided the pressure is constant

In an exothermic reaction heat is

Given out

In an exothermic reaction the products have ______ energy than the reactants

Less

In an exothermic reaction delta H is always

Negative

In an endothermic reaction energy is

Taken in

In an endothermic reaction the products have _____ energy than the reactants

More

In an endothermic reaction delta H is

Positive

What is the Standard Enthalpy of Formation
The enthalpy change when one mole of the compound is formed from its elements under standard conditions with all reactants and products being in their standard states
What is the Standard Enthalpy of combustion
The enthalpy change when one mole of a substance is combusted completely in oxygen under standard conditions with all reactants and products being in their standard states
What are the Standard conditions
298K and 100kPa
What is the Enthalpy of formation of an element
0 kJ mol-1
What is the equation for Energy Change

Energy change (J) = mass of solution (g) x heat capacity (Jg-1K-1) x temperature change(K)




Q= m x c x delta T

What is the Calorimetric Method
One where substances are mixed in an insulated container and the temperature rise is measured
How would you carry out a calorimetric experiment

1)Wash the equipment with the solutions to be used


2) Dry the cup after washing


3) Put polystyrene cup in a beaker for insulation and support


4) Clamp thermometer into place making sure that the thermometer bulb is immersed in liquid


5) Measure the initial temperatures of the solution or both if two are used


6) Transfer reagent into the cup. If a solid reagent is used add the solution first


7) Stir Mixture


8) Measure final Temperature

why would it be difficult to obtain an exact temperature rise if the reaction is slow
Because cooling occurs simultaneously with the reaction

How is the problem of a slow reaction counteracted in a calorimetry experiment to obtain an exact temperature rise
Extrapolate the temperature line backwards to the time where the reagents were added together
What is the big error in the calorimetry experiment

Heat loss




reaction may be incomplete or slow

REMEMBER as it is assumed that all solutions have the same density as water e.g. 25cm3 will weigh
25g
When using delta H = Q/n what MUST be remembered
to divide Q by 1000 to give you kj mol-1
After calculating H=Q/n what must you remember to do
Put the sign to represent the energy change. IF EXOTHEMIC THE SIGN IS NEGATIVE
State Hess's Law
The total enthalpy change for a reaction is the same regardless of the route taken `
What is the definition for Mean Bond Energies
The Mean Bond Energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules