1.4 Energetics
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What is the definition of enthalpy change?
The amount of heat produced it absorbed in a reaction carried out at constant pressure
What are standard conditions for Enthalpy Change?
Pressure of 100 kPa (1 atm) Temperature of 298 K
What is an exothermic reaction?
Give out heat during reaction (negative)
What is an endothermic reaction?
Take in heat during a reaction
What is the definition of activation energy?
The minimum amount of energy required for a reaction
What is the definition of standard enthalpy of formation?
The enthalpy when one mole is formed from its elements with all products and reactants in standard states
What is the definition of enthalpy of combustion?
When 1 mole of a compound is burnt in excess oxygen with all reactants and products in standard states
What is the formula for heat change?
Heat change= mass of water x shc x temp change
What is a calorimeter?
Used to work out enthalpy of a fuel Fuel is burnt to heat a known amount of water Can be used to compare enthalpy of combustion because the errors will be similar
Why is a flame calorimeter better than a normal calorimeter?
• Flame is enclosed- Less heat loss • Flame burns in complete oxygen (reduces chance of incomplete combustion) • Spiral chimney is made out of copper
How are allowances for heat loss made on a graph?
A cooling curve is plotted then extrapolated
What needs to happen to the value of q before enthalpy can be calculated?
Divided by 1000 to get KJ
What is the equation for moles?
Mass / Mr
What is the equation of moles of a solution?
Concentration x volume
What is the overall formula for enthalpy?
Delta H = q / mol
What is Hess’ Law?
The enthalpy change of a reaction is independent of the route taken
What is the purpose of enthalpy cycles?
Enthalpy change can’t always be directly measured Enthalpy cycles help indirectly calculate ot
What is the shortcut for enthalpy of combustion?
C = sum of Reactants - sum of Products
What is the shortcut for enthalpy of formation?
F = sum of products - sum of reactants
What are the main errors for enthalpy of combustion?
• Heat loss to surroundings • Incomplete combustion
When is Hess’s triangles used?
When both formation and combustion is measured
What is the definition of bond enthalpy?
Enthalpy change to break a covalent bond averaged over different molecules
Why do bond enthalpies not always give accurate results?
It’s an average over a range of compounds,not specific to a reaction