8: Energetics I Flashcards
(26 cards)
What is enthalpy change (ΔH)?
The heat energy exchange with the surroundings at constant pressure.
What unit is enthalpy change given in?
kJmol-1
What is standard enthalpy change?
It refers to reactions done under standard conditions
What are the standard conditions?
- 298K (25°C)
- a pressure of 1 bar (100kPa)
- when solutions are involved then a concentration of 1moldm-3
Which direction does the overall reaction move for an exothermic reaction on a graph?
The arrow goes down.
The enthalpy change is negative.
Which direction does the overall reaction move for an endothermic reaction on a graph?
The arrow goes up.
The enthalpy change is positive.
What is the definition for the standard enthalpy change of formation?
Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.
What is the definition for the standard enthalpy change of combustion?
Enthalpy change when one mole of a substance is completely burned in oxygen with all reactants and products in standard conditions.
What is the definition for the standard enthalpy change of neutralisation?
Enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions.
Why is enthalpy change of neutralisation always negative?
Heat is given out when an acid and alkali react. (Exothermic)
What state should water always been when writing out an equation for the standard enthalpy change of combustion?
Liquid
What is the definition for the standard enthalpy change of reaction?
The enthalpy change of a reaction according to the molar quantities in the equation under standard conditions.
In terms of bond energy, what will result in an endothermic reaction?
If more energy is need to break the bonds than to form them.
What happens to energy when a bond is broken?
Energy is absorbed.
What happens to energy when a bond is formed?
Energy is released
In terms of bond energy, what will result in an exothermic reaction?
I’d more energy is released when bonds are formed than what was needed to break them.
What is bond enthalpy?
The amount of energy needed to break one mole of a bond type in a molecule in the gaseous state.
How do you work our enthalpy change using mean bond enthalpy?
Enthalpy change = total energy to break bonds - total energy to form bonds
What is the equation for energy change?
q = mc🔼T
What is q in q = mc🔼T?
Energy change (energy lost of gained) in joules
What is m in q = mc🔼T?
Mass of substance being heated (often water) in grams
What is c in q = mc🔼T?
Specific heat capacity of water
What is the specific heat capacity of water?
4.18 Joules per kelvin grams
What is 🔼T in q = mc🔼T?
Temperature change
