Enthalpy Changes Flashcards
What is enthalpy?
The total chemical energy inside a substance
What is average bond enthaly?
The amount of energy it takes to break one mole of a specified bond in gaseous molecules.
What does breaking and making bonds do?
A change in the enthalpy contained in the bond. Heat is either given out into the surroundings or absorbed into the bond, decreasing surrounding temperature.
In an endotherminc reaction, what is the enthalpy change?
The enthalpy of the products is lower than the enthalpy of the reactants so the enthalpy change is negative.
In an endothermic reaction what is the enthalpy change?
The enthalpy of the products is higher than the enthalpy of the reactants so the enthalpy change is positive.
On an enthalpy change diagram where does the Ea arrow and delta H arrow go?
Ea goes from reactants to peak enthalpy and delta H goes from reactants to products enthalpy.
What are standard states?
The state that a chemical would be in at standard conditions (room temperature and pressure).
What is standard enthalpy change of reaction?
The enthalpy change of a reaction according to the molar quantities in the equation under standard conditions
What is standard enthalpy change of neutralisation?
Enthalpy change when an acid and alkali react to form 1 mole of water, under standard conditions.
What is standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O under standard conditions.
What is standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
What is the concentration of a standard solution?
1 moldm-3
Is bond breaking endothermic or exothermic?
Endothermic as energy needs to be absorbed to break the bond. If more energy is needed to break than make = endothermic reaction.
Is bond breaking endothermic or exothermic?
Exothermic. When new bonds are formed energy is released. If more energy is released when bonds form than absorbed when bonds break the reaction is exothermic.
What practical method is used to measure enthalpy change of combustion?
Calorimetry
What are the steps of calorimetry?
A fuel is burned to raise the temp of the water by a specific amount.
Weigh fuel before and after to work out the mass of fuel burned.
Lid on top of beaker to stop heat loss
Draft shields around the flame to stop it flickering so energy doesn’t transfer to surroundings.
use q=mc∆T
q= energy lost or gained, J
m of water= g (cm3=g=ml)
c=SHC of water or solution, 4.18 J-1°C
T= °C
What is the equation for energy change?
q=mc∆T
q= energy lost or gained, J
m of water= g (cm3=g=ml)
c=SHC of water or solution, 4.18 J-1°C
T= °C
How do you calculate enthalpy change?
q/moles
Divide the energy released by the fuel’s combustion (converted to KJ) by the number of moles of fuel.
The energy change is positive or negative depending on if the reaction is exothermic or endothermic.
How to calculate bond enthalpy change?
Break-make.
Reactants total bond enthalpy - products total bond enthalpy.
What is Hess’s Law?
The TOTAL ENTHALPY CHANGE of a reaction is INDEPENDENT of the route taken.
It helps you work out enthalpy changes you can’t find by doing an experiment.
What are the two main cycles in Hess cyles?
Formation and combustion.
What does a hess cycle look like when given FORMATION data?
Reactants —————–>Products
Elements in
their standard states ∆cH/∆rH on top ∆fH on left arrow pointing UP ∆fH on right arrow pointing UP
What does a hess cycle look like when given COMBUSTION data?
Reactants —————–>Products
COMBUSTION
PRODUCTS ∆fH/∆rH on top ∆cH on left arrow pointing DOWN ∆cH on right arrow pointing DOWN
When the enthalpy change of solution exothermic?
When the sum of enthalpies of hydration are greater than lattice enthalpy
When is enthalpy chane of solution endothermic?
When lattice enthalpy is greater than hydration enthalpy
What affects enthalpy of hydration and lattic enthalpy?
The smaller the ionic radius, the greater attraction between ions and water molecules = more exothermic lattice and hydration enthalpy
The greater the ionic charge, the greater attraction between ions and water molecules
