4.2- ENTHALPY Flashcards
What does the amount of heat given out or taken in by a reaction vary with?
conditions
Examples of conditions that amount of energy given out or taken in by a reaction varies with? (3)
temp
pressure
concentration of solutions
As amount of heat given out or taken in be a reaction vary with conditions, what must be stated?
conditions under which measurements made
What is it called when you measure a heat change at constant pressure?
enthalpy change
What is the symbol for enthalpy?
H
What are the standard conditions for measuring enthalpy changes? (2)
pressure of 100kPa
temperature of 298K
What is the pressure of 100kPa approximatly?
approximately normal atmospheric pressure
What is the temperature of 298K around the same as?
normal room temp, 25°C
What is the standard state of an element?
state in which it exists at 298K and 100kPa
What is it written as when an enthalpy change is measured under standard conditions?
ΔH∘298
298 usually left out
When is a reaction thought to be over? (exothermic)
until products have cooled back to 298K
What is the heat given out to surroundings while reaction mixture cools?
enthalpy change for reaction
In an exothermic reaction what do the products end up with less of?
less heat energy than starting materials
Why do the products end up with less heat energy than the starting materials in an exothermic reaction?
as they have lost heat energy when they heated up their surroundings
What is the ΔH of exothermic reactions like?
negative
What sign do exothermic reaction have?
negative sign
What do some endothermic reactions that take place in aqueous solution do?
absorb heat from water + cool it down
What is an example of an endothermic reaction that happens in an aqueous solution and absorbs heat from water and cools it down?
dissolving ammonium nitrate in water
When is a reaction thought to be over? (endothermic)
products have warmed up to temp at which they started
In endothermic reactions what do the products end up with more of?
more energy than starting material
What is the ΔH of endothermic reactions like as the products end up with more energy than the starting material?
positive
What is the sign for an endothermic reaction?
positive
When does pressure affect the amount of heat energy given out by a reaction?
reaction that involves gases
If a gas is given out, what is required by some of the energy?
some of the energy required to push away the atmosphere
What happens the greater the atmospheric pressure, when a gas is given out?
the more energy is used for this
As more energy is used when there is greater atmospheric pressure and gas is given out, what does this mean?
less energy remains to be given out as heat by reaction
What does increasing the atmospheric pressure, leading to more energy used when a gas is given out and less energy remaining to give out as heat by reaction show the importance of?
importance of having a standard of pressure for measuring energy changes
What else affects the enthalpy change of a reaction?
physical states (gas, liquid or solid) of reactants + products
Example of how physical states of reactants + products affect enthalpy change of reaction?
heat must be put in to change liquid to gas + is given out when gas changed to liquid
As physical states affect enthalpy change of reaction, what must be included in the equations?
state symbols
What are enthalpy level diagrams sometimes called?
energy level diagrams
What are enthalpy level diagrams used to represent?
enthalpy changes
What do enthalpy level diagrams show?
relative enthalpy levels of reactants (starting materials) + products
What does the vertical axis of an enthalpy level diagram represent?
enthalpy
What does the horizontal axis of an enthalpy level diagram represent?
extent of reaction
