Flashcards in Chapter 9 - Enthalpy FINISH LATERRRR Deck (23):
What is the enthalpy change 🔼H of a reaction?
The energy change at constant pressure.
What happens in exothermic and endothermic reactions?
In exothermic reactions heat is released (heat EXits)
In endothermic reactions heat is absorbed (heats ENters)
What is the enthalpy change, 🔼H for exothermic and endothermic reactions.
For an exothermic reaction 🔼H is negative.
For an endothermic reaction, 🔼H is positive.
What is the activation energy?
The minimum energy required for a reaction to take place. Usually only a small proportion of the particles will have sufficient energy to react.
What do enthalpy profile diagrams look like for exothermic and endothermic reactions?
(Enthalpy on y-axis) (progress of reaction on x-axis)
Exothermic reaction is a step down graph.
Endothermic reaction is a step up graph.
Examples of exothermic reactions
Combustion and many oxidation reactions
Examples of endothermic reactions
What is the enthalpy change of reaction 🔼rH
The heat energy change at constant pressure when the reaction is carried out with the amounts of reactant (in mol) given by the equation.
What are the standard conditions for enthalpy change of reaction
Concentration of solutions 1 mol dm-3
At a started temperature, usually 298k
Indicated by the symbol 🚫(little London Underground not a stop sign lol)
What is the standard state of the substance.
The physical state (solid, liquid or gas) of a substance at 100kPa and 298K is the standard state of that substance.
Write an equation for burning propane, and state the enthalpy change of this reaction.
C3H8(g) + 5O2(g) —> 3CO2(g) + 4H2O(l)
The standard enthalpy change of this reaction is the heat energy change when one mole of propane gas burns in five moles of oxygen gas forming three moles of carbon dioxide gas and four moles of liquid water (the standard state) at a pressure of 100kPa and a temperature of 298K.
What is the formula for calculating the heat released or absorbed?
Q=mc ^ t
What do the letters in the Q=mc(triangle)t represent and what unit are they measured in?
Q is the heat change in Joules (J)
m is the mass of water/solution in grams (g)
c is the specific heat capacity of water (4.18 J K- mol-1)
(triangle)t is the temperature change (K or degrees Celsius)
What is enthalpy change measured in? How do you work it out?
The heat change per mole of reactant.
So work out heat change using Q=mc(triangle)t and then divide that by the number of moles of the reactant
If a reaction is exothermic what will the enthalpy change be? (positive or negative)
If a reaction is endothermic what will the enthalpy change be? (positive or negative)
What is the enthalpy change of neutralisation?
The enthalpy change of neutralisation is the enthalpy change when one mole of water is formed in a neutralisation reaction.
Suggest two practical ways of minimising heat loss in experiments
Use a polystyrene cup (as it's a poor conductor of heat) and a lid.
State the ionic equation for neutralisation.
H+ + OH- --> H2O
What is the enthalpy change for a strong acid and base?
How do you calculate percentage error?
(true value - experimental value) / true value x100
What conditions must there be for Hess' law to apply?
For Hess' law to apply, the reactants must start in the same conditions for both routes and the products must end in the same conditions for both routes.