2.1 Thermochemistry and 2.2 Rates of reaction Flashcards
what is enthalpy change?
the heat energy measured at constant temperature
what are the standard conditions for enthalpy change?
pressure- 1atm
temperature- 298K (25°C)
what is the definition for the standard enthalpy change of formation?
the enthalpy change when one mole of a substance if formed from it constituent elements in their standard states under standard conditions
what is the definition for the standard enthalpy change of combustion?
the enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions
what is the definition for the standard enthalpy change of reaction?
the enthalpy change between a number of moles of reactants shown in the equation for the reaction
what does Hess’ law state?
Hess’ law states that the total enthalpy change for a reaction is independent of the route taken from the reactants to the products
what is the equation for enthalpy change of reaction?
ΔH= ΣΔfH(products)-ΣΔfH(reactants)
what is the equation for enthalpy change of formation?
ΔfH= ΣΔfH(products)-ΣΔfH(reactants)
what is the equation for enthalpy change of combustion?
ΔcH= ΣΔcH(reactants)-ΣΔcH(products)
what is meant by bond enthalpy?
the energy needed to break a specific type of covalent bond in the molecules of a gaseous species
why do calculation results involving formation and combustion enthalpies differ to calculation results involving mean bond enthalpies?
the calculations using formation and combustion enthalpies are much more accurate. the mean bond enthalpies are averaged out over all the different types of compounds so they are not specific to the compound being investigated
which equation can be used to work out the energy transferred in a reaction?
q=mcΔT
q= energy released/absorbed (J)
m= mass of solution (g)
c= specific heat capacity of water (J g-1°C-1)
ΔT= temperature change (°C/K)
why are energy change reactions carried out in an insulated container/calorimeter?
to prevent heat energy loss to the surroundings
Describe an experiment that can be conducted to investigate the enthalpy change for the thermal decomposition of potassium hydrogencarbonate
- Weigh 3 g of potassium carbonate into a test tube.
- Use a burette to add 30 cm° of 2 mol dm 3 hydrochloric acid to a polystyrene cup in a beaker.
- Measure the initial temperature of the acid.
- Add the potassium carbonate to the cup. Continually measure the temperature and stir. Record the highest temperature reached.
- Use the equation q=mcT to calculate the enthalpy change.
what is meant by rate of reaction?
the change in concentration of a reactant or product per unit time
what is the collision theory?
for a collision to be effective, the molecules must collide in the correct orientation and with enough energy to react
what is meant by activation energy?
the minimum amount of energy required for a reaction to take place by breaking of bonds
describe and explain the gradient of the curve on a rate of reaction graph
initially the gradient is very steep because the rate of reaction is fastest at the start (there are more reacting particles so more frequent successful collisions).
The gradient decreases over time as the reactants are used up.
The curve eventually levels off when the reaction is complete (one or all of the reactants have been completely used up)
does reactants or products go on the left of an reaction profile diagram?
reactants on left
products on right
what conditions can be changed to increase the rate of a reaction?
- increase temperature
- increase pressure
- increase surface area of reactants
- increase concentration of reactants
- add a catalyst
- add light
