Module 3.2 Flashcards
Enthalpy change equation
Products - reactants
Exothermic reaction
Heat energy is released from the chemical system
- the enthalpy of the product is smaller than the reactant
Endothermic reaction
Chemical system has taken in energy from the surroundings so temperature of the surroundings has decreased
- enthalpy of the product is greater than the reactant
Standard enthalpy change of combustion
The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions
Standard enthalpy change of neutralisation
The enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions
Why is the standard enthalpy change of neutralisation the same in any strong acid and strong alkali ?
- because the key parts of the reaction a H+ in the acid and the OH- on the alkali which are the same
Standard enthalpy change of formation
the enthalpy change when one mole of a compound is formed from its element under standard conditions
Standard enthalpy change of a reaction
Enthalpy change associated with a stated equation
Activation energy
The minimum energy required for a reaction to take place
standard states and conditions of enthalpy change
- pressure=100kpa
- Temperature=298 k (25degrees)
- concentration= 1 mol dm3
enthalpy change from calorimetry equation
- q= m x c x temp change
how to calculate enthalpy change of combustion
q = m x temp change x temp change (k)
- divide by moles of fuel used
why is the experimental value lower than the actual enthalpy change?
- Heat loss to surroundings (use a lid and polystyrene cup over glass to prevent this)
- Heat is absorbed by the container
- incomplete combustion
- evaporation
what is the advantage of using a bomb calorimeter?
- minimises heat lost, uses pure oxygen to ensure complete combustion is achieved therefore the experimental value is more accurate.
how to calculate enthalpy change of neutralisation
- q= m x c x temp change
- workout more of water produced by finding the moles of the reactant and using balanced mole equation to find moles of water produced.
- energy/ moles
- add - if its exothermic
average bond enthalpy
- the energy needed to break one mole of a specific bond in a molecule in the gaseous state
how do we calculate bond enthalpy
- write a chemical equation using display formulae
- tally number of bond in reactants and products and substitute
- reactants - products
Hess law
- enthalpy change in a chemical reaction is independent of the route it takes.
Hess cycle to calculate enthalpy change
- write out all the elements involved in the reaction. if combustion co2 and h20
- fill in the enthalpy data
- change the sign . right if formation left if combustion
- add enthalpy change of reactant and product
Collison theory
- for a reaction to occur the particles must collide in the right direction and have the minimum anount of kinetic energy
Formulae for rate of reaction
Amount of reactant used/product made/ time
Effect of concentration on the rate of reaction
- increased concentration= increase rate of reaction
- the molecules are closer together so there is a greater chance of them colliding with sufficient energy to overcome the activation energy
- more successful collisions
Effect of pressure on the rate of reaction
- pressure of gas increases= rate of reaction increases
- same number of molecules occupies a smaller volume
- more successful collisions
Effect of temperature on the rate of reaction
- increase temperature = increased rate of reaction
- increases kinetic energy and move faster
- higher proportion of molecules have an energy that is greater than the active energy
- more successful collisions
Effect of catalyst on the rate of reaction
- more particles hace enough energy to reach the activation barriar
- more successful collisons = rate of reaction increases
Catalyst
Substances that increased the rate of reaction by providing an alternative pathway that lowers activation energy . The catalyst is chemically by the end of the reaction
Hetergeneous catalyst
- a catalyst that is a different phase from the reactant. E.g. solid catalyst used in liquid reactants
increasing the SA of this increases rate of reactions as more particles can react with the catalyst at the same time.
homogeneous catalyst
- catalyst that is in the same phase as the reactant. generally AQ and AQ.
they are used up in a reaction forming an intermediate species from the reactant combining which reacts to form a product, the catalyst is remformed again
benefits of using a catalyst
- reduces energy demand resulting in reduction in co2 emissions and lowering cost
- catalytic converters reduces toxic emission from vehicles preventing photochemical smog by converting toxic carbon monoxide and nitrogen oxide into co2 and n2,
- improves stainability
which ways can we measure the rate of reaction
- measure how long it takes preticipate to form
- volume of gas produced using a gas syringe
- mass lost
Dynamic equilibrium
Equilibrium exist in a closed system when the rate of the foward reaction is equal to the rate of the reverse reaction and the concentration of reactants and products do not change
Effect of catalyst on equilibriums
- speeds up the rate in which a reaction reaches a equilibrium
- increases the rate of both foward and reverse reactions by the same amount resulting in a unchanged position of equilibrium
What is meant when kc value is greater than 1
- the reaction favours the products
Kc is smaller than 1
- the reactant favours the products
Kc=
Products/ reactants
