Energy changes Flashcards
What is a reversible reaction?
It is a reaction which proceeds in two directions –> forward and reverse direction. The reactants combine to form products. The products can form reactants.
exothermic reaction definition
An exothermic reaction is a reaction in which the amount of energy taken in is less than the amount of energy released (given out). Net energy is released. The enthalpy change is negative.
EquilibRium
The rate of the forward reaction is equal to the rate of reverse reaction. When equilbrium is reached, the concentration of the reactants and products remain constant.
Haber process equation
- 3H2 + N2 (reversible) 2NH3
- iron oxide catalyst
Conditions for reversible reactions
- Rf = Rr
- closed system
- Le Chatelier’s principle
When equilibrium is reached, the concentration
of the reactants and products remain constant
Uses of ammonia
- fertiliser industry:crop production
- explosive industry- TNT (Trinitrotuolene)
- household detergents
- It is a very weak alkali- it could neutralise acid in soil
- smelling salts: ammonium carbonate- medical use of ammonia- alfactory nerve will sting - to help people who have fainted
endothermic reaction definition
a reaction during which the amount of energy absorbed (taken in) is greater than the amount of energy released. Net energy is absorbed. The enthalpy change is positive.
conditions for a reversible reaction
- rate of forward reaction = rate of reverse reaction
- closed system
- Le Chatelier’s principle
Heat of reacion (delta H) enthalpy change definition
The difference between the energy of the reactant (s) and the energy of the product (s).
Activation energy definition
The minimum energy needed to start a chemical reaction. It is the energy needed to break the ‘bonds’ of the reactants.
catalyst
-It is a chemical substance that could SPEED UP/ increase the rate of a chemical reaction, without itself undergoing any chemical change.
Catalyst properties (3)
- a catalyst only changes the rate but not the yield
- catalysts lower the activation energy and provide an alternative energy pathway
- advantages: energy costs are lowered. Less electricity is needed. Less fossil fuels are burned. Environmentally friendly.
Industrial catalysts and uses
- iron/iron oxide: Haber-Bosch process (NH3)
- nickel: hydrogenation (margarine and chocolates)
- vanadium pentoxide (V2O5): sulfuric acid (H2SO4 conc)
- platinum (Pt): nitric acid (HNO3 conc)
What are bond energies used for?
To determine whether a chemical reaction is exothermic or endothermic
When equilibrium is reached, the
concentration of the reactants and products remain constant
Applications of exothermic reactions
- hand warmers
- self-heating cans
eg: - many oxidation reactions: respiration
- combustion reactions: burning fuel (methane + oxygen–> carbon dioxide + water)
- many neutralisation reactions (hydrochloric acid + sodium hydroxide –> sodium chloride + water)
Applications of endothermic reactions (much less common than exothermic reactions)
- sports injury packs: inside the pack is a bag of water and a substance such as ammonium nitrate. When the pack is squeezed, the water bag bursts and the ammonium nitrate dissolves in the water in an endothermic process.
eg: - calcium carbonate (heat) –> calcium oxide + carbon dioxide
- ## Reaction of acids with metal hydrogencarbonates (citric acid and sodium hydrogencarbonate are in sherbet sweets and react in your mouth in an endothermic reaction.
Energy must be supplied to do what?
- To break the bonds in the reactants
Energy is released when what?
- when bonds in the products are made
simple cell
- It is a source of electrical energy
- simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltage.
zinc and copper eg- simple cell
- zinc is the more reactive metal and forms ions more easily, readily releasing electrons
- the electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes
- the electrons then flow around the circuit to the copper electrode which is now the more positive electrode
- the difference in the ability of the electrodes to release electrons causes a voltage to be produced.
- the greater the difference in the reactivity of the metals, the greater the voltage produced.
- The electrolyte ysed also affects the voltage.
Why does the electrolyte used also affect the voltage?
Different ions react with the electrodes in different ways.
non- rechargeable batteries (eg: alkaline batteries)
- electrochemical cells include the familiar batteries used in everyday appliances and cars
- they work by connecting two or more cells in series, which combine to give a larger overall voltage.
- over time, the electrodes degrade as the reactions that occur there are irreversible.
- cells produce a voltage only until one of the reactants is used up and when this occurs the battery dies or goes flat.
- the products formed cannot be reverted back into reactants as the reaction is irreversible and the battery must be replaced.
