enthalpy

what happens when chemical reactions happen?

some bonds are broken and some bonds are made

what is enthalpy change?

the heat energy transferred in a reaction at constant pressure

define enthalpy?

the energy stored within bonds

state the units of enthalpy?

kj mol-1

define chemical bonds?

forces of attraction that bind together atoms in compounds

what does bond breaking reccquire?

energy

what does the “chemical system” refer to?

atoms molecules or ions making up the chemicals

can enthalpy be measured?

no

can enthalpy change be measured?

yes

define the surroundings?

• the apparatus, the laboratory, everything which isnt the chemical system

define the universe?

includes both system and surroundings

what does the law of conservation of energy state?

energy cannot be created or destroyed

when a chemical reaction involving enthalpy change occurs, where is heat energy transferred between?

heat energy is transferred between the system and the surroundings

what does heat loss in a chemical system result in?

heat gain to the surroundings, temperature increase

what does heat gain in a chemical system result in?

heat loss from the surroundings, temperature decrease

how do you calculate enthalpy change?

change of products- change of reactants

define activation energy?

the minimum amount of energy required to begin breaking bonds and start a chemical reaction

state the standard conditions?

• standard temperature of 298 K

• standard pressure of 100 kPa

• concentration of 1.0 mol dm3( for reactions in aqueous conditions)

what are changes in enthalpy affected by?

temperature and pressure

define disproportiontion?

a type of redox reaction where the same species is both oxidised and reduced

what does an enthalpy diagram show?

a diagram of a reaction to compare the enthalpy of the reactants with the enthalpy of the products

what occurs in an exothermic reaction?

• enthalpy change is negative

• heat is given out to the surroundings

• reacting chemicals loose energy

• heat lost by chemicals = the heat gained by surroundings

give examples of an exothermic reaction?

• respiration

• combustion

• neutralisation

what occurs in an endothermic reaction?

• enthalpy change is positive

• heat is taken in from the surroundings

• reacting chemicals gain energy

• heat gained by chemicals= the heat lost by surroundings

give examples of endothermic reactions?

• photosynthesis

• thermal decomposition

define standard state?

• the physical state of a substance under standard conditions or 100kpa and 298 k

examples of standard state?

• Mg(s)

• H(g)

• H2O(l)

state the 4 types of enthalpy change?

• standard enthalpy change of reaction

• standard enthalpy change of formation

• standard enthalpy change of combustion

• standard enthalpy change of neutralisation

define the standard enthalpy change of reaction?

• the enthalpy change when the reaction occurs in molar quantities shown in the chemical reaction under standard conditions

define the standard enthalpy change of fornation?

• the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

define the enthalpy change of combustion?

• the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

define the standard enthalpy change of neutralisation?

• the enthalpy change when an acid and alkali react together under standard conditions to form 1 mole of water

is bond breaking endothermic or exothermic?

endothermic, enthalpy change is +ve

is bond making endothermic or exothermic?

exothermic, enthalpy change is -ve

define average bond enthalpy?

the energy needed to break one mole of bonds in the gas phase

state the enthalpy change equation?

Q= mc^T

what is q?

the heat lost or gained in joules (j)

what is m?

the mass of water in the calorimeter in grams (g)

what is c?

specific heat capacity of water (4.18 jg -1 k-1)

what is ^T?

the change in temperature of the water in solution in kelvin (K)

what 2 conditions are needed for a reaction to take place?

• a minimum amount of kinetic energy

• must collide in the right direction

what 3 factors affect the rate or reaction?

• increasing concentration

• increasing temperature

• use of catalyst

how does concentration affect rate of reaction?

• increasing the concentration or reactants in a solution means particles will be closer together

• this increases successful collisions

• so more chances to react

how does temperature increase rate or reaction?

• provides particles with more kinetic energy so move faster

• molecules then collide more faster

how do catalysts increase rate or reaction?

• catalysts lower the activation energy by providing an alternative reaction pathway with lower activation energy#

• the catalyst remains unchanged at the end of the reaction

state the 2 types of catalysts?

• heterogeneous catalysts

• ]homogeneous catalysts

define homogeneous catalyst?

• has the same physical state as the reactants

• the reactants combine with the catalyst to make intermediate species

• this then reacts to form the products and reform the catalys

define heterogeneous catalyst?

• has a different physical state as the reactants

how are catalysts environmental friendly and sustainable?

• lower temperatures and pressures can be used so energy is saved

• less co2 is then released and fossil fuels are preserved

• also reduce waste so better atom economy

what is the spread of particle energies known as?

the boltzmann distribution curve

what is the area under the curve equal to?

the total number of molecules

explain the effect of temperature on the boltzmann distribution?

• as temperature increases boltzmann distribution increases

• this is as particles have more kinetic energy and are able to overcome high activation energy at higher speed

• increasing rate of reaction

state the rate of reaction equation?

• the amount of reactant used or product formed/ time

steps to measure change in mass?

• if the product is a gas its formation can be measured using a mass balance#

• start a timer when the reaction starts

• take measurements at regular time intervals

• make a table with a column for time and a column for mass

• the reaction is finished when the reading on the balance stops decreasing

steps to measure volume of gas given off?

• use gas syringe to measure volume of product formed

• measure the volume of gas in a syringe rather than a balance

which equation works out rate of reaction?

change in y/ change in x