Module 3.2.2 - Reaction rates

The rate of a chemical reaction is the change in concentration of a reactant/product per unit time

-Concentration of reactant (liquid) -Temperature - Use of a catalyst - Surface area (solids only) - Pressure (gases only)

For a chemical reaction to occur; molecules must collide with each other, with energy greater than/equal to the activation energy, in the correct orientation (successful collision) If molecules dont have collide with enough energy/wrong orientation = ineffective collision

As concentration of reactant molecules increase, rate of reaction increases. At higher concentrations there are more molecules in a given volume, meaning more frequent successful collisions

When the pressure of a gas increases, the gas molecules are pushed closer together; the number of gas molecules in a given volume increases and more frequent successful collisions occur. SAME EFFECT AS INCREASING CONCENTRATION OF GASES

As temperature of reaction mixture increases, rate of reaction increases. At a higher temperature, the average (kinetic) energy of the molecules increases, a greater proportion of molecules have energy greater than/equal to the activation energy meaning more frequent successful collisions occur

Increases the rate of a reaction and is not used up by the overall reaction

The catalyst has a different physical state from the reactants

The catalyst of a reaction has the same physical state as the reactants

A catalyst increases the rate of a reaction by providing a different reaction pathway/mechanism for the reaction, which has a lower activation energy

They have economic importance and are beneficial for sustainability

Reduced energy demand from combustion of fossil fuels -> Less CO2 emissions from combustion of fossil fuels-> Less cost due to lower energy demand

Haber process (Manufacture of ammonia) uses Fe(s) catalyst Manufacture of poly(ethene) uses Ziegler-Natta catalyst Catalytic converter to remove toxic gases from car exhaust, Rh/Pd/Pt alloy catalyst

The distribution of energies of molecules at a particular temperature

• x axis; energy, y axis; number of molecules • starts at origin (0,0) • asymptotes with x-axis at high energies • has asymmetrical shape towards the left

• Peak is the mode energy as the greatest number of molecules have that energy • Total area under the curve is equal to the total number of molecules in the mixture • Shaded area under curve represents the proportion of molecules that have the energy greater/equal to activation energy • asymmetrical shape as more molecules have lower energies

At a higher temperature, more molecules have higher energies as the average energy of molecules increases. Boltzmann flattens and shifts to the right and the shaded area under the curve is greater A greater proportion of molecules have energy greater than/equal to the activation energy so more frequent successful collisions occur

A catalyst increases the rate of reaction by lowering the activation energy. A greater proportion of molecules have energy greater than/equal to activation energy so more frequent successful collisions occur.

• measuring the mass of a reactant/product at regular time intervals • using titration at regular time intervals on a sample • using a colorimeter/UV (visible spectrophotometer) at regular time intervals • measuring the gas evolved at regular time intervals • measure the change in pH / electrical conductivity at regular time intervals

Draw a tangent to the curve at time, t Find the gradient of the tangent to the curve

Draw tangent at time = 0 and calculate the gradient