Chapter 18: Rates of reaction

Rate of reaction

How is rate of reaction found?

by measuring amount of moles of a reactant used up

Or by calculating how many moles of a product are made

Catalyst

a substance that speeds up the rate of a chemical reaction without being consumed by the overall reaction by providing an alternate pathway with a lower activation energy

activation energy

the minimum energy required to start a reaction by the breaking of bonds

What is a homogenous catalyst?

a catalyst with the same physical state as the reactant

How do homogenous catalysts work?

The catalyst and reactant react to form an intermediate

The intermediate breaks down to form the product and regenerate the catalyst

what is a heterogenous catalyst?

a catalyst with a different physical state from the reactants

How do heterogenous catalysts work?

The molecules are absorbed (weakly bonded) into the surface of the catalyst where a reaction occurs

The product molecules are then deabsorbed from the surface of the catalyst

Benefits of catalyst use

allows more specificity in products

Increased profitability due to quicker production

Lower temp requirements (reduces production costs and fossil fuel demands)

4 Drawbacks of catalyst use

Some are expensive

Some are very specific and can only catalyse 1 reaction

Some are toxic (must be disposed of carefully)

Some are poisoned by impurities (will become inefficient over time)

How does temperature increase affect the Boltzmann distribution?

Peak moves to a higher energy with a lower height

Greater proportion of molecules exceed activation energy

Why does temperature increase affect rate of reaction?

energy of molecules increase

More molecules have exceed activation energy

Proportion of effective collisions increase, more frequent collisions

So rate of reaction increases

How does the presence of a catalyst affect the Boltzmann distribution?

the curve remains the same, but the activation energy is lowered

Why does the presence of a catalyst affect the rate of reaction?

a greater proportion of molecules have energy equal to or exceeding the activation energy

So more frequent effective collisions

Rate of reaction increased

Ways to measure reaction rates

pH, pressure, volume, mass, colour change

Precipitate formation

general rate equation

rate= k[A]^m[B]ⁿ

How is the overall order of a reaction calculated?

adding up the individual orders of reaction within the reaction

0 order concentration-time graph

straight line

Negative gradient

1st order concentration-time graph

Curve, slope decreasing

Negative gradient

2nd order concentration-time graph

Curve, slope decreasing

Negative gradient

Difference between 1st and 2nd order concentration-time graph

1st order has constant half-life

2nd doesn’t

Half life

the time taken for the concentration of a reactant to fall to half of its original value

Equation for k for a first order reaction

k= ln(2)/t_1/2

Where t= half life

0 order rate-concentration graph

0 gradient straight line

1st order rate-concentration graph

straight line through origin

Positive gradient

2nd order rate-concentration graph

curve

Increasing positive gradient