Rate equations yr13

Rate of reaction units

Mol dm^-3 s^-1

Define order of reaction

• The power to which the concentration of x affects rate

What does order of reaction tell you?

• The power to which the concentration of x effects rate

What does the rate equation show and what is it?

• Mathematical link between the concentration of each reactant and the reaction rate

• Rate = K [A]^x [B]^y

What is 1st order of reaction

• Rate is directly proportional to the concentration of x

• E.g. If concentration is doubled (X 2)→ rate is doubled (X 2¹ )

What is 2nd order of reaction?

• Rate is proportional to the concentration of X squared

• E.g If concentration is doubled ( X 2) → Rate is increased by a factor of 4 ( X 2² )

What is 0 order?

• Rate does not change despite change in concentration

• E.g. if concentration was doubled (X 2) → rate increases by 2⁰ (Which is x 1 so no change)

Anything raised to the power of 0 …

• Is not in the rate equation

If a rate/ concentration graph is unclear, how can you determine the order?

• Use the equation

• Log rate = m log[A] + log k

• You work out the log of rate and the log of concentration of the reactant and log of k

• This gives you a straight line and gradient m gives you the order

Define overall order

• Sum of all the powers to which the reactants are raised to

Why is knowing the overall order helpful?

• Helps determine the rate determining step for a reaction

• Helps determine the mechanism for a reaction

• Gives an understanding of how the rate of reaction would respond to changes in concentration

What is the role of catalysts and involvement in rate equation?

• Increase the rate of reaction but are not used up in the reaction

• Appear in the rate equation NOT in the chemical equation

What happens when a reactant is in excess?

• If the concentration of a reactant is much larger than another, its concentration will only change by a very small proportion

• Thus the concentration of excess reactant is constant

• Meaning the concentration of the chemical (in excess) is unlikely to affect the rate if reaction

• Excess reactant treated as a zero order

How can you make a concentration time graph to determine the order of a reactant?

• Make the reaction mixture and start timer

• Remove a sample at regular time intervals, to determine leftover reactant concentration

• Quench the reaction by neutralising it or placing it in ice

• Analyse the concentration of leftover reactant via titration

What does the gradient of a concentration time graph tell you?

• Rate of reaction

Explain a concentration time graph depicting 0 order

• Depicts a constant decline of the concentration of the reactant

• Gradient is unchanged, even though reactant concentration decreases

• Therefore rate is constant

• Shows 0 order as reactant concentration decreases at a constant rate

Explain concentration time graph for first order

• Sloping curve with a constant half life→ time taken for concentration of reactant to half

• Order can be found out by using tangents

• Gradient = rate

• Rate is directly proportional to concentration

Explain concentration time graph for second order

• Curve declines steeply at first then levels out→ increasing half life ( time increases for concentration to halve)

• Gradient = Rate

What does the rate constant(K) demonstrate?

• Links concentrations to the rate of reaction

Larger the value of K…

• The faster the rate of reaction

What does the value of K vary with ?

• Temperature

• Larger the temperature, the larger the value of K (Rate constant)

Units of rate constant (K)

• Depends

When your trying to deduce order of a reactant from a table of experiments what do you do ?

• Find pairs of experiments where only 1 of the reactants has had a change in concentration.

What is a mechanism and how does it link to the rate determining step?

• A series of steps through which a reaction progresses often forming intermediate compounds

• Each step can have a different rate of reaction

• Slowest step is called the RDS

• And controls the overall rate of reaction

• (So if you want to speed up a reaction focus on RDS)

Chemicals in the RDS…

• Will be in the rate equation

What should you know about intermediate products and the rate equation?

• Intermediate products do not appear in the rate equation

• And instead the reactants that produced the intermediate are included in the rate equation

• (Check notes for additional guidance)

The rate equation demonstrates the link between concentration of reactants and reaction rate so any other factor affecting rate must be affecting…..

• The value of K

What is the Arrhenius equation and units ?

What does the Arrhenius equation depict?

• Shows that the rate constant varies with temperature

How do you rearrange the Arrhenius equation to workout Ea?

• Ea = (In A- InK) x RT

How else can you rearrange the Arrhenius equation, where it mimics a straight line equation?

• May have to use this equation to plot a graph→ To find out Ea

• Ink on y axis and 1/T on x

• In K has no units and 1/T → k^-1