4.3 How Fast? - Rates Flashcards
What is reaction rate?
The change in amount of reactants or products per unit time.
What is the purpose of a rate equation?
A rate equation tells you how the rate is affected by different concentrations of reactants.
What is the order of reaction with a respect to a reactant?
The order tells you how the concentration effects the rate.
How do you work out the overall order of a reaction?
The overall order of reaction is the sum of the order of the reactants.
What is the rate constant, k?
It relates concentrations to the rate. A greater k means a faster reaction. It is constant for a certain reaction at a particular temperature.
What is the half-life of a reaction?
The time it takes for the concentration of a reactant to half.
What is the rate-determing step?
The slowest step in a reaction mechanism. Reactants that appear in the rate equation are involved in the rate-determining step.
What is the activation energy of a reaction?
The minimum amount of kinetic energy that particles need to react.
What is a homogenous catalyst?
A catalyst that is in the same state as a reactant.
What is a heterogenous catalyst?
A catalyst that is in a different physical state from reactants.
Name one example of a homogenous catalyst.
Enzymes.
Name threes examples of heterogeneous catalysts.
Vanadium peroxide in the contact process of making sulfuric acid. Nickel in the hydrogenation of vegetable oils. Platinum in catalytic converters.
Name fives properties that may changes as a reaction progresses.
Gas Volume. Loss of mass. Colour change. Clock Reaction. Electrical conductivity.
How can you use gas volume to follow the progress of a reaction?
Collect it in a gas syringe and record volume at regular time intervals.
How can you use loss of mass to follow the progress of a reaction?
Carry out the reaction on a balance and record mass at regular time intervals.
How can you use colour change to follow the progress of a reaction?
Using a colorimeter.
How can you follow the progress of a clock reaction?
Measure the time it takes for colour change to happen. A shorter time means a faster rate.
Why can you use electrical conductivity to follow the progress of a reaction?
As the number of ions change, the electrical conductivity of the solution will also change.
How can you work out the rate from a concentration-time graph?
Draw a tangent and find the gradient.
If you increase a reactant’s concentration by x and the rate is unchanged, the order with respect to that reactant is…
0
If you increase a reactant’s concentration by x and the rate increases by x, the order with respect to that reactant is…
1
If you increase a reactant’s concentration by x and the rate increases by x^2, the order with respect to that reactant is…
2
If a concentration-time graph for a reactant has a straight line, the order with respect to that reactant is…
0
If a concentration-time graph for a reactant has a line with constant half-life, the order with respect to that reactant is…
1
If a concentration-time graph for a reactant has a half-life that isn’t constant, the order with respect to that reactant is…
2
If a rate-concentration graph for a reactant has a horizontal line, the order with respect to that reactant is… because…
0 because changing the concentration does not affect the rate.
If a rate-concentration graph for a reactant has a straight line through the origin, the order with respect to that reactant is… because…
1 because the rate is proportional to the concentration of the reactant.
If a rate-concentration graph for a reactant has a curve, the order with respect to that reactant is… because…
2 because the rate is proportional to [reactant]^2
How would you find the activation energy of a reaction?
Plot ln(1/t) vs 1/T. Find the gradient and multiply it by -R where R is the gas constant (8.31 JK^-1mol^-1)
What does a catalyst do?
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged after the reaction.
Primary halogenoalkanes are hydrolysed by OH- ions by which mechanism?
Sn2 because two molecules are involves in the rate determining step.
Tertiary halogenoalkanes are hydrolysed by OH- ions by which mechanism?
Sn1 because only one molecule is involved in the rate determining step.
What is the rate equation for the reaction of bromoethane with hydroxide ions and why?
rate=k[CH3CH2Br][OH-] because the hydrolysis happens in one step using both the bromoethane and hydroxide ion.
How do you calculate the half-life of a zero order reactant?
[reactant]/2k
How do you calculate the half-life of a first order reactant?
ln2/k
How do you calculate the half-life of a second order reactant?
1/k[reactant]
