Changes to Rate of Reaction

Kinetics

What is the rate of reaction?

The change in concentration of a reactant or product per unit time.

What is the equation for rate of reaction using concentration?

Rate of reaction = change in concentration ÷ time taken.

How can the rate of a reaction be measured?

By measuring how quickly a reactant is used up or how quickly a product is formed.

What does the gradient of a concentration-time graph represent?

The rate of reaction.

How do you calculate the rate of reaction at a particular point on a graph?

Draw a tangent to the curve at that point and calculate its gradient.

How is the gradient of a tangent calculated?

Gradient = change in y ÷ change in x.

What does a steeper gradient on a concentration-time graph indicate?

A faster rate of reaction.

Why does the gradient usually become less steep as a reaction proceeds?

The reactants are being used up, so their concentrations decrease and the reaction slows down.

What happens to the gradient when a reaction is complete?

The gradient becomes zero.

How is the initial rate of reaction determined from a graph?

Draw a tangent to the curve at time = 0 and calculate its gradient.

What are the units of rate when concentration is measured in mol dm⁻³ and time in seconds?

mol dm⁻³ s⁻¹.

How can reaction rate be calculated by measuring gas production?

Rate = volume of gas collected ÷ time taken.

What happens to the time taken to produce a fixed amount of product when reaction rate increases?

The time taken decreases.

What does collision theory state?

Reactant particles must collide for a reaction to occur, but not every collision results in a reaction.

What two conditions are required for a collision to result in a reaction?

The particles must collide with sufficient energy and in the correct orientation.

What is activation energy, Eₐ?

The minimum energy that colliding particles must possess for a reaction to occur.

What happens when particles collide with less energy than the activation energy?

They bounce apart and no reaction occurs.

Which collisions can result in a reaction?

Collisions with energy equal to or greater than the activation energy and the correct orientation.

Why is activation energy needed in a chemical reaction?

Energy is required to break some bonds before new bonds can form.

Why do many collisions between reactant particles not result in a reaction?

The particles may have insufficient energy or may collide in the wrong orientation.

What is meant by the correct orientation of colliding particles?

The particles collide in a position that allows the necessary bonds to break and new bonds to form.

Why must ethene and hydrogen bromide collide in a particular orientation to react?

The hydrogen end of H–Br must approach the C=C bond of ethene for the reaction to occur.

What happens if HBr approaches ethene in the wrong orientation?

The molecules collide and bounce apart without reacting.

What is a steric factor?

The effect of molecular shape on whether particles can collide in a suitable orientation for reaction.

What is steric hindrance?

The slowing of a reaction because large groups within a molecule block an attacking species from reaching the reaction site.

How can large groups within a molecule affect a reaction?

They can prevent another particle from approaching the reaction site closely enough to react.

Why can reactions occur slowly even though particles collide extremely frequently?

Only a small proportion of collisions have sufficient energy and the correct orientation to result in reaction.

Why do many reactions involving organic compounds need to be heated or refluxed for long periods?

They may have high activation energies, so only a small proportion of collisions have sufficient energy to react.