hess's law, rates of reaction, and factors effecting rates of reaction

three lessons in one

what is hess’s law

if two or more chemical equations can be added together to produce an overall equation, the sum of the enthalpy equals the enthalpy change of the overall equation

what are two rules for using hess’s law

1. if you reverse a chemical reaction, you must also reverse the sign of \Delta\char"0397

2. if the coefficients in a balanced equation multiplied by a factor, the value of \Delta\char"0397 is multiplied by the same factor.

what are chemical kinetics

a branch of chemistry that studies rates of reactions

what are reaction rates

the change in concentration of a reactant or product per unit of time

What are the five factors affecting rates of reactions?

1. chemical nature of the reactants

2. molar concentration of reactants

3. temperature

4. surface area

5. presence of a catalyst

What is the first factor affecting rates of reaction? What does it mean?

Chemical nature of the reactants. This means that some substances are naturally more reactive than others. Eventhough all conditions may be the same in a reaction, the rate is relative the reactivity of the substance.

What is the second factor affecting rates of reaction? What does it mean?

Molar concentration of reactants. This means that if the intial concentration of a reactant is increased then the reaction rate also usually increases. Similarly, if the intial concentration of a reactant is decreased then the reaction rate also usually decreases

What is the third factor affecting rates of reaction? Why is it significant and what does it mean?

Temperature. Temperature has the greatest effect of rates of reaction. For every 10°C increase, the rate of reaction approximately doubles. And for every 10°C decrease, the rate of reaction approximately half as fast.

What is the fourth factor affecting rates of reaction? What does it mean?

Surface area. This means that an increase in reactant surface area increases the rate of reaction.

What is the fifth factor affecting rates of reaction? What does it mean?

Presense of a catalyst. This means that if there is a catalyst the rate of reaction generally increases as it reduces the activation energy needed to convert reactants into products.