If the surface area of a solid is increased, its reaction with a gas will occur faster. This will increase the rate of the reaction because when the solid has a higher surface area, more of its particles (atoms or molecules) will be exposed to the gas and will therefore be able to participate in the reaction. A large block of a solid (low surface area) will have more particles that are inside of the large block. These particles will not exposed to the gas and therefore will not be able to participate in the reaction. This makes the reaction occur slower for solids with lower surface area.