Aided by continued momentum in its soft-drink and snack-food units, PepsiCo Inc.'s second-quarter profits rose 21 percent on a 16 percent increase in revenues.

PepsiCo Chairman Donald M. Kendall said the firm earned $72.6 million (78 cents a share) for the quarter ended June 16 compared with $59.8 million (65 cents) a year earlier. Revenues rose to $1.18 million from $1.02 million.

For the first half, PepsiCo's earnings rose 22 percent to $117.7 million ($1.26) from $96.8 million ($1.05) as revenues climbed 18 percent to $2.198 billion from $1.857 billion.

Delta Air Lines reported fiscal 1979 earnings of $136.7 million ($6.88 a share) compared with $131.1 million ($6.60) for fiscal 1978.

Robert Oppenlander, vice president for finance, told the airline's board of directors in Atlanta yesterday that operating revenues for fiscal 1979 were $2.4 billion compared with $2 billion in fiscal 1978. Operating expenses for the 1979 period were $2.2 billion compared with $1.8 billion in fiscal 1978.

Earnings for the fourth quarter ended June 30 were $43.7 million ($2.20) compared with $43.2 million ($2.17) for the same quarter in 1978.